£>tate College of Agriculture iat Cornell ^Hnibersitp Sttiaca, M. S- Htfirarj* Cornell University Library QD 549.08H 1919 A handbook of conoid-chemlstrvj the ^ »1 Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924003685363 A HANDBOOK OF COLLOID-CHEMISTRY OSTWALD A HANDBOOK OF COLLOID-CHEMISTRY THE RECOGNITION OF COLLOIDS, THE THEORY OF COLLOIDS, AND THEIR GENERAL PHYSICO-CHEMI- CAL PROPERTIES BY DR. WOLFGANG OSTWALD PRIVATDOZENT IN THE XJNIVEKSITY OF LEIPZIG SECOND ENGLISH EDITION TRANSLATED FROM THE THIRD GERMAN EDITION BY DR. MARTIN H. FISCHER PROFESSOR OF PHYSrOLOGY IN THE UNIVERSITY OF CINCINNATI WITH NUMEROUS NOTES ADDED BY EMIL HATSCHEK CASS INSTITUTE, LONDON WITH 63 ILLUSTRATIONS PHILADELPHIA P. BLAKISTON'S SON & CO. 1012 WALNUT STREET Copyright, 191Q, by P. Blakiston's Son & Co. th::e: maple phess y o h h; f j^ PREFACE TO THE SECOND EDITION Yielding to pressure from the publishers who find need at this time for a new printing of this volume, we have reluctantly met their request to revise and bring up to date the original translation as made by the one of us with the assistance of Ralph E. Oesper and Louis Berman. To make such revision when the author of the volume is alive in another country con- stitutes a delicate if not impossible task. We have met the situation by leaving entirely untouched those large portions of the volume which contain the author's individual views. Some errors in quotation and in mathematical formulas as present in the original text have been corrected. Beyond this, one of us (Hatschek) has added numerous paragraphs intended to bring to the reader various important advances in colloid- chemistry which have been made since 191 2, especially such as have to do with the mechanical properties of colloids, more particularly their viscosity. Martin H. Fischer. Emil Hatschek. Cincinnati and London. June, 1918. TRANSLATOR'S PREFACE TO THE FIRST EDITION The day is past when the importance of colloid-chemistry to the worker in the abstract or applied branches of science needs emphasis. The endeavor of the "pure" chemist to reduce all substances to crystalloid form and from the knowledge of their behavior to resynthesize the phenomena of nature has been a good one, but the limitations of such a point of view have grown daily more apparent. It happens that nature has chosen the colloid form in which to show her face. Crystalloid behavior is the exception, colloid behavior the rule, in the cosmos. Whether we deal with the regions above the earth, as the color of sky, the formation of fogs, the precipitation of rain and snow, or with the earth itself in its muddied streams, its minerals and its soils, or with the molten materials that lie under the earth, the problems of colloid-chemistry are more to the fore than have ever been the crystalloid ones. To the abstract thinker in science colloid-chemistry there- fore, because of its universality, represents the larger field. But the practical worker knows, too, that in a better knowledge of the properties of those very materials which the orthodox chemist has too often cast aside in his jellies, pastes and glues, is found the explanation of so much that interests him. Is it any wonder then that colloid-chemistry appeals to the agriculturalist, the metallurgist, the dealer in precious stones, the tanner of skins, the manufacturer of wood pulps and paper, the dyer, the his- tologist, the steel worker, the weaver of textiles, the smelter, the manufacturer of paints? Not only the inorganic world but the organic also has chosen the colloid realm in which to manifest itself. Living matter, whether of plants or animals, and under normal or pathological conditions, is chemistry in a colloid matrix; whence colloid- chemistry comes to concern every botanist and zoologist, the X PREFACE physiologist, the pathologist and the practical man in medicine and surgery. Under the circumstances, does this volume, known the world over as the authoritative and classical text, need an introduction to any of our people who think in the day's work? It can only seem somewhat strange that three large German editions and seven years were required before its first issue in the tongue of Thomas Graham and the brilliant modern group of English- speaking colloid-chemists. Wolfgang Ostwald's writings repre- sent in colloid-chemistry what those of Charles Gerhardt represent in organic, Justus Liebig in agricultural, and Wilhelm Ostwald in physical chemistry. Martin H. Fischer. EiCHBERG Laboratory of Physiology, University or Cincinnati. June, 1915. TABLE OF CONTENTS PRACTICAL INTRODUCTION Page §1. Identification of Colloid Systems by Elementary Methods. i 1. General Considerations. . . . . . i 2. The Colloid State is Independent of Chemical Composition. . 2 I. ELEMENTARY GENERAL COLLOID ANALYSIS 3. Chemically Homogeneous and Heterogeneous Liquids . 3 4. True Solutions, Mechanical Suspensions and Colloid Solutions 4 5. The Properties of Mechanical Suspensions 4 6. The Instability of Mechanical Suspensions . . S 7. Differentiation of a True from a Colloid Solution 6 8. The Tyndall Phenomenon 7 9. The Distinction of True from Colloid Solutions on the Basis of Their Mechanical Properties g 10. Dialysis Experiments. 10 11. Transition Systems .12 II. ELEMENTARY SPECIAL COLLOID ANALYSIS 12. Suspensoids and Emulsoids . . 12 13. Viscosity. . 13 14. Coagulation. .13 15. Influence of Concentration 14 16. The Electric Properties of Colloids. 14 17. The Mutual Precipitation of Colloids 16 18. Electrophoresis 16 19. Summary. 16 PART I GENERAL COLLOID-CHEMISTRY CHAPTER I THE GENERAL CONSTITUTION OF COLLOID SYSTEMS )2. The Colloids as Heterogeneous Systems . . . 21 ±. The Concept of Heterogeneity. . , . . 21 2. Physical and Chemical Heterogeneity. 22 !3. Colloids as Disperse Heterogeneous Systems . . . 23 XU CONTENTS Page 1. The Phases are in Contact with Each Other under Conditions Which Permit the Development of Much Surface between Them. 23 2. The Phases are so Distributed within the System That Externally the Whole Appears Homogeneous . 23 §4. The Disperse Phase and the Dispersion Medium . 25 §5. Specific Surface in Dispersoids; Degree of Dispersion 26 §6. Classification of the Dispersoids According to Their Degree of Dispersion . 29 1. Classification of Zsigmondy. . • • 29 2. Classification of Dispgrsoids According to Their Degree of Dis- persion 31 3. Defects of this Principle of Classification 34 4. Polydispersoids 35 5. Dispersoids Varying with Changes in Concentration . . 35 6. Temperature-variable Dispersoids • 3^ 7. Complex Dispersoids. 3° 8. Transition Phenomena . 39 §7. General Colloid-chemical Nomenclature 4° CHAPTER 11 RELATIONS BETWEEN THE PHYSICAL STATE AND THE GENERAL PROPERTIES OF COLLOID SYSTEMS §8. Classification of Dispersoids According to the States of Their Phases . 4 J.. The Physical State of the Disperse Phases as a Principle of Classi- fication. . . 42 2. Classification of the Dispersoids According to the Physical State of Their Phases 43 §9. Transition Phenomena. Complex Dispersoids . . . 44 1. General Considerations. Influence of Temperature and Degree of Dispersion 44 2. Influence of Concentration upon State in Complex Dispersoids 45 A. Complex Systems H.wing the Composition Liquid -|- Liquid (0) Influence of Concentration upon the State of the Dispersoid as a Whole . 47 (6) Influence of Concentration on the State of the Disperse Phase 48 B. Complex Dispersoids Having the Composition Liquid + Solid (a) Influence of Concentration on the State of the Dispersoid as a Whole . 48 (6) Influence of Concentration on the State of the Disperse Phase. 49 510. Colloid Systems as Suspensoids and Emulsoids 49 1. General Considerations. . 49 2. The Empirical Establishment of Two Classes of Colloids 50 3. The Theoretical Characterization of the Two Classes of Colloids . 51 4. The Frequency of Occurrence of Complex Emulsoids. . 53 5. Relation of These Two CoUoid Classes to Molecular Dispersoids . 54 6. Suspensoids and Emulsoids . ... .54 §11. Transition Phenomena between Suspensoids and Emulsoids . 55 CONTENTS Xlll Page §12. The Cn-stalline (Vectorial) Constitution of tlio Disperse Pliase. . 56 1. The Concept of Crystallinity 56 2. Direct Proof of Crystallinity in Colloids 58 3. Indirect Proof for the Crystallinity of Colloid Phases. 58 4. Dependence of Crystallinity upon Size of Particles. 61 5. CrystaUinity of Emulsoids . 64 CHAPTER III GENERAL ENERGETICS OF THE DISFERSOIDS §13. Surface Energies . 66 I. Forms of Energy Characteristic of Dispersoids ' . 66 ^. Surface Energy of the First Order . 66 3. Surface Energy of the Second Order 67 4. The Relation of Surface Energy of the Second Order to Other Forms of Energy . . 71 §14. Dependence of Surface Energies upon Specific Surface . 72 1. General Considerations. 72 2. Surface Energy of the First Order and Specific Surface. 74 3. Surface Energy of the Second Order and Specific Surface. 74 4. Dependence of Surface Tensions upon Specific Surface. 76 §iS- Reciprocal Effects of the Two Surface Energies . 77 1. General Considerations. 77 2. Discontinuous Increase in Surface 78 3. Theory of Dispersion . . 80 4. Consequences of the Energetic Theory of Dispersion . 82 5. Discontinuous Diminutions in Surface 84 6. Theory of Condensation 88 §16. Influence of the Specific Surface upon the Relations between Surface Energies and Other Forms of Energy 90 I. Specific Surface and Volume Energy; Capillary Pressure 90 ^. Specific Surface and Changes of State 91 3. Specific Surface and Electrical Energy 92 4. Specific Surface and Chemical Energy 93 5. Specific Surface and Radiant Energy. . 96 CHAPTER IV DISTRIBUTION OF THE COLLOID STATE AND THE CONCEPT OF COLLOID CHEMISTRY §17. The Fundamental Independence of the Colloid State of the Chemical Nature of the Phases ... .99 1. Statistical and Experimental Development of the Idea of the Universality of the Colloid State. . . . . -99 2. Universality of the Colloid State as a Necessary Consequence of Characterizing Colloid Solutions as J>isperse Systems. . . loi §18. IsocoUoids . . 102 XIV CONTENTS Page §19. Multiplicity of the Colloid State of One and the Same Substance. Ex- ample: Colloid Ice . . . . . .... 106 1. Isocolloids of H2O. . 107 2. Chemically Heterogeneous H 2O Colloids . . 109 §20. The Concept of Colloid-chemistry. . m PART II SPECIAL COLLOID-CHEMISTRY CHAPTER V MECHANICAL PROPERTIES OF COLLOID SYSTEMS I. Relations of Volume and Mass in Colloids §21. Volume and Density Relations in Colloids . ... 115 1. Volume Relations of Colloid Systems. 115 2. Density and Space Relations in Colloid Systems. , 120 3. The Concentration Function of Density in Colloid Systems. 124 4. Thermal Coefficient of Expansion in Colloids 126 §22. Vapor Tension, BoiUng Point and Freezing Point of Colloid Solutions. 128 i. General Remarks .... 128 .i. Measurements of Vapor Pressure of Colloid Solutions 129 3. Elevation in Boiling Point of Colloid Solutions . 130 4. Depression of Freezing Point of Colloid Solutions , 131 §23. Mass-relations in Colloids . . 132 i. Concentration of Colloid Systems 132 2. Experimental Work on Saturation in Colloid Solutions. . . . 134 3. Theoretical Considerations Bearing on the Saturation of Colloids 136 4. Supersaturation in CoUoid Systems .... . . 138 §24. Molecular Weight of Substances in the Colloid State as Pleasured by Changes in the Constants of the Dispersing Medium. . 140 1. General Remarks 140 2. Examples of the "Molecular Weights" of Substances in the Colloid State as Determined by Changes in the Constants of the Dispersing Medium ..... 142 II. Internal Friction and Surface Tension of Colloids §25. Internal Friction of Colloid Systems. 145 I. General Remarks . 145 i. Internal Friction of Suspensoids. . 146 3. Effects of External Conditions upon the Viscosity of Suspensoids. 150 4. Mechanical Theory of the Viscosity Relations in Suspensoids. . 152 5. Viscosity of Emulsoids. Literature . 155 6. Viscosity Changes in Emulsoids with Time 157 7. Effect of ilechanical Treatment on Viscosity of Emulsoids. . . 161 8. Influence of "Inoculation" on Internal Friction of Emulsoids. 161 9. Rate of Shear and Viscosity of Emulsoids. . . 162 CONTENTS XV Page 10. Influence of Thermal Plistory on Viscosity of Emulsoids . .164 11. Influence of Concentration on Internal Friction of Emulsoids. 165 12. Influence of Temperature on Viscosity of Emulsoids . . 168 13. Influence of Added Substances on Viscosity of Emulsoids. 169 14. Effect of Added Substances on Internal Friction of Emulsoids; Behavior of Protein Solutions. . . . 173 15. Influence of Added Substances on Viscosity of Emulsoids. Effects of Non-electrolytes and Mixture of Dispersing Media 177 16. Theory of Viscosity of Emulsoids 178 17. Viscosity and Electrical Charge of Disperse Phase. 179 18. \'iscosity and Degree of Dispersion; Viscosity of Coarse and Com- plex Dispersions. . . . 181 19. Viscosity and Type of Disperse Phase 184 §26. Surface Tension of Colloid Solutions. 185 1. General Remarks . 185 2. Experimental Facts 186 CHAPTER VI MECHANICAL PROPERTIES OF COLLOID SYSTEMS III. JIOVEMENT IN CoLLOID SvSTEMS .\XD ItS RESULTS §27. Brownian Movement . . 191 1. General Remarks 191 2. The Independence of Brownian Movement of External Sources of Energy. . . 194 3. More Exact Determination and ^Measurement of Brownian ilovc- ment. 197 4. Uniformity of Brownian ilovement 200 5. Influence of the Specific Surface of the Particles. 201 6. Influence of the Concentration of the Dispersoid. 202 7. Influence of the Viscosity of the Dispersion JMedium. 202 8. Influence of Temperature. . 203 9. Influence of Added Substances 205 10. Influence of Electrical Charge. 205 11. Influence of Gravity on the Distribution of Oscillating Particles. 206 12. Validity of Stokes' Law for Highly Dispersed Particles . . . 209 13. Kinetic Theory of Brownian Movement 210 14. Determination of the "Molecular Weight" of Dispersed Particles from Their Brownian Movement. . 214 §28. Diffusibility of Colloids . 215 1. General Remarks . 21s 2. Experimental Study of Diffusion of Colloids. 216 3. Experimental Facts Regarding Diffusion of Colloids 218 4. Influence of Degree of 1 dispersion on Diffusion W-locity 220 5. Theory of Colloid Diffusion. . . . . 222 6. Effect of Added Substances on Colloid Diffusion. Spurious Diffu- sion of Colloids ... . . j.'4 XVI CONTENTS Page §29. Dialysis of Colloid Systems. 227 1. General Remarks . .227 2. Methods of Dialysis . 228 3. Experimental Facts Regarding the Dialysis of Colloids. . 229 4. Special Observations Regarding the Dialysis of Colloids . 232 §30. Osmosis of Colloid Systems. . 236 1. General Remarks and Literature. 236 2. ]Methods of Measuring the Osmotic Pressure of Colloids 238 3. Instability of Osmotic Pressure of Colloids .... 240 4. Influence of Concentration on Osmotic Pressure of Colloids. 243 S- Influence of Temperature on Osmotic Pressure of Colloids 247 6. Influence of Added Substances on Osmotic Pressure of Colloids. 249 7. On the Theory of Osmotic Pressure of Colloids . 258 8. Determination of the "Molecular Weight" of Colloid Systems by Osmotic Means .... 263 9. On the Moleculo-kinetic Theory of Osmosis in Colloid Systems 266 ADDEXoni — Other Types of ^Iovement in Dispersoids. 267 §31. Filtration and Ultrafiltration of Colloid Systems. 268 1. Filtration of Colloid Systems 268 2. Ultrafiltration of Colloid Systems 269 Author Index 273 StJBjECT Index 279 PRACTICAL INTRODUCTION §1. Identification of Colloid Systems by Elementary Methods (The Elements of Qualitative Colloid-chemical Analysis) I. General Considerations. — The teachings of colloid-chem- istry are by no means so familiar to all who encounter colloid substances in their scientific or practical work that the questions : "How can we recognize a colloid?" or "When is a body said to be a colloid?"' are no longer raised. These questions have often been put to me, not only by such men of science as physicists, physical chemists, physicians and mineralogists, but by technolo- gists who for years perhaps have worked exclusively in such practical colloid problems as the manufacture of rubber. Even the organic and inorganic chemists frequently encounter phe- nomena, particularly when they work with highly polymerized and highly complex substances that remind them of what they know of the properties of colloids, and make them ask how they can de- termine quickly and simply whether colloid-chemical principles will help them in the solution of their problem or no. As a matter of fact I am of the opinion that such questions have not been asked frequently enough, say in organic chemistry, where examination of the colloid behavior of one and the same organic substance in different solvents would throw much light on the properties observed.^ The youth of colloid-chemistry itself justifies such questions, and their discussion is by no means either useless or superfluous. An answer to the question: "How do we know when we • We need but call to mind the modern problem of the relation to each other, in solvents of various kinds, of color, chemical constitution, and molecular state as studied by A. Hantzsch and his pupils. It seems to me that a colloid-chemical (dialytic or ultramicroscopic) examination of such variously colored solutions would bring light especially in those cases in which molecular weight determinations have been exhausted without result. The failure of Beer's law governing the proportion- ality between thickness of layer and light absorption when applied to colloids and to solutions of dyes, o.xime salts and organic ammonium salts as well as other facts seem to me to indicate that suitable colloid-chemical investigations in this field will bring to light as surprising facts as did those of J. Amann (Koll.-Zeitschr., 6, 235, 7, 67 (1910)) on the colloid and molecular solubility of iodine in various solvents. , I 2 COLLOID-CHEMISTRY are dealing with a colloid?" would consist in a presentation of the elementary properties and the experimentally observed be- havior of colloid substances. Such an analysis would constitute the elements of a qualitative colloid-chemical analysis. A possible method of procedure in attempting to discover the colloid nature of any substance is indicated in the following: 2. The Colloid State is Independent of Chemical Composition. — At first sight one might hope to obtain an answer to the question under consideration by constructing a comprehensive table of all the colloid substances or groups of substances known. As a matter of fact such attempts^ have been made even recently, but never with the full approval of competent workers in the field. It was soon noticed that we cannot speak of colloid sub- stances in the same way as we (still) do of "liquid-crystalline" or "radio-active" substances. We have been compelled to rec- ognize that colloid properties are in no way connected with any particular type of chemical constitution to the end that only certain elements or certain compounds, for example, appear as colloids. We can speak of "colloids" only as we speak of "crvs- tals," "amorphous" substances, "soluble and insoluble" sub- stances, or better still of "gaseous, liquid, and solid" substances. All substances can appear as colloids under appropriate conditions. This peculiarity of colloid-chemistry, through which it thus pre- sents itself not as a study of colloid substances but as a study of the colloid state, will be discussed in detail later. But it is of great importance for even an elementary characterization of col- loid substances to know that depending upon experimental con- ditions one and the same chemical compound can appear either as a colloid or as a non-colloid. Generally speaking, the knowledge of the chemical constitu- tion of a substance furnishes no trustworthy indication as to whether or not we are dealing with a colloid. Only one law has thus far been deduced governing the relation between chemical constitution and colloid state: The more complex chemically the compound, the greater the probability tJiat it is in a colloid state. Thus most of the native proteins appear in a colloid state; and the chemical composition of the original colloid, namely gelatine, is so complex that we are still largely ignorant concerning it. 1 See, for example, KoU.-Zeitschr., 2, 53 (1907). PRACTICAL INTRODUCTION 3 Solid and liquid and even gaseous bodies may appear in the colloid state. "^ The liquid colloids are the most numerous and the most important, and thus far have been most studied. When- ever we deal with the class properties of the colloids we there- fore usually refer to these. I. ELEMENTARY GENERAL COLLOID ANALYSIS 3. Chemically Homogeneous and Heterogeneous Liquids. — If we wish to inquire into the possible colloid nature of a given liquid, it is well to decide first whether it is chemically Jiomoge- neous or chemically Jielerogeneous. In an ideal case a chemically homogeneous liquid has the following properties: i. It is susceptible of hylotropic change, that is, it can be evaporated or frozen without changing its com- position at any time during the manipulation. 2 . The hylotropic transformations take place within narrow limits of temperature and pressure; there is only one boiling temperature and one congelation temperature; we speak of melting and boiling points. Among further properties of an ideal liquid is to be mentioned the fact that the temperature coefficient of its molar surface energy equals 2.12.- As is well known, there are a great many substances, or- ganic liquids, more particularly, which fulfill these requirements in part only. They are the mixtures of isomeric, mctamcric, and polymeric substances; to which we may add the so-called asso- ciated liquids. Even though all these liquids show the same elementary analysis in every state of aggregation, yet they can be separated by fractional distillation, for example, into parts having different boiling points; or, notwithstanding the analytic- ally identical composition of the liquid undergoing distillation and the distillate, it is noted that the former is not completely evaporated at any definite temperature. These facts are illus- trated by the behavior of polymerized liquids such as styrol- metastyrol. Again, as in associated liquids, the molar surface energy is found to be less than the normal. The following rule may be stated regarding the relation of these properties to the possibility of the appearance of a colloid 1 See page 43. ^ See the textbook of Wilh. Ostwald, Grundr. d. allgem. Chemie, 4 Aufl., 1519, for a discussion of the general concepts of physical chemistry employed here. 4 COLLOID -CHEMISTRY state in liquids of constant composition: The more a liquid approaches the ideal of chemical homogeneity, the less probable that it is ill the colloid state. Therefore, if from general physico- chemical examination we know a liquid not to be "normal" with regard to exactness of boiling point, molar surface energy, etc., it is possible that we are dealing with a "physical mixture," and therefore with a molecular or colloid solution. Colloid liquids showing the same analytical composition with every hylotropic transformation are by no means rare. Thus far these have been little studied from a colloid-chemical point of view. Details regarding the peculiarities of these so-called isodispersoids, more especially the isocolloids,^ will be given later. Nearly all the colloid solutions investigated thus far belong to the class of the chemically heterogeneous liquids discussed below. Since the fundamental properties of colloid hquids depend, not upon chemical composition but upon physical conditions which are especially to be encountered in chemically heterogeneous liquids, we shall also discuss in the succeeding paragraphs the general data by means of which we recognize the colloid character of chemically homogeneous liquids. 4. True Solutions, Mechanical Suspensions and Colloid Solu- tions. — Chemically heterogeneous liquids can be separated by changes in temperature and pressure (distillation, freezing, etc.) into at least two components of different chemical composition. When we have thus determined in our unknown that we are deal- ing with a chemically heterogeneous liquid it may appear in any one of three states: (a) The unknown may be an ordinary or "true" (molecular- disperse) solution of one or more substances. (&) It may be a coarse "mechanical suspension" of one or more substances which form true solutions to a limited degree only, if at all. (c) It may be a colloid solution. 5. The Properties of Mechanical Suspensions. — In a qualita- tive analysis for the determination of the "degree of dispersion"^ ' Examples of such liquid isocoUoids are oils, petroleum, paraffin, styrol- metast3Tol, liquid sulphur at temperatures above 170°, highly polymerized liquids, etc. ^ See page 29 for a discussion of the concept "degree of dispersion;" the three classes of systems mentioned above are distinguished from each other by their different degrees of dispersion. PRACTICAL INTRODUCTION 5 in a heterogeneous liquid the second of the above possibilities can be disposed of most easily. Typical "mechanical sus- pensions" of substances but slightly soluble in liquids, as sus- pensions of quartz, kaolin, or oil in water, are turbid in trans- mitted light, and their individual particles can be recognized under the microscope (though sometimes only with high magni- fications and special optical means). If no microscope is available, filtration is the next simplest method by which a suspension can be recognized. Ordinary filter paper holds back particles having a diameter greater than about 5m; a hardened filter (Schleicher and Schiill, No. 602 e.h.), those about 2/1 in diameter. Clay cylinders and the so-called Pukall filters which are frequently employed in bacteriology will even hold back particles about 0.4 to 0.2^ in diameter.^ The size of the particles in question can therefore be roughly measured by the employment of such differently permeable filters. When applied to emulsions, that is, suspensions of droplets of one Uquid in another filtration is successful only when the suspended droplets are not materially deformed during filtration. As the investiga- tions of E. Hatschek on the filtration of emulsions show, this difi&culty does not appear if the droplets are moderately viscous, as are the droplets of castor oil or olive oil; or when their surfaces are in a condition which gives the droplets themselves sufficient stiffness. Such stiffness may result from the formation of thin elastic membranes about the droplets, of the nature of the well-known saponin or peptone films, ^ or it may be due — and this seems most important — to the small size of the droplets with its accompanying increase in surface energy. As E. Hatschek has shown, it is often possible to separate emulsions into their components by means of appropriate filters. 6. The Instability of Mechanical Suspensions. — Another characteristic of coarse suspensions of solid and liquid particles is their instability, that is their tendency to separate "spon- taneously," i.e., by the action of gravity, into their components. If we can exclude the stabilizing effects of additions of viscous substances such as gelatine, tragacanth, etc., as well as the peculiar 1 For details regarding permeability and size of pores in various filters see H. Bechhold, Zeitschr. f. physik. Chem., 64, 342 (1908). "E. Hatschek, KoU.-Zeitschr., 6, 254 (1910); 7, 81 (1910). 6 COLLOID-CHEMISTRY "protective action" of small amounts of soap, saponin, albumose, etc., separation occurs in typical coarse suspensions in accordance with the difference in the densities of their components.^ Con- siderable acceleration in separation can be eilected by moderately centrifuging the mixture. A hand-centrifuge such as is employed in the study of blood does very well. The suspended component then separates out, in accordance with the difference in density, either in the form of a precipitate or of a supernatant layer. After such a separation has been accomplished either by gravity or with the assistance of a centrifuge, the original system can in most instances be restored by shaking the components together again. When the suspended particles are in a very finely divided condition, indefinite or negative results are obtained by these procedures. Under such circumstances two possibilities still remain: the liquid in question is either a "true" or a "colloid" solution. 7. Differentiation of a True from a Colloid Solution. — It is generally harder to distinguish a true from a colloid solution than to distinguish a coarse suspension from either, yet this problem is precisely the one that arises most frequently. We must there- fore discuss the methods involved in detail. A. Optical Differences. — Absolutely clear liquids are formed as a rule by substances in molecular or true solution. If a liquid {which is not chemically homogeneous, and which is not a coarse suspension) is seen to be turbid, we may suspect that it is a colloid solution. The existence of a slight turbidity may be recognized on inspection of a rather thick layer of the liquid in a thin-walled glass vessel against a dead black background (black paper, or better, black velvet). If the liquid is colorless but turbid, the background shining through it assumes a grayish-white appearance. In the case of colored liquids (in the examination of which it may be necessary to employ a particular dilution or thickness of layer) .an optical effect appears which is similar to that observed on mixing water-colors with small quantities of opaque white (the colors become milky). Different varieties of 'In this experiment it is well to use very long tubes and relatively "dilute" systems. The temperature must be kept constant to prevent mixing of the layers by convection currents. Closed basement rooms may be used if a suitable thermo- stat is not available. PRACTICAL INTRODUCTION nephelometers have been constructed for the more exact deter- mination of the degree of turbidity.^ 8. The Tyndall Phenomenon. — A far more dehcate method of demonstrating the presence of a very fine turbidity consists in the use of the so-called Tyndall phenomenon. It is well known that when, for example, the air of a room is intensely illuminated, say by sunlight, from one side only, dust particles are rendered visible which cannot be seen when illumination is equal on all sides. This is the prototype of the so-called Tyndall phenomenon, the theory of which will be discussed later. Extraordinarily iine turbidities Tyndall phenomenon. can be rendered visible by such means; in fact this holds true to such an extent that special measures become necessary if we would obtain, for example, an absolutely "optically empty" distilled water; ordinary distilled water regularly shows individual dust particles. Tyndall experiments can be best carried out where sunlight and a darkened room are available. The phenomenon becomes beautifully evident if we but let a sharply defined ray of light, • For simpler forms of such apparatus see H. von Oettingen, Zeitschr. f. physik. Chem., 33, i (igoo); J. Friedlander, ibid., 38, 430 (igoi). 8 COLLOID-CHEMISTRY entering a darkened room through a hole bored in the shutter of a window, pass through the Hquid in question contained in a thin-walled test tube. Very good results are also obtained if a projection lantern is used from which the light rays are concen- trated as much as possible by means of a condenser and dia- phragm (see Fig. i). A powerful incandescent lamp^ enclosed in a box that is impervious to light and provided with a small opening, and if possible with a condenser, is generally satisfac- tory also. The thinnest and clearest glass vessels must be used. Special advantages are offered by vessels with parallel walls which reflect light least. When we work with hot or very cold liquids we use cotton-stoppered, double-walled tubes from which the air has been exhausted (Dewar tubes). When cold liquids are used in these, the tubes may be immersed in alcohol to avoid the condensation of water on their outer walls. It should now be noted that it is not the presence of many more or less evident particles which may be recognized either macroscopically or microscopically that distinguishes a colloid from a molecular-disperse (true) solution. It is rather the in- tensity of the unbroken light-cone passing through the solution which betrays the state of liquid. It is safe to say that liquids which show no definite Tyndall light-cone or show it only in high concentrations are molecular-disperse solutions. Practically all colloid solutions give a positive Tyndall effect. The Tyndall phenomenon is not to be confounded with fluorescence. When a ray of light enters certain solutions, such as those of some dye-stuffs and alkaloids, the path ' of the beam betrays itself in brilhant colors ' even though these solutions may not be in the colloid state. The fluorescence can be dis- tinguished from the Tyndall effect by looking at the light-cone with a Nicol prism. If we look at the Tyndall cone of a colloid solution through a Nicol prism we find that it disappears when the prism is rotated, to light up again at a certain angle. Fluorescent light remains visible at all angles. Emphasis should be laid on the fact that the Tyndall effect is of particular value in the recognition of isocolloids. It should further be mentioned that the microscopic ex- 1 Not only electrically lighted but gas-lighted projection apparatus as used in photographic enlarging, when combined with a condenser is suited for this purpose. PRACTICAL INTRODUCTION amination of a Tyndall cone with the highest avaihiljk' muKnili- cations at times permits us to see the individual particles which in their totalit}- gi\-e rise to the light-cone. This is called ullra- microscopy. Since for ultramicroscopy special apparatus and powerful sources of light arc necessary which are by no means generally available, and since the technique of ultramicroscopy is by no means simple, we cannot further discuss the subject in this elementary outline of colloid anah'sis. 9. The Distinction of True from Colloid Solutions on the Basis of Their Mechanical Properties. — B. Mechanical Differences. — Dijfu- sioii and dialysis experiments provide us with two further simple methods for distinguishing molecular-disperse (true) from colloid solutions. These might be called the "classical" methods for the qualitative analysis of solutions, for it was by them that Thomas Gra- ham in 1861 first distinguished be- tween the ''states" of different solu- tions and thus introduced the concept "colloid." (c7) Diffusion- Experiments. — Per- haps the simplest and most con^'enient experimental method of estimating the diffusion velocity of a dissohx-d substance depends upon the fact that moder- ately concentrated jellies of gelatine, agar, etc., offer onl\- slight or no resist- ance to the dilfusKm yl substances j„ents w,th gelatine hcIs at through them, as determined h\ com- '^'"'^ ''f --i ''°""- ("> (■^'''"'-■'d) coiiK'o red; (0) (moleeularly parison with the diffusion of these same dispersed) safranm. substances through the pure sohent. For such tests we prepare a 5 percent gelatine, or ))el ter a j ]H'rcent agar solution, lill some test tubes al3(.)ut halfwa)- with the hot solution, and allow it to congeal. It is well to use gelatinizing substances that have been thoroughl_\- washed and purijieil. The lO COLLOID -CHEMISTRY solution under examination is then poured upon these gelatine or agar layers and the tubes are left standing, variations in temper- ature being avoided as far as possible. A true solution in water, either of a dye such as safranin, or of a colored salt such as copper sulphate is taken as a control. If the solution undergoing analysis is colored a picture similar to that shown in Fig. 2 may be seen after a day or two. While non-colloids, that is molecular-dis- perse or true solutions, gradually spread down into the jelly, colloid solutions do this only very slightly or not at all. In other words, substances in the colloid state practically do not diffuse at all. At the best they diffuse with extreme slowness when compared with the behavior of substances in m.olecular solution. If it is feared that a liquid of high specific gravity may by mechanical means force itself into the jelly, a small tube half filled with gelatine or agar may be placed mouth downward into the solution contained in a second larger vessel. The tube is removed after a few days and carefully washed when it also will show the phenomena that have been described. If the liquid under examination is light colored or colorless the test tube con- taining the gelatine or agar is dipped for an instant into hot water so that the jelly slips out. This is then divided into several slices of equal size, and the individual slices are examined analyt- ically for their content of the substance in question. This method is not generally applicable to the analysis of isocolloids, nor when marked chemical or colloid-chemical reac- tions take place between the jelly and the liquid under examina- tion. Under such circumstances it is necessary to resort to other methods. 10. Dialysis Experiments. — {b) Dialysis, a process closely re- lated to diffusion, depends upon the fact that many animal, vegetable and artificial membranes hold back substances in colloid solution while they allow substances in molecular solution to pass through them whenever such a membrane separates the liquid under examination from the pure dispersion medium (the solvent) . Parchment bags, so-called diffusion thimbles made in one piece (see Figs. 3 and 4), pig and fish bladders, and artificially prepared colloid membranes form the most convenient as well as the most frequently employed of these. The last-named are made by dipping a large, well-cleaned test tube into collodion dissolved in PRACTICAL INTRODUCTION ether and alcohol, permitting the collodion layer formed to harden slightly by evaporation, repeating the process if necessary, and then hardening the whole by washing in water. The collo- dion bag is then carefully drawn off the tube.^ When only small amounts of liquid are to be analyzed coUoidally, diffusion thimbles (Schleicher and Schiill) arranged as shown in Fig. 3 are par- ticularly useful. For this purpose a small Erlenmeyer flask is used, into the neck of which the diffusion thimble fits snugly; the flask is first filled with pure solvent while the liquid under examina- tion is poured into the thimble which is then closed with a cork stopper. In this way, aided by the slight swelling of the thimble which usually occurs, evaporation and the entrance of dust into Figs. 3 and 4. — Simple arrangement for dialytic analysis. the liquid are largely prevented. It is evident that if the solutions under examination are alcoholic or ethereal in character, collodion thimbles cannot be used. When deahng with such volatile liquids it is advisable to employ glass-stoppered vessels in which the dialyzer is placed or suspended as shown in Fig. 4. The dialyzer distinguishes colloid from crystalloid solutions in thai il does not allow the former to pass through the membrane into the outer liquid. Occasionally we find that a colloid "phase" will pass 1 Details of various methods of preparation may be found in A. Cotton and H. Mouton: Les Ultramicroscopes, 117, Paris, 1906; L. Bigelow, Journ. Am. Chem. Soc, 29, 1576 (1907); J. Duclaux, Journ. Chem. Phys., 7, 43° (i9°9)i W. Biltz and A. von Vegesack, Zeitschr. f. physik. Chem., 63, 369 (1909). 12 COLLOID -CHEMISTRY together with a molecularly dissolved phase into the outer liquid. But this happens only at first. After the outer liquid has been renewed once or twice, no more of the colloid phase comes through. Sometimes a dissolved substance will penetrate a collodion membrane when it is held back by the less porous parchment paper. In such cases we are evidently dealing with a "highly disperse" (finely divided) colloid, or to put it in another way, with a substance occupying a position midway between the colloid and molecular-disperse state. So-called ultrafilters are used for more exact determinations of the degree of subdivision, but they cannot be discussed here be- cause they are rather complex (see later). 11. Transition Systems. — It will nearly always be possible to determine by one or more of the methods described whether a substance in solution is in the colloid or in the molecular-dis- perse state. At the same time it must be admitted that we encounter cases in which one and the same liquid yields different results with different methods. Thus a pure congo red shows only a faint Tyndall cone, yet it scarcely diffuses through parchment paper. Protein solutions behave in a similar way in certain concentrations, etc. For a complete analysis it is therefore not only advisable but necessary to employ several methods. But even then it may occasionally be doubtful whether we are dealing with a colloid or with a molecular-disperse solution. These cases constitute the extremely interesting transitional types between the two kinds of solution. Their state can be completely analyzed only by application to them of the more refined methods of colloid and physical chemistry — ultramicroscopy, ultrafiltration, molecular weight determination, etc. n. ELEMENTARY SPECIAL COLLOID ANALYSIS 12. Suspensoids and Emulsoids. — When one undertakes detailed work with substances in the colloid state one soon dis- covers that the indiyidual illustrations arrange themselves in two classes of systems which differ markedly from each other, in spite of the fact that all are possessed of the same general prop- erties that we have already discussed. These two groups of colloid solutions are the suspension colloids (suspensoids) and PRACTICAL INTRODUCTION 13 the emulsion colloids (emulsoids), or as they are also called, the lyophobic (hydrophobic) and lyophilic (hydrophilic) colloids. The theoretical basis for such nomenclature will be discussed later. In passing, it should be noted that the two terminologies are not entirely synonymous, though for practical purposes they may be so regarded. When by the general methods previously discussed we have discovered that we are dealing with a colloid solution we have next to determine whether it is a suspensoid or an emulsoid. Of the many means of doing this we describe the following because they are particularly characteristic and simplest in character. 13. Viscosity. — The viscosity of a suspension colloid, par- ticularly in low concentration, is imperceptibly greater than that of the pure dispersion medium (the pure solvent). In contra- distinction, the \dscosity of an emulsion colloid even in low con- centration is much greater than that of its dispersion medium; in fact at higher concentrations this becomes so great that the colloid solution assumes an oily or even a gelatinous consistency. Further, the \dscosity of an emulsion colloid generally increases rapidly with decrease in temperature which is not the case with a suspension colloid. The simplest way of estimating experi- mentally the viscosity of a colloid solution and its variations with temperature and concentration is to measure the time of outflow of a constant volume of liquid from a standard volumetric (lo cc.) pipette. Roughly, the viscosity is inversely proportional to the time of outiiow. 14. Coagulation. — It is characteristic of colloid solutions that the substance in colloid solution may be easily precipitated or "coagulated" through various agencies (see Figs. 5 and 6). Elec- trolytes such as neutral salts are particularly effective. The suspension colloids are easily coagulated when minute quantities of salts, especially those having a polyvalent ion, are added to them, while the emulsion colloids are precipitated only after the addition of much larger quantities of salt. This is particularly true of hydrosols, that is of colloids having water as the dispersion medium. If, for example, aluminium sulphate (ordinary alum serves the same purpose) is selected as the coagulant, it is found that almost all suspension colloids are precipitated by this as soon as it is present in a i percent concentration. Much higher 14 COLLOID -CHianSTRY Concentrations are necessary to j)recii3itate the typical emulsion colloids. In fact the coagulati(jn of man\- emulsion colloids is not Iji'ought about until the neutral salts ha\-e been added to the point of saturation. In makim,' these Cjualitati\"e anah'scs erne must not use siills of Ihc licavy mclals, for they frequently produce entireh' abnormal coa.uiulalion elTects. 15. Influence of Concentration. — Une will occasional!}' en- counter instances in whicli iieither \'iscosit\' nor coagulation deter- minations will ser\-e to distinguish clearh' a suspension colloid from an emulsion colloid. It is then ad\-isable to compare with each other rather dilute solutions of suspensi(.)n colloids anri ^. — - .\i .ii-coayi:lated. Fi'^. 0, — C- ia;.,'ulateil throu^'h addition of 2 r>ercent sulphuric acid. Coagulation of an aciueous suspension i.tf lamp-black- (After R. E. Free.) rather concentrated s(dutions of emulsion colloids. Wc encounter here also a series of interesting transitional t\-pes which can be accurateh' anah"zed (jnh' through quantitati\'e stud\'. The suspenstjid or emulsoid state is not a constant or integral property of a chemical siibstijiicc. it is the result of a series of physico- chemical A'ariables which Ijring about a particular state in a chemical substance. 16. The Electric Properties of CoUoids. — Colloifl solutions have a characteristic electric behavior which explains man\' of PRACTICAL INTRODUCTION IS tlieir peculiar propcrtirs. Mdsl substaiK cs in (olloid sokilioa assume au electric charge luwaid Iheir disptTsinn medium, tlioiii^fh the magnitude nf this charge \aries greatl)'. We are aldt' tij distinguish l>et\\een negati\el\' and ])(isiti\'el\' (harged sub- stances in colloid solutions. Tlie simplest metlmd of deternu'ning "with \\diich of these we are (hiding in a gi\'en case is t 0. 1 mm. = I X 10-2 cm. lo" 600 square cm. 6.io2 O.OI mm. = I X io~' cm. 10' 6000 square cm. 6.10' I.O M = I X lo"* cm. io'2 6 square m. 6.io< O.i M = I X io~' cm. loi' 60 square m. 6.10' O.OI M = I X io~* cm. I018 600 square m. 6.io« 1 .0 t^t^ = I X io~' cm. lO^l 6000 square cm. 6.10' 0.1 MM = I X io~* cm. lO^' 6 hectares 6.108 O.OI I'ti = I X io-» cm. 10" 60 hectares 6.10" O.OOI MM = I X lo-i" cm. ioS° 6 square km. 6.io'» Particles somewhat less than lo/xfj. in diameter may be distin- guished optically by means of the Ultramicroscope of H. Sieden- topf and R. Zsigmondy. A i cm. cube of metallic gold subdivided up to the limit of ultramicroscopic visibility would therefore have a total surface of over 600 square meters and a specific surface of 6.10* Even at this point we begin to enter the sphere of molecu- lar dimensions. Lobry de Bruyn and Wolff' for instance, calcu- lated an approximate diameter of 5/iyu for the starch molecule. If a cubic centimeter of dry starch could be subdivided into molecules, that is if it could be "dissolved" in the ordinary sense of the word, the starch would present a total surface of several thousand square meters toward the solvent. When we deal with the molecular dimensions of gases and of substances in crys- talloid solution, assuming for their average molecular diameter the value of i.io~* we obtain values of several hectares for i cubic centimeter of dissolved substance. Thus in 100 cc. of a 10 per- cent sugar solution there would be an "internal surface" of about 50 hectares when the smallest possible surface, the surface of a sphere, is assigned to the sugar molecule. Finally, if it is assumed that ions and electrons are also separated through surfaces from their dispersion media (and an electrical heterogeneity and the existence of electrical surfaces must be postulated in these) the absolute and especially the specific surfaces attain enormous values, 1 Lobry de Bruyn and Wolff, Rec. Trav. chim. Pays, Bas., 23, 155 (1904). 28 GENERAL COLLOID-CHEMISTRY It should further be noted that the increase in the surface of a cube with progressive subdivision may be expressed by the formula : ml in which SO is the total surface in square centimeters, a, the length of one edge in centimeters, and m^, the number of cubes. The original volume was taken as equal to i cc. (H. Mayer). ^ icm^ Since the unit of specific surface = ,> then if a is taken as ^ •' icm" equal to i, the calculated values obtained for surface represent at the same time the specific surface or the degree of dispersion.^ The concept of specific surface may conveniently be replaced by the somewhat clearer one of "degree of dispersion." Thus we may say that the degree of dispersion increases greatly with progressive subdivision of a given phase, etc. As is well known, the surfaces of solid and liquid bodies of even ordinary dimensions already exhibit a whole series of peculiar phenomena, the intensity of which increases in direct proportion with the absolute and specific surfaces of the bodies. As examples might be mentioned the condensation of gases on solid surfaces, the manifold effects of surface tension in liquids, the fact that the majority of electrical phenomena appear at surfaces, etc. It should be remembered, however, that in such behavior the abso- lute surface is less responsible for these phenomena than the specific surface. Thus a few milligrams of platinum black have an effect upon an explosive gas mixture which is not equaled by that of several square meters of sheet platinum, for while they may have approximately equal absolute surfaces the former has an enormously greater specific surface. We are driven to conclude that all the phenomena observable at ordinary surfaces increase enormously in intensity and that they may even change qualita- tively when we come to deal with dispersoids with their immense internal surfaces. There are also certain forms of energy that play an insignificant role in macroheterogeneous systems, but an enormous one in dispersoids. ' H. Mayer, KoUoidchem. Beihefte, I, 62 (1909). ^ See also Wilh. Ostwald, Grundr. d. allg. Chemie, 4 Aufl. 531, Leipzig, igog. GENERAL CONSTITUTION OF COLLOID SYSTEMS 29 §6. Classification of the Dispersoids According to Their Degree of Dispersion I. Classification of Zsigmondy.— It is evident that either the degree of dispersion or the number of phases in a system may be used for classifying the dispersoids. The mere number of phases is relatively unimportant as a means of classification, for the majority of the dispersoids and of the colloids in particular are either diphasic or triphasic. Classification on the bp-sis of the degree of subdivision permits finer distinctions. R. Zsigmondy^ has developed a classification on this basis. According to him the field of colloid-chemistry occupies a middle position among the dispersoids thus far known. Particles about o.i/u in diameter, that is, particles with a specific surface of about 6.10^ (see Fig. 8), are stated by R. Zsigmondy to represent the lower limit of dispersion. The size of such particles is about that of the particles in emulsions and suspensions which no longer undergo separation. The value o.iju about represents the limit of microscopic visibiHty. According to Zsigmondy the field of colloid-chemistry begins with particles of this size and extends to particles about i/iyu in size, that is, to such as have a specific surface or degree of dispersion of about 6.10^, assuming that the particles are cubiform. The value in/j, is rather lower than the diameter of the smallest particles hitherto observed by ultramicro- scopic means (about 6yu/i). On this basis of classification the colloids represent dispersions of a magnitude varying between 6.10^ and 6.10^. H. Siedentopf^ and R. Zsigmondy' have proposed a nomen- clature for the individual particles of typical dispersoids which is based upon their degree of dispersion. Particles visible under the microscope are termed "microns," while those which can be seen only by the application of ultramicroscopic methods are called "submicrons" or " ultramicrons." The disperse phase of colloid solutions would therefore be made up of submicrons (ultramicrons). It can be shown in several ways that particles exist whose size we know to be beyond that of ultramicroscopic visibility. They must therefore be less than 6/^^ in diameter. ' R. Zsigmondy, Zur Erkenntnis der Kolloide, 22, Jena, 1905. " H. Siedentopf, Berl. klin. Woch., Nr. 32, (1904). ' R. Zsigmondy, Zur Erkenntnis der Kolloide, 87, Jena, 1905. 3<=> GENERAL COLLOID-CHEMISTRY These particles to which molecules and the products of their dissociation belong, are called "amicrons." The accompanying Fig. 8 (based chiefly on the data of R. Zsigmondy) is designed to illustrate approximately the rela- FiG. 8. — Comparison of particles of different size. The large circle corresponds to the diameter of ti human red blood corpuscle (about 7.5 m) ; the large pentagon to that of a starch granule of medium size (about 7.0 m). The particles enclosed in the frame are, in comparison with the rest of the figure, enlarged 333 times. The figure has been constructed from data and tables given in R. Zsigmondy (Zur Erkenntnis der KoUoide, Jena, 1905). The values for the mastic suspension are taken from J. Perrin's studies [KoUoidchem. Beihefte i, 221 (1910)]. tive sizes of the particles in typical dispersoids which have been the object of study. According to this diagram human blood corpuscles, starch granules, kaolin, and mastic particles would be microns, gold particles would be submicrons, while the finest gold GENERAL CONSTITUTION OF COLLOID SYSTEMS 31 particles, starch molecules, etc., which cannot be made out ultra- microscopically would be amicrons. It seems of interest to give here the estimated diameters of a number of molecules. The smallest molecule seems to be that of hydrogen gas, 0.067 to o.i^gixix; water vapor has a molecular diameter approximating o.ii3;uyu; carbon dioxide one of about o.22>$nix,^ etc. Different methods of calculation yield different values, yet all approach the magnitude o.ijuyu or i.io— * cm. The molecular diameters of hydrated ions have recently been measured in several ways.^ The molecular diameter of NaCl was found to be o.26jum; that of sugar, o.'jy.n, etc. 2. Classification of Dispersoids According to Their Degree of Dispersion. — It follows from Zsigmondy's classification that dispersoids having a very small or a very high degree of dispersion do not belong to the systems to be specially considered in this book. Such dispersoids should have special names. Dispersoids with a degree of dispersion of less than 6.10^, that is microscopic sus- pensions, emulsions, and foams, might be called '^true or coarse dispersions," while dispersoids with a degree of dispersion higher than 6.10^ might be termed "molecular dispersoids." Roughly, the molecular dispersoids correspond with Thomas Graham's " crystalloids." As this term is based upon a property which does not necessarily determine the degree of dispersion it is not as free from objection as that which I suggest. Since molecules may dissociate into smaller particles, like ions, we obtain systems which may be designated as "ionically disperse" or as "ionic dispersoids, " as suggested by The Svedberg.^ It should be remembered, how- ever, that ions are by no means always the products of dissociated molecules, and especially is this not true if such appear in colloid solutions. Such ions need not therefore have a higher degree of dispersion than the colloid particles themselves. This will be discussed later in the section on the electrochemistry of the colloids. It has further been found that the specific surface of the ' These figures are taken from a table on p. 64 of the excellent publication of W. Mecklenburg, Die exper. Grundlegung der Atomistik, Jena, 19 10. The \arious methods of calculation may also be found there. ^ See, for example, the summaries of G. H. Washburn, Jahrb. d. Radioaktivitat, 5, 493 (1908); 6, 69 (1900). ' The Svedberg, Stud. z. Lehre v. d. koll. Losungen. Nov. Act. R. Soc. Scient. Upsaliensis, Ser. IV, II, i (i907)- 32 GENERAL COLLOID-CHEMISTRY colloids may vary within the limits calculated by Zsigmondy, that is to say, between 6.10^ and 6.10^. We may therefore expect to find that colloid solutions contain particles of different sizes. Experimental study has confirmed this expectation. Not only have different colloids very different degrees of dispersion, but one and the same substance may exist in different degrees of subdivision in a given dispersion medium. As an example may be cited a series of carefully studied aqueous gold dispersoids in- vestigated by R. Zsigmondy.'- Table 2. — Aqxteoxis Gold Dispersoids o? Dipeerent Degrees of Dispersion Designation of the solution* Color of the dispersoid Calculated average size of particles in mm AU37° AU92 AU97 Aug 2' AU91" Auss" Au, Gold suspension a. Gold suspension b. Gold suspension c. . Rose Bright red. Bright red. Bright red. Molet red. , Molet red. . Purple red. Violet red. . Bright red. Bluish About About About 6.0 10. o 15-3 17.0 23,0 32.0 38.0 45-0 950 130.0 *The designations are those of Zsigmondy {I.e.). Zsigmondy and other investigators have prepared gold dis- persoids in which the size of the particles could not be determined. They must therefore have been smaller than 6/iju. This variability in the degree of dispersion within the Hmits characteristic of colloid solutions has been recognized in the literature of colloid-chemistry by distinguishing between sub- stances having a "strong or a weak colloidality," and different "degrees of colloidality." Substances have also been designated as s}-stems "slightly, intermediately, highly, or completely colloid," or "coarsely disperse, finely disperse," etc. The term highly colloid is synonymous with highly disperse, etc. It is also at times advisable to distinguish between supermolecularly dispersed phases (as in the case of ions) and submolecularly dispersed phases. ' R. Zsigmondy, Zur Erkenntrds d. KoUoide, 104, Jena, 1905. GENERAL CONSTITUTION OF COLLOID SYSTEMS 3Z The following outline gives graphically a classification of the dispersoids according to their degree of dispersion. DISPERSOIDS True or coarse dispersions Colloid solutions. (suspensions, emulsions, etc.). Size of the particles of the disperse phase greater than O.I /J. Specific surface <6.io.* Size of the particles of the disperse phase between o.iM and IMM- Specific surface between 6.10' and 6.10'. > CoUoidality decreases Molecular and supermo- lecular dispersoids (solu- toids).! Size of the particles of the disperse phase about i/j/i or less. Specific surface >6. 10'. Degree of Dispersion increases P. P. von Weimarn- has repeatedly emphasized that the so-called "supersaturated solutions" (in which we are justified in assuming the existence of larger molecular aggregates) occupy a position between the colloid and the molecular disperse-systems. But there seems to be no reason for believing as von Weimarn does that supersaturated solutions always represent transitions between colloid and molecular-disperse systems; or for believing that such transition types must appear every time we pass from a high degree of dispersion to a lower one or vice versa. The con- cept of supersaturation embodies in itself no information regarding degree of dispersion which alone is the criterion for the tj^e of 'This name was proposed by P. P. von Weimarn, Koll-Zeitscher, 7, 155 (1910). It should be noted that von Weimarn wishes the terms "colloid," "colloid solution," etc., avoided and replaced by the more general terms "dispersoid" "dispersed solu- tion," etc., while the term "colloid chemistry" is to be replaced by "dispersoid chemistry." In spite of the fact that I was the first to propose the extension of the study of the colloids to that of the disperse systems and first suggested a, suitable nomenclature, yet, for obvious reasons I do not deem it advisable to eliminate the use of the term "colloid." Even the fact that the word "colloid" originally had a different meaning, namely, a more special one than it now has, does not justify the proposed measure. The word "molecule," for example, has not disappeared from science even though its exact meaning has changed frequently and consider- ably. A dispersoid chemistry, in other words, a chemistry dealing with disperse systems of all degrees of dispersion, does of course exist. Nevertheless, persistence in the use of the term colloid for at least that portion of this more general science with which this work deals seems to be justified on historical and other grounds. See my preface to the second German edition of this work. 2 See P. P. von Weimarn, Koll.-Zeitschr., 6, 179 (1910); and for greater details KoUoidchem. Beihefte, i, 331 (1910). 34 GENEILA.L COLLOID-CHEMISTRY classification here under consideration. Supersaturation consti- tutes a possible but not the sole means of preparing submolecular dispersoids. Such may be prepared by "direct methods of dispersion." 3. Defects of this Principle of Classification. — The following should be noted regarding the classification of the dispersoids according to their degree of dispersion. The degree of dispersion is manifestly a continuously variable quantity, and so it is self-evident that it may have any possible value between the extremes which characterize individual classes of dispersoids. As a matter of fact, transitional values between those which characterize the field of colloid solutions and those which characterize the molecular dispersoids, or between those of the former and those of the coarse dispersions are not only con- ceivable but have been demonstrated experimentally. The exist- ence of transitional values may be deduced from Table 2, for at its top are gold dispersoids with particles approaching molecular values in size, whUe at its bottom are suspensions which can be resolved under the microscope. An analogous series of dispersoids in which the degree of dispersion varied between points lying beyond either side of the field embraced by the colloid solutions was, among others, prepared by H. Picton and S. E. Linder^ at an early period in the history of colloid-chemistry. Dispers- oids of arsenious trisulphide in water were used. The size of the particles in these could not be determined directly, but that their degree of dispersion varied greatly was clearly demonstrated by their different degrees of diffusibility. In the face of these facts it must be admitted that the classi- fication of the dispersoids according to their degrees of dispersion is an arbitrary one. But while this is so, there is undoubtedly a practical justification for the distinctions proposed. The dispersion values given were chosen because with changes in them abrupt changes occur in other properties of the dispersoid also. Thus the particles of a dispersoid with diameters of less than o.i/i are not only no longer visible under the microscope, but at this degree of dispersion they tend to show diffusibility and they no longer settle out spontaneously. On the other hand while these particles do not pass through a dialyzing membrane, produce no changes in the freezing and boihng points of their ' H. Picton and S. E. Linder, J. Chem. Soc, 61, 148 (1892); iJii., 67, 63 (1895). GENERAL CONSTITUTION OF COLLOID SYSTEMS 35 dispersion medium, etc., all these properties alter in value, greatly and suddenly, when molecular dimensions are ap- proached. These t/wcontinuous changes of other properties therefore form the true basis for the classification of the dis- persoids on the ground of their degree of dispersion. But that a quantitative characterization of the dispersoids according to their degree of dispersion is important is evidenced by the fact that dispersion in itself must be regarded as the chief character- istic of the substances with which this book deals. 4. Polydispersoids. — It has frequently been found in. deter- mining the degree of dispersion in dispersoids, such as colloid solutions, that the individual particles of the disperse phase are of different sizes, in other words the degree of dispersion of the disperse phase must be described as multiple. Accord- ing to L. Michaelis,^ examples of such systems are found in the aqueous solutions of certain dyes, such as fuchsin, methyl violet, etc. In these there is a molecular-disperse phase in addi- tion to a phase observable under the microscope or ultra-micro- scope. jMany protein solutions probably behave in an analogous way, as may be inferred from their behavior on ultrafiltration (see later); but even the individual, ultramicroscopically observable particles in dispersoids (as in those of gold) are frequently of different sizes. It follows therefore that in practice we can only speak of an average dispersion value. The importance of the simultaneous existence of particles of different sizes in one and the same dispersion medium for many questions of colloid- chemistry, for example in that of their stability, will be discussed in detail later. These systems in which the disperse phase is composed of particles having different degrees of dispersion may be called polydisperse systems or polydispersoids. 5. Dispersoids Varying with Changes in Concentration. — In a number of molecular as well as colloid dispersoids the re- markable fact has been observed that the degree of dispersion varies progressively with changes in concentration. In all the cases thus far studied it decreases with increasing concentration. Cane sugar, for example, in dilute solution has all the typical 'L. Michaelis, Deutsche medizin. Wochenschr, Nr. 24 (1904); Virchow's Arch., i79i 19s (190s). 36 GENERAL COLLOID-CHEMISTRY attributes of a molecular dispersoid. But when cane sugar solu- tions of higher concentrations are investigated by applying the Tyndall test to them it is found that they show an intense light- cone this proving themselves submolecularly disperse. Entirely analogous observations have been made on solutions of various salts such as aluminium sulphate, and on those of certain dyes, proteins, etc. No doubt careful investigation will demonstrate the wide-spread nature of this remarkable fact. It should be added that such a progressive decrease in the degree of dispersion by simply changing the quantitative relations of the dispersoid to the dispersion medium may be demonstrated by yet other than purely optical methods. Further details will be given in discussing the individual physico-chemical properties of the colloids especially in the chapter on their internal changes of state. Analogous phenomena are encountered in studying the properties of mo- lecularly and supermolecularly dispersed systems, being then de- scribed as "polymerizations, condensations," etc. We will call these systems, among which many colloid disper- soids appear, "concentration-variable systems." 6. Temperature-variable Dispersoids.- — Just as the degree of subdivision of a dispersoid may vary with changes in concentra- tion, it may also vary with changes in temperature. As far as we know now, raising the concentration of a dispersoid produces the same type of change as lowering its temperature. A disperse system therefore tends to become less disperse when the tem- perature is lowered. Such anomalous behavior in "true" solu- tions has generally been explained by saying that the substances "polymerize" or "condense."^ An analogous behavior, resulting in diminutions of degree of dispersion, is found in even greater degree in colloid systems. Here we can only point out the fact; it is to be dealt with in detail in the chapters on internal changes in state, more particularly in that on gelation. Dispersoids showing this property are called "temperature- variable dispersoids." 7. Complex Dispersoids. — There exists another class of com- plex systems which is interesting for both the theory and the practice of colloid-chemistry. It is characterized by the fact that each constituent of such systems, both disperse phase and dispersion ' For examples and literature see H. Schade, Koll.-Zeitschr., 7, 26 (1910). GENERAL CONSTITUTION OF COLLOID SYSTEMS 37 medium, is in itself a dispcrsoid. Evidently the degree of disper- sion in these individual dispersoids must alicays be higher than that of the compound dispersoid. And in fact, the best-known ex- amples of such systems are those in which the individual disper- soids have a molecular degree of dispersion, while the compound dispersoid is colloid or molecular-disperse in character. The best examples of such "complex dispersoids" are found among the emulsions, that is among those systems in which both phases are liquid. Pretty instances are formed by the so-called critical mixtures of liquids and their analogues. As is well known, it is possible at suitable concentrations and at suitable temperatures to make a dispersoid of two liquids which have a limited molecular solubility in each other. As an example may be quoted the production of an emulsion of phenol in water. Since all liquids are mutually soluble to some extent at least, this type constitutes the bulk of the dispersion S3'stems having a liquid- liquid composition. Such complex dispersoids are characterized by the fact that changes in the concentration or in the temperature of the macrodisperse system are accompanied by changes in the composi- tion of the microdispersoids. Thus when droplets of phenol are dis- persed in water, both phases contain phenol as well as water. If the concentration of the emulsion is changed through the addition of one of its components, for example water, the composition of both microdisperse phases is also changed. As more water is added the phenol phase becomes progressively richer in water until a Hmit is reached (until the phenol is saturated with water), etc. Variations in temperature produce analogous effects. Another peculiarity of these complex dispersoids which should be emphasized is that in addition to the fact that the composition of the individual phases changes with variations in concentration or temperature, their degree of dispersion does also, and apparently whenever a change is produced in the total concentration. Thus the droplets in mixtures of liquids of limited mutual solubility become progressively smaller as the mixtures approach the so- called critical concentration, and disappear altogether at the "critical point;" in other words, the droplets become molecular- disperse. But at constant temperature the critical concentra- tion is always less than the concentration of the fluids in a coarsely disperse state. Here again there exists exactly the same variation 38 GENERAL COLLOID-CHEMISTRY in degree of dispersion with change in concentration that was previously described.^ What was said above regarding simple dispersoids holds for the influence of temperature on composition and degree of dispersion in complex dispersoids also. The complex dispersoids are con- centration-variable and temperature-variable systems. Dispersoids with a liquid dispersion medium and a solid disperse phase may also form complex systems, but up to the present time these have been studied but little. It is self-evident that a solid particle floating in a liquid may either take up part of it into itself, or attach a layer of it to itself. Such behavior may be observed macroscopically when solid gelatine is pulverized and thrown into cold water. Each particle then "swells," that is, it absorbs water, but if the temperature is low enough it does not lose entirely the properties of a solid. But the properties of a solid, such as constancy of form and elasticity, become less marked as the solid particles take up more water or as the temperature rises. This very important behavior, which therefore consists in an approximation of the previously solid state to that of a liquid, will be discussed in detail later (see page 44). Let it further be pointed out that complex dispersoids may be expected to appear more frequently in systems composed of two liquid phases than in those composed of a liquid and a solid phase. This depends upon the fact that a greater mutual molecular miscibility may be assumed to exist in the case of two liquids than in the case of a liquid and a solid phase, and second, upon the fact that the "solubility" of two liquids is m,utual. Both phases will therefore be disperse in a liquid-liquid dispersoid. On the other hand, while we may be able to speak of the "solubility" of a soHd phase in a liquid dispersion medium we shall only rarely be able to speak of the "solubility" of the dispersion medium in the solid disperse phase. It seems of interest to mention in this connection that many reasons have recently been found for assuming the existence of similar phenomena in molecular and ionic dispersoids. Such complexes are called "solvates," or if they occur in aqueous ' It should be noted that concentration is always regarded as the quotient of the amounts of the t^ —. ^— ^ . In the range above the critical point this fraction dispersion medium is reversed, in that the dispersion medium becomes the disperse phase and vice versa. GENERAL CONSTITUTION OF COLLOID SYSTEMS 39 solutions, "hydrates." In these compound disperse phases there also occur variations in composition with changes in concen- tration or in temperature entirely similar to those discussed above. 8. Trsinsition Phenomena. — The transition phenomena ob- served in passing from the members of one class of dispersoids to those of another having a different degree of dispersion are par- ticularly interesting. Our knowledge of the properties of dis- persoid systems is at present distributed in such a way that we may say we know a great deal about typical molecular dispersoids, somewhat less about typical colloids, and still less about typical coarse dispersions. But the atypical representatives of all three classes, that is, the transition forms between coarse dispersions and colloids on the one hand, and between colloids and molecu- lar dispersoids on the other, have been almost entirely neg- lected. There is an historical reason for this state of affairs. As is well known, the founder of colloid-chemistry, Thomas Graham, was so impressed by the differences between typical colloids and typical molecular dispersoids that he declared the two to represent "different worlds of matter." He endeavored in consequence to contrast them as much as possible. The ma- jority of his successors followed him in this, and only recently has the effort been made to cease discovering rare and sharp distinctions between colloids and molecular dispersoids. As a matter of fact no such sharp distinctions exist. But the realiza- tion of this fact was important in that it yielded a new point of view, on the basis of which it became possible to formulate the concept of the dispersoid, and with it to obtain a rational sys- tematization of these bodies^ (see later). It must be empha- sized, however, that even today comparatively few investigations are carried out with the conscious purpose of studying these transition phenomena, more especially the changes which the individual physical and physico-chemical properties exhibit with progressive variations in the degree of dispersion. §7. General Colloid-chemical Nomenclature Dispersoids are called sols if they have the properties de- scribed in the foregoing paragraphs, if their degree of dispersion 1 See Wo. Ostwald, KoU.-Zeitschr. i, 291, 331 (1907). 40 GENEEAL COLLOID-CHEMISTRY lies between 6.10^ and 6.10^, and the disperse phase is uniformly distributed throughout the dispersion medium. This name origi- nated with Thomas Graham.^ When we speak of "a colloid" we nearly always mean one in this condition, in other words, one in the sol condition. There exists also what Graham first called the gel condition. A sol becomes a gel when its degree of dispersion is decreased in such a way that it passes beyond the lower limits characteristic of the colloids, in other words, when the system becomes microscopically heterogeneous. A usual, though not an absolutely necessary accompanying phenomenon of gel formation is a loss of the uniform distribution of the disperse phase in the dispersion medium. The sol "precipitates," "clots" "coagulates," "cements," etc. It is sometimes said that the sol "gelatinizes," but it is best to reserve this word for another process which can be distinguished from the ordinary "precipitation," "clotting," or "coagulation." The' phenomena opposed to "coagulation," in other words, those which result in an increase of the degree of dispersion and tend toward an approximately or absolutely uniform distribution of the disperse phase in the dispersion medium are summed up under the term "peptization." This term was also first suggested by Graham. All variations in the degree of dispersion and in the properties connected with it are designated by a term introduced by Wolfgang Pauli — "changes of state in colloid systems." When a change in the state of a colloid may be reversed by reversing the conditions which brought that change about, it is said to be "reversible." Thus when a colloid which has been precipitated by a salt goes back into solution on removal of the salt, the col- loid change is said to be "reversible." On the other hand, if this does not occur it is "irreversible." The reversibility of such a change of condition is not determined, in the main, by the nature of the colloid itself, but rather by the character of the conditions which produce the coagulation. Thus the precipitation of typical protein sols by neutral salts is reversible, but their pre- cipitation by heat is irreversible. We cannot therefore speak of reversible and irreversible colloids, as is still frequently done, but only of reversible and irreversible changes in state in the colloids. Another inappropriate word is "solid-sol," by which is really meant iTh. Graham, Ptil. Trans. Roy. Soc. (1861); Liebig's Ann., 121, i (1862). GENERAL CONSTITUTION OF COLLOID SYSTEMS 4 1 a gel which will redissolve in the dispersion medium from which it has been precipitated or dried. According to the chemical name of the dispersion medium we also distinguish between hydrosols and liydrogcJs, alcosols (alcohol- sols) and alcogels, sulphosols (sulphuric acid sols) and sulphogels. If the dispersion medium is an organic liquid, the dispersoid is called an organosol, etc. The chemical name of the disperse phase is used as a prefix, thus: gold-hydrosol, silicic acid-alcogel, ice- xylosol, etc. CHAPTER II RELATIONS BETWEEN THE PHYSICAL STATE AND THE GENERAL PROPERTIES OF COLLOID SYSTEMS §8. Classification of Dispersoids According to the State of Their Phases I. The Physical State of the Disperse Phase as a Principle of Classification. — As soon as we attempt to carry out practically the classification of the dispersoids an the basis of their degree of dispersion we find that this principle does not always sufiice. Two dispersoids, for example, may be identical in that the size of the particles of their disperse phases is the same, yet their other prop- erties may differ so widely that the similarity appears as a mere incident. This may be illustrated by the great difference be- tween a quartz or kaolin suspension in water and an emulsion of oil in water even when the average size of the particles in the two is the same. But for the colloids in particular such a classi- fication according to degree of dispersion seems to be entirely in- adequate. First, the extremes between which the degree of dis- persion may vary in colloids are not widely separated. According to the classification of Zsigmondy the degree of dispersion in col- loids can only vary between 6.10^ and 6. 10'' while that of coarse dispersions and molecular and supermolecular dispersoids is limited on one side only. Second, different colloid solutions are known in which the particles are of the same size, but which in other points differ so markedly from each other that the similarity in degree of dispersion seems merely accidental or unimportant. What we need therefore is an additional principle of classification which is not based upon the size' of the particles. Such an one lies close at hand. Since colloid systems are heterogeneous or polyphasic — a fact discussed in detail in the preceding chapter — we may use the physical character (the physical state) of the phases composing the systems as a basis for classification. We may classify disper- 42 GENERAL PROPERTIES OF COLLOID SYSTEMS 43 soids according to the character of their phases quite as justly as according to the number or the degree of dispersion of their phases. Theoretically, such a classification would be as valuable as the other two. But when we consider that an entire series of properties changes, whenever the physical character of a phase changes, a classification based on this character is evidently a more natural one than that based upon any arbitrarily chosen single property. The succeeding paragraphs will show that a classification of dispersoid systems, more particularly of colloid systems, according to the physical state of their disperse phases is at least as important as their classification on the basis of their degree of dispersion. 2. Classification of the Dispersoids According to the Physical State of Their Phases. — The most important dispersoids that need to be considered are diphasic. By joining the three physical states of matter in pairs we obtain the following possibilities: 1. Solid + SoHd. 4. Liquid + SoHd. 7. Gas + Solid. 2. Solid + Liquid. 5. Liquid + Liquid. 8. Gas + Liquid. 3. Solid + Gas. 6. Liquid + Gas. 9. (Gas + Gas.) No example exists of a dispersoid having the composition Gas + Gas, for gases are freely and completely miscible with each other in all proportions. Examples of the other classes are: 1. SoHd + Solid. — Intercalations of foreign particles in many minerals (microliths, etc.), carbon particles in iron, of coloring matter in mineral salts and precious stones; "solid" colloid solu- tions, mixed crystals, solid solutions. 2. Sohd + Liquid. — Liquid intercalations in many minerals, water of occlusion, inclusion and crystallization. 3. Solid + Gas. — Gaseous inclusions in many minerals (meer- schaum, pumice stone, lava, tufa), solutions of gases in solids (hydrogen in iron, etc.). 7. Gas + Sohd. — Smoke, for example tobacco smoke; con- densing metallic vapors (F. Ehrenhaft); cooling vapors of am- monium chloride; cosmic dust, etc. 8. Gas + Liquid. — The fog formed at the liquefaction point of gases or in the condensation of steam; atmospheric fog, clouds, Tyndall's photochemically produced liquid fog, etc. All these will be discussed in detail later. 44 GENERAL COLLOID-CHEMISTRY The classes of dispersoids mentioned under 4, 5, and 6 are by far the most important and deserve special attention in colloid chemistry. The dispersion medium is liquid in all three cases, the disperse phase is solid in the first instance, liquid in the second, and gaseous in the third. The most familiar examples of these three classes of dispersoids are the coarse dispersions known as suspen- sions, emulsions, and foams. §9. Transition Phenomena. Complex Dispersoids I. General Considerations. Influence of Temperature and Degree of Dispersion. — As soon as we study the problem closely, it becomes evident that transitions are encountered in this scheme of classification also. This is more particularly true in passing from the sohd to the liquid state, a transition which is produced more frequently and more easily, as is well known, than the transi- tion from liquid to gas. Thus one and the same dispersoid may be an emulsion or a suspension, depending upon the temperature. At ordinary temperatures a few drops of an alcoholic mastic solution or of an alcoholic solution of rosin form a suspension when poured into an excess of water, but if the system be heated to the melting point of the resins a mixture of two liquid phases or an emulsion results. Between these two temperatures all possible transitions between solid and liquid may appear. In the last analysis the difiiculty of drawing a sharp line between suspensions and emulsions is identical with the more general one of formulating a precise definition of the sohd as contrasted with the liquid state of matter. Since the discovery of liquid crystals and crystalline liquids the only criterion we seem to have left to characterize the solid state is its enormous internal friction. When we deal with droplets of microscopic size, their liquid character [that is to say, the presence of free (positive) surface energy in them] can be demonstrated rather easily by deformation experiments. But it is much harder to ascertain the physical state of a disperse phase when higher dispersion values come into play. As E. Hatschek^ has shown particularly well, the amount of energy necessary to produce the deformation of a liquid droplet increases very rapidly with increase in degree of dispersion. ' E. Hatschek, Koll.-Zeitschr., 7, 81 (1910). GENERAL PROPERTIES OF COLLOID SYSTEMS 45 While its own weight suffices to squeeze a macroscopic oil globule through a glass tube half its own diameter a pressure of 4.5 atmos- pheres is necessary under analogous experimental conditions if the oil droplet is 0.2/1 in diameter. Such a drop is still visible under the microscope. The explanation for this lies in the great increase in the surface energy of a given volume with its progressive subdivision. (For details see later.) The greater the degree of subdivision of a liquid disperse phase, the more does it approximate a solid in its mechanical behavior. P. P. von Weimarn/ starting with other, mainly moleculo- kinetic, conceptions, has reached a conclusion the converse of this. The properties of solid disperse particles approach those of a liquid as their degree of dispersion increases. We may say, therefore, that differences between liquid and solid disperse phases become progressively less marked as the degree of dispersion increases. In entire agreement with this conclusion is the fact that the importance of the original physical state disappears entirely when the phases become molecular-disperse. Thus all solutions of acetic acid are the same whether they are prepared from the vapor or from the liquid or solid f.orms of acetic acid.^ 2. Influence of Concentration upon State in Complex Dis- persoids. — Further analysis shows that in addition to tempera- ture and degree of dispersion the concentration, that is to say the quantitative relation of the components of a dispersoid to each ' other, may have an important influence upon its state. Two possibilities may be distinguished: 1. The state of the dispersoid as a whole may vary with the concentration. 2. The state of the individual phases of the system may vary with the concentration. We seem at first sight to deal here with rare and complex phenomena, but while it is true that only suggestions of them are found among the best-known dispersoids, namely the molec- 1 P. P. von Weimarn, Koll.-Zeitschr., 6, 32 (1910); 7, 155 (1910). 2 It is therefore impossible and so purposeless to try to distinguish molecularly dis- perse systems (solutoids) from each other on the basis of the "solid" or "fluid" nature of the disperse phase as P. P. von Weimarn {I.e.) proposes. But this state- ment is not to be construed as meaning that the processes of formation or of solution are identical in the two cases. The heats of solution are certainly different. Since in a classification of disperse systems we only classify the like results of very different physical and chemical processes, the advantage of utilizing these differences in modes of preparation for purposes of classification is not evident. 46 GENERAL COLLOID-CHEMISTRY ularly and coarsely disperse, they play an important part in certain colloids. Let us first consider the behavior of the simpler dispersoids of this type when their concentration is varied. (a) The state of a molecular-disperse solution is determined by the state of the solvent. The reason for this is that the original state of the dissolved substance has lost its importance when a molecular-disperse phase results, as already explained above. In the case of those atypical solutions which are more or less solid, there are good reasons for supposing that in them the disperse phase has not remained in a molecular state of subdivision, but has been polymerized, associated or condensed into submolecular and then into colloid particles, as discussed above. Thus soap solutions are molecular-disperse and liquid in low concentrations, but are colloid and solid in higher concentrations. If the con- centration is varied while the molecular degree of dispersion is maintained a separation of the disperse phase in liquid or solid form generally occurs, in other words saturation is attained. {h) The state of a coarsely disperse system depends in an interesting way upon concentration. In certain extremes of concentration, bodies having a. liquid dispersion medium and a gaseous, liquid, or solid disperse phase may assume some of the properties of solids, such as constancy of form and elasticity. Solid powders mixed with a little water illustrate this. Sand with a definite but by no means immeasurably small water content may be cut into slices. Emulsions of mineral oil in soap, or of water in mineral soaps form stiff pastes in certain concen- trations;^ if the concentrations are slightly changed a coarse mixture of liquids is again formed which may be poured from one vessel to another. As is well known, foams prepared with small quantities of a liquid dispersion medium may have a definite shape and exhibit considerable solidity. Thus, a slice of pasteboard weighing two grams will not sink into a well-whipped saponin foam. It is characteristic of all these systems that they contain a great excess of the disperse phase, though even here an optimum concentration exists beyond which the system again loses its solid state. ^ In these '^highly concentrated dispersoids" the dispersion ^See S. U. Pickering, Koll.-Zeitschr., 7, 11 (1910); D. Holde, ibid., 3, 270 (igi6). 2 See Wo. Ostwald, KoU.-Zeitschr., 6, 185 (1910). GENERAL PROPERTIES OF COLLOID SYSTEMS 47 medium surrounds the disperse phase with a "Hquid film" which deprives the disperse particles of their mobility, and so imposes upon them the properties of a solid. (c) Particularly interesting and important phenomena appear among the complex dispersoids. The more important character- istics of these bodies, such as variations in concentration in the individual dispersoids with temperature and total concentration, arid variations in the degree of dispersion with the same factors, were mentioned above. In considering the influence of concen- tration on the state of complex dispersoids we have therefore to deal with an exceptionally large number of factors which may all act in the same general direction but some of which may also counteract others. A. COMPLEX SYSTEMS HAVING THE COMPOSITION LIQUID -I- LIQUID {a) Influence of Concentration upon the State of the Dis- persoid as a Whole. — In Liquid -|- Liquid systems the same in- fluence of concentration described above for simple, coarsely disperse suspensions may exhibit itself, and a body having the properties of a solid may result. Here also a sudden "setting" may take place in certain limited regions of concentration. But to this there must be added the influence of the total concentra- tion not alone upon the concentration of the individual dispersoids, and consequently upon their physical properties, but also upon the degree of dispersion of the system as a whole, and thus upon its state. If we try to imagine what must be the effect of these in- dividual factors upon the behavior of such complex dispersoids we can only suspect that the changes in state of the system as a whole with changes in concentration or temperature must be smoother in the case of the complex dispersoids than in the case of the simple ones. For, generally speaking, it is probable that the consequences of any change will manifest themselves less clearly when a great number of factors working partly in the same, partly in opposite directions, are affected by it than when only one or a relatively small number of such determining factors are affected. This is particularly true of systems that occupy an intermediate position among the dispersoids, in other words of the colloids. It follows from the principle of continuity which justly 48 GENERAL COLLOID-CHEMISTRY pla}'s a great role in all science that colloid systems having the composition Liquid + Liquid must occupy an intermediate posi- tion between coarse dispersions and molecular dispersoids in this respect also. (b) Influence of Concentration on the State of the Disperse Phase. — Exceptionally complicated and interesting relationships become apparent when we take into consideration the fact that in complex systems the state of aggregation of one of the individual dispersoids, the disperse phase for example, may be changed in the same way as that of the compound dispersoid by a change in concentration. For there is no reason for excluding the possi- bility that a floating droplet having the composition Liquid -j- Liquid may "set" in certain concentrations and at certain tem- peratures as does the dispersoid as a whole at other concentrations, perhaps. Just as soap, water, and parafiin oil may form a body having the properties of a solid under certain conditions, so a disperse particle having a similar composition may suddenly stiffen even though the dispersion medium itself may still be Kquid. A great variety of possibilities exists here which we shaU encounter again later. B. COMPLEX DISPERSOIDS HAVING THE COMPOSITION LIQUID + SOLID (a) Influence of Concentration on the State of the Dispersoid as a Whole. — It is easily seen that when a complex system has the composition Liquid + Solid (a suspension of swollen gelatine particles, for example) it may assume the properties of a solid as its concentration rises. There need only be enough particles in a given volume of liquid so that in the process of swelling they interfere with each other's movement in order to get a stiff jelly. Figuratively speaking, the particles at higher concentrations struggle with each other for the dispersion medium, and as the amount of liquid at their disposal decreases the particles adhere more and more iirmly to each other. This may be observed ex- perimentally if the Uquid dispersion medium is gradually removed by evaporation. In this way a typical solid is finally obtained.^ Conversely, such a complex system will tend to approach a normal liquid in character as the number of suspended particles con- 'The crystalline structure of solid bodies is disregarded here. GENERAL PROPERTIES OF COLLOID SYSTEMS 49 tained in the unit' volume is lessened. If the dispersion medium is also heterogeneous, that portion of it which is made up of the disperse phase must also increase when the total concentration is increased so that the augmented viscosity resulting from this will further aid in giving the dispersoid as a whole a solid consistency (b) Influence of Concentration on the State of the Disperse Phase. — The state of the solid disperse phase more nearly ap- proaches that of a liquid the greater the amoui>t of dispersion medium it has absorbed. To sum up we may say that a great variety of relations exists between the state of the individual phases composing the dis- persoid and its general properties. Of greatest significance is the fact that a system having the properties of a solid may he formed from a disperse mixture of non-solid phases. Other disperse systems having the composition Gas -j- Solid or Gas + Liquid may assume the properties of liquids. Thus smoke or fog some- times exhibit phenomena of (positive) surface tension ("rings" of smoke) ; and at least most of the hydrostatic properties of liquids may be easily demonstrated in extremely concentrated systems of this composition, as in not too fine, dry sand. §10. Colloid Systems as Suspensoids and Emulsoids I. General Considerations. — Once we accept the physical heterogeneity of colloid systems we are compelled to consider the state of their phases. The dispersoids that must first be dealt with are those which have a liquid dispersion medium. Special attention must therefore be paid to the part played by the state of the disperse phase. In considering the three types of systems which belong to this group we may disregard the class Liquid + Gas, for these (foams composed of extremely small bubbles) are not t3^ical representa- tives of the colloids. This should not be taken to mean that such systems. are unknown or are incapable of existing. The turbidity which appears in Hquids about the critical vaporization point is probably dependent upon the formation of a great number of gas bubbles in a high degree of dispersion. A detailed compari- 4 so GENERAL COLLOID-CHEMISTEY son. of the properties of these systems with those of others has not been made as yet, although their optical behavior, the effect of the walls of the vessels containing them, etc., demonstrate the close relationship of these to the dispersoids. A detailed theoretical and practical study of the whole problem, more especially of the critical cloudiness of liquid mixtures, would yield fruitful results, for great similarities have already been shown to exist between these systems and certain colloids (see later). We have therefore to concern ourselves only with colloid solutions having the composition Liquid + Solid and Liquid + Liquid. According to our classification we may expect to en- counter two classes of colloids, and our problem narrows itself down to the relations existing between suspensions and emulsions on the one hand and colloid solutions on the other. On a priori grounds it would seem possible that the coarse dispersions men- tioned might yield two types of colloid solutions when the degree of dispersion is properly increased. The question as to what properties such systems exhibit is worth our attention. 2. The Empirical Establishment of Two Classes of Colloids. — The existence of two classes of colloids differing markedly from each other has recently become apparent on purely empirical grounds and entirely independently of any theoretical considera- tions. Protein and gelatine solutions represent one extreme, Zsigmondy's aqueous gold dispersoids the other of these two types of colloid solutions. Different names are employd in the literature for these two sets of colloids, different investigators having made use of different properties for their characterization. V. Henri^ calls them "stabile" and "instabile" colloids, A. A. Noyes,^ "col- loidal solutions" and "colloidal suspensions," J. Perrin,^ "hydro- philic" and "hydrophobic" colloids, and also "hydrosoles stables et colloides hydrophiles." After Wolfgang Ostwald* pointed out that the latter names are too narrow since the character of the dispersion medium may vary, H. Freundhch^ and W. Neumann suggested the more comprehensive terms "lyophilic" and "lyopho- bic" colloids. All these terms are either based upon the existence 1 V. Henri, Zeitschr. f. physik. Ch., 51, 29 (1905). 2 A. A. Noyes, Journ. Amer. Chem. Soc, 27, 85 (1905). ' J. Perrin, J. Chim. Phys., 3, 50 (1905). *Wo. Ostwald, ZKoU.-eitschr., 1, 291, 331 (1907). ' H. Freundlich and W. Neumann, KoU.-Zeitschr., 3, 80 (1908). GENERAL PROPERTIES OF COLLOID SYSTEMS 5 1 of some striking individual difference between the two groups or else are expressive of particular theoretical conceptions. Noyes has analyzed the two classes of colloids from a broad experi- mental point of view. He characterizes one type as "viscous, gelatinizing, colloidal mixtures not (easily) coagulated by salts," the other as "non-viscous, non-gelatinizing, but easily coagulable mixtures." A possible addition to Noyes' happy empirical descrip- tion is that the former usually have a lower surface tension than their pure dispersion medium, while in the latter the tension is practically unchanged. Further, electrical factors usually play a more important role in the latter than in the former. This is evidenced, for example, by the great precipitating power which polyvalent ions have upon the non-viscous colloids. Examples of the "non-viscous, etc.," colloids are the metallic sols, sulphide sols, many dyes (congo-red for instance), iron hy- droxide in dilute solution, etc. The best illustrations of the other class are: the proteins and related substances, gelatine, agar, cholesterol, silicic acid, " meta "-phosphoric acid, stannic acid, meta-hydroxides in concentrated solution, so-called gelatinous salts (sulphates, phosphates, carbonates, etc.), dye-stuffs like night-blue, etc. The question now arises whetlier these two classes of colloid solutions do actually represent dispersoids having the composition Liquid + Sohd and Liquid + Liquid. 3. The Theoretical Characterization of the Two Classes of Colloids. — Even the theoretical conceptions which led to the use of the expressions "lyophilic" and "lyophobic" imply the exist- ence of a close relationship between the properties denoted by these names and the state of the disperse phase. This is par- ticularly true of the "lyophilic" colloids which we characterize as systems having the composition Liquid -|- Liquid, for when we say that the disperse phase is here composed of "exceedingly swol- len particles" or of "particles united to a great number of Hquid molecules," we imply that its state is liquid. Statements Hke this of J. Perrin:^ "Un granule d'hydrosol stable contiendrait. une tres forte proportion d'eau, 90 percent par exemple" can hardly be interpreted differently. Yet while such statements may be regarded as approaching a characterization of the two classes of ' J. Perrin, J. Chim. Phys., 3, 84 and 87 (1905). 52 GENERAL COLLOID-CHEMISTRY dispersoids on the basis of the state of their disperse phases, a detailed analysis of these two classes based on experimental study has been attempted only recently. This is particularly true of the "lyophilic" colloids. The colloids described as "lyophobic," "unstable," etc., were characterized as dispersoids having the composition Liquid + Solid early in the history of colloid-chemical investigation as an almost self-evident conclusion to be drawn from a consideration of their properties. The similarities between colloid metals, for example, and coarse dispersions having the composition Liquid -1- Solid are so great and the relations between them so intimate and striking that even before the formulation of the concept of "colloidality,"i B. J. Richter,^ M. Faraday,^ and J. Berzelius^ thought of the former as suspensions of particles of the same character as "reguline" metals. B. J. Richter showed as far back as 1802 that gold in its well-known "aqueous solu- tions" does not exist here in some unknown molecular condition, but in a finely divided metallic (soKd) state. This conclusion was afterwards confirmed by Faraday and Zsigmondy, and extended to other colloids of this group by other investigators. We might now try to correlate these two empirically estab- lished types of colloid^ systems with the dispersoids having the composition Liquid + Solid and Liquid + Liquid by detaiHng the similarities between the two sets of systems. But such a com- parison would presuppose knowledge of a great number of the special properties of colloid systems which only the succeeding portions of this book will bring. The results of comparison would not be convincing, for the facts to be employed could only be pointed out here in brief. We shall therefore for the present only assume that the two types of colloid systems under discussion differ from each other in the matter of the state of their disperse phases and in the properties which result from this fundamental '■ Wo. Ostwald, KoU.-Zeitscher., i, 291, 331 (1907). The statement of A. Muller, [Allg. Chemie d. Koll., 147, 186, Leipzig (1907)], that G. Quinclie [Ann. d. Physik. (4) 9, 797, loog, etc. (1907)] first classified colloids according to the type of the dis- perse phase is not correct. The statements of Quincke which no doubt led to this historical error, do not refer to colloid systems but to coarsely dispersed ones, Quincke holds all colloid solutions (including those of arsenious trisulphide) to be mixtures of two fluid phases {I.e., 1009, 1034, etc.). See in this connection G. Bredig [Ann. d. Physik., i, 11, 221 (1903)]. 2 B. J. Richter, see Wilh. Ostwald, KoU.-Zeitschr., 4, 5 (rgog). ' M. Faraday, Philos. Mag. (4) 14, 401, 512 (1859). * J. Berzelius, Lehrb. d. Chemie, 2 Aufl., 2, 244 (1823). GENERAL PROPERTIES OF COLLOID SYSTEMS 53 difference. We shall of course not disregard the necessity of proving this assumption later. '^ 4. The Frequency of Occurrence of Complex Emulsoids.— Let us anticipate a particularly important generalization which follows from characterizing the "lyophilic" colloids as systems having the composition Liquid + Liquid. The behavior of such colloids demonstrates clearly the liquid character of the disperse phase in them and emphasizes, first, that the "lyophilic" colloids are complex dispersoids, that is to say, their individual phases are in themselves dispersoids of a higher degree of dispersion, and, second, that the composition of these individual dispersoids as well as their degree of dispersion varies greatly with concentration, temperature, etc. Further, in consequence of the complex char- acter of these systems the state of the disperse phase may pass pro- gressively from liquid through semisolid to solid. The possibilities of variation in the state of a complex dispersoid with degree of dispersion, temperature, concentration, etc., as discussed on p. 45, appear very clearly in these colloids and undoubtedly constitute one of the chief reasons why their behavior is so much more varied than that of colloids having the composition Liquid + Sohd.- The value of this conception will also show itself in dis- cussing the individual phenomena characteristic of these systems, when it will become evident that the different theoretical views held regarding the properties of these two classes of colloids may not only be correlated but be simplified and explained if the differ- ences in the tjpes of the disperse phases and the consequences thereof are kept in mind. 1 An exact classification and description of both classes of colloids according to the type of the phases was indicated in the first German edition of this book. Since then I have found only further and very excellent support for this view. I hope in the near future to publish a monograph on the physical theory of colloids of the composition Liquid -|- Liquid (see the following footnote). 2 Even in the first German edition of this book (pp. in, 328, 356, 374, etc.) I emphasized that lyophilic colloids are not "only" systems of the composition Liquid + Liquid, but also dispersoids of a "higher order," or, in the words used above, complex dispersoids. This has not been taken into account by those writers who have objected to my characterizations by pointing out that colloid mercury which consists of two fluid phases has no lyophilic properties. They have errone- ously ascribed to me the view that all Liquid -|- Liquid systems have such properties, while actually I merely held the narrower opinion that lyophilic colloids belong to these systems. But it may be pointed out again that for the reasons given on p. 47, complex systems may be expected in systems of the type Liquid + Liquid with greater certainty than in those of the type Liquid -|- Solid. A complex composition is therefore more general and commoner in a Liquid + Liquid dispersoid. If colloids of the composition Liquid -|- Liquid were unknown it would be necessary to seek them and they would no doubt be easily found. 54 GENERAL COLLOID-CHEMISTRY 5. Relation of These Two Colloid Classes to Molecular Dis- persoids. — Many investigators have pointed out that greater similarities exist between "lyophilic" colloids and typical mo- lecular dispersoids than between the latter and colloids of the type Liquid + Solid. Without going into details which must be reserved for later, we may emphasize that such relations may be expected on the mere basis of our characterization of the "lyo- philic" colloids as Liquid + Liquid systems. Physical chemists have recently become increasingly certain that the highly disperse phases of molecular and supermolecular solutions must be con- ceived of as combined with a number, sometimes a, very consid- erable number (100 and more), of the molecules of the solvent as solvates. Even though, as emphasized before, we cannot speak of the state of aggregation of a molecule, such a union of solvent with molecule cannot be conceived of physically other than as a highly disperse hquid. As a matter of fact, recent workers on the theory of solution speak of "droplets."^ This widespread im- pression of the existence of a closer relationship between "true solutions" and "lyophilic" colloids than between the former and "lyophobic" systems therefore corresponds with the conceptions above presented. 6. Suspensoids and Emulsoids. — In view of the relations exist- ing between these two classes of colloids and the corresponding coarse dispersions it seems expedient to give the former special names. R. Hober^ introduced the name suspension colloids for the colloids of the type Liquid + Solid. An analogous term for the second class would be: emulsion colloids. The abbrevia- tions suspensoids and emulsoids have been suggested by P. P. von Weimarn.^ These will be employed in the succeeding pages of this book. If one wishes to characterize a colloid in greater detail one may speak of " polysuspensoids " (systems composed of solid phases having different degrees of dispersion), of "com- plex emulsoids," etc. No objection can, of course, be raised against expressions like "lyophilic emulsoids." Only it should be remembered that the terms "suspensoids" and "emulsoids" in contradistinction to "lyophiUc" and "lyophobic colloids" have ' See K. Drucker, Zeitschr. f. physik. Chem., 67, 634 (1909). ' R. Hober. Physik. Chem. d, Zelle., ;; Aufl., 208, Leipzig, 1906. » P. P. Von Weimarn, KoU.-Zeitschr., 3, 26 (1908). GENERAL PROPERTIES OF COLLOID SYSTEMS 55 the advantage of expressing more definite and hence more fruitful views regarding the properties of the dispersoids. A view which connects the state of the disperse phase with the general conception of the dispersoid seems incomparably more concrete, more useful, experimentally, and more Suggestive than, for example, the conception of "lyophilia." §11. Transition Phenomena between Suspensoids and Emulsoids As already mentioned, it is possible for a phase to pass smoothly from a solid to a liquid state and vice versa. Often such progressive changes may occur during the process of coagulation in one and the same system, as in a complex dispersoid. Thus an originally liquid disperse phase may be precipitated in an almost solid condition by appropriate means of coagulation. Such a transition from emulsoid to suspensoid demonstrates par- ticularly well the properties which result from a change in the state of the disperse phase. According to J. Friedlander,^ for ex- ample, two kinds of systems may be prepared from alcohol, rosin and water, both of which are turbid, thus proving them disperse heterogeneous systems. The first of these is made by pouring a few drops of an alcoholic solution of rosin into an excess of water when the rosin, which is practically insoluble in water, separates out as a solid disperse phase while the alcohol, in greater part at least, is dissolved in the water. The second is made by adding a few drops of water to a concentrated alcoholic solution of rosin. In this case the first drops of water probably dissolve in the rosin- alcohol, but further amounts can dissolve only in the alcohol or, perhaps, succeed in withdrawing this from the solution so that small droplets of water-alcohol (liquid) appear in the liquid, alcoholic solution of rosin and make it turbid. A disperse hetero- geneous system with a solid disperse phase as well as one with a liquid disperse phase may therefore be prepared from the same three components by appropriate changes in their concentration. Friedlander found the behavior of the two systems to be entirely different. "Such a turbid mixture (a concentrated alcoholic solution of rosin to which a little water has been added) behaves very differently from the ordinary rosin suspension in that it is » J. Friedlander, Zeitschr. f. physik. Chem., 38, 430 (1901). 56 GENERAL COLLOID-CHEMISTRY not coagulated by an increase in temperature or on the addition of electrolytes. When the temperature is lowered the rosin phase becomes solid but is not coagulated, for a rise in temperature restores the system to its previous condition. Although pre- viously irreversible, the system is now completely reversible."^ Friedlander further found the internal friction of the second kind of system to be greater than that of the first. In this respect the second system closely resembles typical emulsions such, as those of isobutyric acid in water. A detailed study, qualitative and quantitative, of these systems would evidently be of great interest for the classification and characterization of disperse systems on the basis of the state of the disperse phases entering into their composition. Transitions from suspensoids to emulsoids and vice versa exist also among the colloids proper. Nearly all protein solutions, for example, are emulsoid in character; they are viscous, flocculated only by large quantities of electrolytes, etc. Yet O. Hammarsten,^ found that a neutral solution of salt-free serum globulin is coagulated by minute quantities of salt (o.i to 0.3 percent NaCl); and according to W. Erb^ the same is true of a plant protein, vitellin. According to H. Freundlich and W. Neu- mann,* many dyes show an emulsoid character in aqueous solu- tions and a suspensoid character in alcoholic solutions. Solutions of these substances in mixtures of the two dispersion media must evidently exhibit transitions between suspensoids and emulsoids similar to those which Friedlander discovered. Systematic in- vestigations in this field would also be of importance for the theory of the colloid state. Finally, we will here point out that one and the same substance may appear either in the suspensoid or in the emulsoid state in one and the same dispersion medium depending only upon the conditions under which it is prepared. §12. The Crystalline (Vectorial) Constitution of the Disperse Phase I. The Concept of Crystallinity. — As is well known, most sohd substances as well as a limited number of liquids are character- ' J. Friedlander, I.e., 432, 433. ^ O. Hammarsten, Pfliiger's Arch., 18, 38; see also Zeitschr. f. physiol. Chem., 395 (1905)- ' W. Erb, Zeitschr. f. Biol, 41, i (1901). *H. Freundlich and W. Neumann, KoU.-Zeitschr., 3, 80 (1908). '^ GENERAL PROPERTIES OF COLLOID SYSTEMS 57 ized by the fact that when their viscosity is sufficiently great their optical, elastic, dielectric, etc., properties are dependent upon the arrangement of their molecules in space. Besides the vectorial nature of these and other properties of such systems, which we usually designate as crystalline, these systems assume a definite external shape when their internal friction is sufficiently great. In the most characteristic cases this external shape is made up of a series of plane surfaces. A detailed discussion of the distrihiition of the cr}-stalline state in nature, or of the question of~ whether so-called amorphous solids are only under-cooled Hquids^ is out of place here. Let it be noted, however, that some investigators like M. L. Frankenheim^ and P. P. von Weimarn are so convinced of the wide distribution of crystallinity or vec- toriality that they have declared the crystalline state "the only internal state of matter." P. P. von Weimarn, especially, believes that the crystalline (vectorial) state is characteristic of all solid, liquid and even gaseous substances, and that generally speaking no amorphous substances exist in nature.^ But evidently there has been confused here the possibility of demonstrating crystalline (vectorial) properties in all manner of substances in every state with the actual existence of vectoriality in these as postulated by P. P. von Weimarn. While all gases may be transformed into liquids and most of these into crystalline solids, only a relatively small number of liquids (and of these only certain ones which exhibit special chemical properties such as "molecular chain formation," etc.) are possessed of experimentally demonstrable crystalline properties when in the liquid state ; and up to the pres- ent time no evidence at all is at hand to indicate the existence of a crystalline structure in gases. From this it follows that the "in- tensity of the vectorial chaining together of the molecules" (P. P. von Weimarn) is so slight in all gaseous and most liquid sys- tems that it is of no importance. The assumption of vectoriality in these systems is in consequence superfluous, for it leads to no fruitful deductions. It must further be emphasized that the concept of crystal- 1 See especially the recent and extensive discussion with refcrcncLS to the litera- ture by C. Doelter, Koll-Zeitschr., 7, 29, 86 (1910). ^ P. P. Von Weimarn. See his numerous discussions in KoU.-Zeitschr., 2, and subsequent volumes, especially 6, 32 (igio). ' The earlier literature is extensively discussed and in part cited verbatim by O. Lehmann, Molekularphysik, 1, 716, Leipzig, 1888. S8 GENERAL COLLOID-CHEMISTRY linity or vectoriality is as ambiguous a one as is that of hetero- geneity (see the next paragraph). For a system may be vectorial or crystalUne in certain of its properties while it is isotropic in others. All solid crystals, for example, are vectorial in shape, but crystalline liquids have, generally speaking, only an optical vectoriality. On the other hand, all solid crystals of the regular system, for instance, are not vectorial in their refractive indices. Other types of crystals exhibit different degrees of optical vectori- ality. A characterization of systems according to their vectoriality is therefore somewhat arbitrary, since it is always necessary to state which of the properties are vectorial. The failure of investigators to consider that different kinds of crystalline systems and different kinds and degrees of vectoriality must be distinguished according to the kind and the number of the properties of the vectorial state has undoubtedly contributed its share toward confusing the problem of the relations between crystalline and amorphous, solid and liquid states of substances. 2. Direct Proof of Crystallinity in Colloids.— The most fre- quently applied and simplest practical method of recognizing crystalhne properties is the optical. As indicated in their def- inition, it must be impossible to prove by any direct methods such as the microscopic, that colloids possess a crystalline struc- ture. Ultramicroscopic methods in place of microscopic can only be of limited use, for they give no direct "image" of the object. A whole series of optical facts have, nevertheless, been accumulated in favor of a crystalline constitution of the disperse phase of me- tallic sols. These will be discussed in detail when We consider the optical properties of colloid systems. Upon such and similar grounds, investigators like R. Zsigmondy, H. Siedentopf, A. Cotton and H. Mouton have been led to believe in the possibility if not in the probability of the crystalline nature of metallic sols at least. 3. Indirect Proof for the Crystallinity of Colloid 'Phases.— The Crystallinity Theory of P. P von Weimarn. — Since we have no direct evidence besides the ultramicroscopic upon which to base conclusions regarding the vectorial state of colloid disperse phases we are compelled to resort to indirect means based upon theoretical considerations and extrapolations. Most of these conclusions are based upon the assumption that particles retain their crystal- GENERAL PROPERTIES OF COLLOID SYSTEMS 59 Unity even when their size is progressively changed. Such con- clusions were drawn early in the history of colloid-chemistry; and if the "reguline" state of a metal may be considered as crystalline or cryptocrystalline, B. J. Richter (1862) may be regarded as the first to have urged the view that suspensoid phases have a crystal- line constitution. By far the most convincing evidence in favor of the view that the disperse phase retains its crystalline state, especially in suspensoids as compared with coarse suspensions (which may be demonstrated to be crystalline by direct micro- scopic means) has been given by P. P. von Weimarn (I.e.). The following seem the more important of the numerous reasons he has advanced in favor of this view. (a) Von Weimarn studied the reactions of formation of a great number of inorganic substances (salts, elements, etc.), direct- ing special attention to the influence of the concentrations of the reacting media. It was found that the shape and the degree of dispersion of the product of the reaction varied greatly and con- tinuously with the concentration. Through experiments on more than two hundred substances he was able to show that in medium concentrations^ of the reacting substances definite crystals are formed which attain a maximum size at a definite concentration; at concentrations above and below this the crystals become progressively smaller until in extreme concentrations they pass beyond the dimen- sions of ultramicroscopic visibility. The systems formed in low con- centrations are nothing else than suspensoids identical with those often obtained even before the systematic investigations of P. P. von Weimarn by allowing substances to react with each other in dilute solutions. On the other hand, the systems produced in the highest concentrations correspond with the bodies usually described as "jelHes" or "glasses." P.P. von Weimarn imagines these, too, to have the composition Liquid -f Solid. The conti- nuity with which the size of the crystals decreases with increasing dilution speaks in favor of the view that ultramicroscopic and amicroscopic particles may have a crystalline structure. (b) Another point in favor of the crystalline constitution of ' It should be noted that we are dealing with so-called relative concentrations, in other words, with amounts of dissolved substances calculated in terms of their maxi- mum solubility. In the case of easily soluble materials the concentration ranges therefore correspond with large ranges of absolute molar or percentage concentra- tions. On the other hand, with slightly soluble materials, the whole series of differ- ent precipitates is included in a small absolute range of concentration. 6o GENERAL COLLOID-CHEMISTRY suspensoid phases is their power of starting crystallization in supersaturated molecular-disperse solutions of themselves. Gener- ally speaking, only such solids have this power which are them- selves crystalline. Yet as von Weimarn himself found, highly disperse sols lose this power when their degree of dispersion is sufficiently increased. It is fair to attribute this to the law that the solubiKty of a substance is dependent upon its specific surface, that is to say, rises greatly with extreme subdivision (see p. 74). Highly dispersed particles would therefore not initiate crystalliza-- tion in supersaturated molecular-disperse solutions, because the latter are still unsaturated with regard to them. Wilhelm Ost- wald's finding^ that small quantities of salol, made highly disperse by trituration with an indifferent substance, are unable to effect the crystallization of superfused salol, even though the salol is still demonstrable analytically, may also be thus interpreted. (c) That the particles of sols may coalesce to form micro- crystalHne bodies and even definite crystals after long standing is another fact in favor of the crystallinity of suspensoid phases. Thus P. P. von Weimarn^ found silver crystals to form in a salver hydrosol after this stood a while. The ultramicroscopic observa- tions of M. Traube-Mengarini,^ of J. Amann* and of L. Pelet and A. Wild^ who noted the direct formation of crystalline bodies by a simple coalescence of ultramicroscopic particles in colloid lead (lead oxy hydrate), colloid iodine and colloid dyes are even more convincing evidence of the possibility of a "direct colloid crystallization," that is, a direct fusion of ultramicroscopic par-, tides to form definite crystals. One is inclined to believe that only vectorial particles can have the power of growing into definite crystals, just as one believes that only such may produce crystalli^ zation in supersaturated solutions. Yet we must point out even here that the crystalline character of these "crystalline elements" has been disputed by a whole series of investigators (see below). From these and other facts we may conclude that most sus- pensoids, that is dispersoids having a degree of dispersion of 6.10^ to 6.10^ are possessed of a crystalline disperse phase. But serious 1 Wilh. Ostwald, Z. f. physik. Chem., 22, 289 (1897). 2 P. P. von Weimarn, Koll.-Zeitschr., 4, 317 (1908); 5, 62 (1909). ' M. Traube-Mengarini and A. Scala, Koll.-Zeitschr., 6, 65 (1910). 'J. Amann, Koll.-Zeitschr., 6, 235 (1910). 5L. Pelet-Jolivet and A. Wild, Koll.-Zeitschr., 3, 175 (1908). :.. GENERAL PROPERTIES OF COLLOID SYSTEMS 6 1 objections may be raised to the assumption that all solid disperse particles are crystalline as P. P. von Weimarn, for example, has advocated. Thus, as mentioned above, the crystallinity of large masses of all solids under all circumstances has not been demon- strated experimentally. Even though most substances may be obtained in crystalline form, yet under many circumstances the "vectorial chaining together of the molecules" is so slight or so loose that vectorial properties are no longer observable. While it is true that proteins may be obtained in crystalline form, yet the sohd precipitates from protein solutions, except as produced under special conditions, exhibit no trace of crystallinity. Under such cir- cumstances it is therefore just as suitable to assume that the intensity of vectorial chaining is zero as to postulate a "latent" crystallinity. 4. Dependence of Crystallinity upon Size of Particles. — There remains the possibility that the general assumption upon which all these indirect proofs of the crystalline nature of the col- loid disperse phase depend, namely, the retention of vectoriality in extremes of dispersion, is not vahd for the degrees of dispersion here under discussion. From the behavior of liquids in the proc- ess of sohdihcation we are compelled to assume that solids have a positive surface tension even though its effects do not become clearly evident because of the great internal friction possessed by sohd substances. But, as will be discussed later, the surface energy expressive of this surface tension increases markedly with every increase in the specific surface; in other words, a greater centripetal force acts upon the molecules of highly disperse particles than upon those of coarsely disperse particles. It seems not impossible that such a positive surface tension may produce a deformation in minute crystals, in other words, destroy their structural vectoriality by rounding off their corners and trans- forming them into spheroidal bodies. As shown by the behavior of liquid crystals, the optical vectoriaHty, for example, of such a particle need not be destroyed in such a process. It could there- fore be possible that the free surface tension of solid particles might attain values in extreme degrees of dispersion sufficient to destroy the vectorial chaining together of the molecules responsible for crystallinity. An investigation of the influence of pressure upon the optical properties of crystalline liquids would be of interest in this connection. Further, it mjght be possible that a relation 62 GENERAL COLLOID-CHEMISTRY exists between compressibility and vectoriality of such a nature that easily compressible substances lose their structural vectoriality at lower degrees of dispersion than less compressible ones, etc. It is of importance that such an influence of the free surface tension which increases with the specific surface is not only con- ceivable theoretically but is often demonstrable experimentally. In fact the influence of this factor has been repeatedly observed in that most striking expression of the vectoriality of any system, namely, its crystalline form. As long known from microscopic ob- servation of processes of crystallization, small spherical bodies (globulites) are first seen to appear which in no way resemble crystals.^ It is only after these globuhtes have attained a certain size that they assume crystalline shape. Crystals with rounded edges are seen to appear, and so on.^ According to Link, Franken- heim, Vogelsang, Behrens, Quincke, Biitschli, and many others, crystals are often formed by the coalescence of these microscop- ically isotropic globulites, from which there then result "margar- ites," "honeycombs," etc' It would be of interest to determine whether other changes in the vectoriality of these primary crystals, moi;e particularly changes in their optical properties, also develop as do the structural properties* or whether they exist from the first in even the smallest globulites. Such a microscopic investigation might perhaps be extended to ultramicroscopic refraction studies of colloids. If, for example, vectorial differences in the refraction ' See Wilh. Ostwald, Lehrb. d. allg. Chem., 2 Aufl. i, 1042. ^ See the beautiful microphotographs of P. P. von Weimarn in KoU.-Zeitschr., 2 (1Q08). ' Splendid photographs of such honeycomb structures of crystalline materials are found in O. Biitschli, Untersuchungen iiber Structuren, Leipzig, i8g8. See also the numerous, convincing observations of G. Quincke [Ann. d. Physik. (4), 9, i (1902)] as well as the earlier monographs of H. Behrens, Die Kristalliten, Kiel, 1874; H. Vogelsang, Die Kristalliten, edited by F. Zirkel, Bonn, 1875. A partial reprint of the early views may be found in O. Lehmann, Molekularphysik, 1, 730, Leipzig, 1908. Especial reference should be made to the excellent observations on Asaron of C. Schmidt [Liebig's Ann., 53, 171 (1845)] who observed a perfectly regular coa- lescence of four droplets. For a discussion of the vectorial arrangement of coarsely dispersed particles see R. KruUa [Zeitschr. f. physik. Chem., 66, 126 (1909)]. Wilh. Ostwald [Lehrb. d. allg. Chem., 2, Aufl. 1, 1040, Leipzig, 1903] also recognizes the possibility of a "discontinuous" development of crystals from particles which were originally spherical. But in the end, the question of the state of these "crystal embryos" is still to be regarded as open (see p. 61, in the text). ^ It is important to note that we may not apply to all matter a vectoriality ob- served photographically in the finest precipitates of some solid substances. The degree of effect of positive surface tension upon form depends also upon the internal friction, etc., of the particles and this varies considerably in different cases as evi- denced by the so-called "liquid crystals." See in this connection the work of P. P. von Weimarn cited in the footnote on p. 63 as well as the text on p. 61. GENERAL PROPERTIES OF COLLOID SYSTEMS 63 coefl&cients of many crystals continue to exist when they become extremely small, then the same should be true of the corresponding refraction discs. An investigation of other properties of highly disperse solid particles, such as the thermal and electrical, would also be important. Attention should here be redirected to the conclusion reached above, that solid particles become more and more like liquids as their degree of dispersion increases ; and to the converse of this which P. P. von Weimarn among others has assumed to be the case. It therefore seems possible theoretically that a development of crystals may take place in that the "crystal embryos" are at first liquid and only later become solid as they enlarge, either be- cause of a "progressive" coalescence of molecularly-dispersed particles or through a discontinuous union of submolecular phases. That such seems to be the case is evidenced by the investigations of the observers mentioned above. Wilhelm Ostwald (I.e.), in dis- cussing the analogous process of crystal formation in molten masses, even says: "The precipitation of the insoluble from liq- uids seems always to occur primarily^ in the form of droplets, that is, in the state of an under-cooled liquid." If the dispersion in such a system were to become fixed at such, or more correctly, at a somewhat earlier moment, a highly disperse system (among which colloid systems would be found) containing a Uquid phase would result. In other words, at the beginning of crystallization a structural vectoriality would be lacking. Whether an optical vectoriality exists at this stage remains to be determined. Finally, it seems safe to assume that the form of development of crystals will also vary with the nature of the crystallizing substance. It is evident from all this that the question of the maintenance of crystalUnity, in other words the question of a complete vectori- ahty of the disperse particles, more particularly of the disperse particles of soHds in high degrees of dispersion,^ cannot yet be settled with entire certainty. 1 The italics are mine. This view is also held by G. Quincke (Ann. d. Physik., 9, 10, etc.). ' P. P. von Weimarn, in a recent paper [KoU.-Zeitschr.j 6, 32 (1910)] holds that an influence of the degree of dispersion upon the form of solid particles only becomes effective if their size is less than Sfifi, especially in the case of slightly soluble and difficultly fusible materials. The basis for this is derived from_ a "purely kinetic viewpoint" dependent upon kinetic views regarding the physics of the various "degrees of orientation" of molecules in the body and in the surface layers of a crystal. I confess that to me this argument is not convincing. 64 GENERAL COLLOID-CHEMISTRY 5. Crystallinity of Emulsoids. — Since only a relatively small number of crystalline liquids are known, we may expect to en- counter crystalline emulsoid phases but rarely. In fact, while a number of coarse emulsions having a crystalline disperse phase are known^ not a single example of a crystalline emulsoid is known. This is in part due to the fact that it is rarely possible to make out optically the particles of a disperse phase and thus to investi- gate their vectorial properties, because of the slight difference of refraction between them and the dispersion medium. It is of course not impossible that future investigators may demonstrate the existence of dispersoids having a crystalline emulsoid phase. In this connection the behavior of crystalline liquids when near their "clarification point" should be borne in mind (see the litera- ture quoted in the accompanying footnotes). It must further be remembered that all degrees of vectoriality may be demonstrated, particularly in Uquids.^ Not only do we find examples of different degrees of structural, optical, etc., vectoriality among liquid crystals and crystalline liquids, but as shown by O. Lehmann, many different external factors may in- fluence the kind and the degree of vectoriality. Pressure and tension, the "adsorptive power" of solids, magnetic influences, changes in temperature or of the solvent, the presence of other substances, etc., are all of importance. There are liquids which assume vectorial properties only under the influence of powerful external agencies. Thus, A. Cotton and H. Mouton^ showed that certain organic liquids of high molecular weight become doubly refractive in a strong magnetic field. Similar facts have long been known regarding many typical emulsoids,^ such as con- centrated gelatine solutions (jellies) when under the influence of pressure or tension. As is well known, all the contractile elements of living substance exhibit double refraction.^ Here we deal with a temporary vectoriality which exists only when certain systems are under the influence of transitorily active agencies; or which ^ See the numerous examples in O. Lehmann, Flussige Kristalle, Leipzig, 1906. 2 See the lecture of O. Lehmann, Flussige Kristalle und die Theorien des Lebens, 29, Leipzig, 1906. ^ A. Cotton and H. Mouton, Compt. Rend., 141, 317, 349, etc. (1905). * See the numerous examples investigated by G. Quincke, Drude's Annalen d. Physik., 7, 9, 10, II, 12, 13, 25 (1902 to 1904). ' A recent comprehensive presentation of these relations may be found in W. Engelmann, Ber. Berl. Akad, d. Wiss., 694 (1906), GENERAL PROPERTIES OF COLLOID SYSTEMS 65 is produced through the absorption^ of submicroscopic anisotropic particles. These systems would therefore be classed as possessing the lowest possible grade of vectoriality both with regard to inten- sity and to number of vectorial properties. From all of which it becomes somewhat arbitrary whether we will follow O. Lehmann^ and P. P. von Weimarn' in describing such systems as possessed of an "artificial vectoriality" and as "liquid-crystalline," or not. 1 H. Ambronn, Ber. d. D. Botan. Ges., 6, 229 (1888); 7, in (1889); KoU.- Zeitschr., 6, 222 (igio). ^O. Lehmann, Verh. d. D. physik. Ges., 10, 321 (1908); 10, 406 (1908). 'P. P. von Weimarn, KoU.-Zeitschr., 3, 166 (1908). CHAPTER III GENERAL ENERGETICS OF THE DISPERSOIDS §13. Surface Energies 1 . Forms of Energy Characteristic of Dispersoids. — The fore- going pages have dealt with the general and topographical char- acterization of dispersoid systems, more particularly colloid systems. It is our next problem to discuss the more important forms of energy which play a role in these — for like all physical systems, dispersoids exhibit phenomena which are attributable to changes in their thermal, radiant, electrical, chemical, etc., energies. Evidently, physical systems may be classified on the basis of the forms of energy which appear most frequently or most prominently in them. Thus, gases are best characterized by the behavior of their volume energies, while electrical phenomena seem to be especially characteristic of dilute salt solutions. The form of energy most characteristic of the dispersoids is directly deducible from their definition. A development of much surface is the fun- damental property of dispersoid systems. But the absolute value of this surface is a direct measure of the capacity factor of the so-called surface energies. One therefore anticipates that the properties of these and of closely related forms of energy must play an important part in the dispersoids. Especially true is this of all changes in the dispersoid state which involve an increase or a decrease in the degree of dispersion; for according to definition every change in the magnitude of the surface must be regarded as the result of free surface energies or of their compensation by other energies. Wilhelm Ostwald pointed out the importance of the surface energies for the theory of colloid phenomena even before their dispersoid character was established on theoretical and ex- perimental grounds. 2. Sixrface Energy of the First Order. — Surface energy as usually discussed is made up of two components; a capacity factor as measured by the absolute surface, and an intensity factor as measured by surface tension. This t3^e of surface energy en- 66 GENERAL ENERGETICS OF THE DISPERSOIDS 67 deavors to decrease the surface of a system if free energy is avail- able. For reasons to be discussed in the succeeding paragraphs we shall call this, surface energy of the first order and its intensity factor, positive surface tension. Its most important properties are the following. If surface energy of the first order is freed in any way it is changed into other forms of energy, especially heat, the surface of the system decreasing at the same time. Conversely, if heat is introduced into a system capable of developing free surface energy of this order, the surface tension is decreased. Roughly, the decrease in surface tension is proportional to the increase in tem- perature. If an electric surface is produced, in that two phases having different electric charges which are not permitted to neu- tralize each other are brought in contact with each other, the surface tension of the phases decreases. Further, the value of the surface tension varies with the chemical character of the phases which are in contact with each other. General laws regarding the relation between magnitude of surface tension and chemical character of the phases have not yet been discovered. The surface tension of a dispersion -medium may be lowered or raised by the molecular- disperse or colloid subdivision of a phase in it (for details see page 140). The value of the total surface tension of dispersoids is dependent upon the age of the surface. If the disperse phase lowers the surface tension of the dispersion medium, the value of the tension decreases with time; but if the disperse phase increases the surface tension of the dispersion medium, Httle or no change is observable. The ultimate value of the surface tension attained after a longer period of time is called the static surface tension, in contradistinction to the dynamic surface tension observable in freshly produced systems. We shall discuss the reasons for such changes later. Details regarding positive surface tension and the many methods of measuring it with its correlated surface energy of the first order must be sought in text-books of physics and phys- ical chemistry.^ 3. Surface Energy of the Second Order. — For reasons which we are unable to discuss in detail here we are compelled to recog- nize the possibility of the existence of another form of surface ' A recent and in part exhaustive presentation of the relation of positive surface tension to other physical and chemical factors may be found in H. Freundlich, Kapil- larchemie, Leipzig, 1909. 68 GENERAL COLLOID-CHEMISTRY energy, namely, surface energy of the second order. As is well known, two forms of volume energy are characteristic of gases: one which is transformed into other varieties of energy when the volume of the gas increases, and a second which is analogous to surface energy of the first order in that it also is converted into other forms of energy when the volume of the gas decreases. The intensity factor of this second, less well-known form of volume energy is the so-called "internal pressure." In liquids this at- tains a value estimated at several thousand atmospheres. Rea- soning by analogy we may suspect that a form of surface energy exists which has the tendency to change itself into other forms of energy whenever the surface of a system increases. The intensity factor of this type of surface energy might be designated expansive or negative surface tension. What evidence is there for the actual existence of such a second type and are we familiar with phe- nomena which may advantageously be explained through its properties?^ As a matter of fact, certain phenomena are known which can only be explained by assuming the existence, of such a surface energy of the second order — an expansive surface tension. These are the increases in surface which occur in strictly diphasic systems. The simplest and clearest expressions of an expansive surface tension are observed when small volumes of liquid, such as drop- lets or streamlets, are electrified. The phenomena have long been known under the names "electric heart," "electric fountain," etc.^ The accompanying Fig. 9 taken from O. Lehmann illus- 1 We frequently encounter in the literature, as in the writings of Maxwell , Mens- brugghe, Wilh. Ostwald, Fuchs, van't HofE-Donnan, M. Heidenhain, J. Perrin, L. Michaelis, F. Haber, etc., discussions of the possible existence of, and of the effects of the intensity factor of this kind of expansive surface tension. Since the begin- ning of 1905, partly without the knowledge of the studies of these authors and partly before their papers appeared, I have occupied myself with this concept of surface energy of the second order. Since it led to conclusions which were somewhat surprising and far reaching, I did not dare to publish a monograph entitled "Unter- suchungen zur Theorie der Oberflachen- und Volumenenergien " even though the manuscript had been revised for the third time by the summer of 1905. It has been revised and enlarged several times since then and its contents subjected to rigid reexamination. Because similar views have been frequently expressed, and encour- aged by scientific friends, I have at last decided to publish these investigations, even though far from complete, under the title, "Die energetische Atomistik, tjnter- suchungen zur Theorie der Oberflachen- und Volumenenergien" (Theodor Stein- kopff, Verlag, Dresden). Further details regarding the properties of surface energy of the second order and its role in dispersoid systems may be found there. ^ Regarding phenomena of this type see O. Lehmann, Molekularphysik, 1, 824, Leipzig, 1888; H. Freundicb, KapUlarchemie 212, 255, 260, Leipzig, 1909. I do not, of course, agree with the theories of the latter which differ fundamentally from mine; see Wo. Ostwald, KoU.-Zeitschr., 7, 142 (igro). GENERAL ENERGETU'S OF THE 1)[SI'I';|.;S{ il DS 69 tratcs the "disruptive" surfaee increase a,t,'ainsl luri^entinc \vlii( h liquid (molten) sulphur shows when electrified. The left liand figure shows the effect of a weak, the right-hand that of a strong charge. The liquid sulphur surrounding the rod-shaped electrode first assumes conical sha[ie at the tip of the electrode (this already means increase of surface) and then breaks up into indi\idual droplets. Through strong electrification sexeral such "points of discharge" all showing the same beha\-ior, ma\' be produced. \\hen the electrode is placed in a \'ertical ])osilion, and when the / / Fig. 9. — Increase in surface, when eleetriealh" eharKcd, of melted sulphur against turpentine oil. (After (.'. Lilnu:iiin .) The left-hand figure shows the effect of a weak, tlie ri^;ht-iiand, the etTect of a stronger charge. charge is high and the licjuid has little \isc(isit)-, the j)heniimen(.in of the "electric fountain" is produced (see the figure in (J. Leh- mann's \-olume). The "electric heart" is the name applied to the changes in form obser\'ed when the \'oIume of liquid is weakK elec- trified (see the ])oint of the electrode in the figure to the lel'l). Such increases in surface have also been ol.)ser\'ed whi'ir .v()//(/ phases are brought in contact with liquid ones e\'en when no electric energy is a\'ailable. Of recent investigations of this prol)- lem those of I\I. ^J'raube-Mengarini, A. Scala,' and J. .\mann'- ' M. Traube-Mcngarini and .\. Scala, Koll.-Zcitsclir., 6, (15 (iijio). ^ J. Amann, KoU.-Zeitschr., 6, 2,35 (1910). 70 GENERAL COLLOID-CHEMISTRY deserve special mention. These authors were able to observe microscopically and ultramicroscopically the breaking up of coarsely disperse particles of lead or iodine, in a suitable medium, into smaller but not amicroscopic (molecular-disperse) particles. J. J. von Kossonogow^ found that electrification promoted these effects. Another striking illustration of an increase in the surface of a "solid" phase is seen in the production of lead sponge from lead plates when a suitable current is passed through them (see pp. 71 and 82). In complex dispersoids the phenomena characteristic of an expansive increase in surface remain essentially the same, but they are complicated through the simultaneously occurring changes in concentration and secondary chemical effects. Expan- sion phenomena are observed when fatty acids come in contact with alkaline solutions; when cholesterin, etc., come in contact with various pure solvents, etc. The so-called "myelin forms" produced under such conditions will be discussed later. If we bear in mind that all possible transitions exist between coarsely disperse, colloid, and molecular-disperse solutions, we are driven to the ultimate and, perhaps, most important conclusion of all, namely, that the process of molecular or "true" solution is also to be regarded merely as such a spontaneous and extreme increase in surface in a diphasic system. - ' J. J. Kossonogow, KoU.-Zeitschr., 7, 129 (1910) where earlier publications are listed. ^ Even the most modern textbooks of physics state that the only physical require- ment for solution resides in a reduction of the positive surface tension to zero. But this really tells us nothing concerning the character of solution, for to prove the absence of any energy potential gives no clue to the source of the work necessary for solution. Especial emphasis, therefore, must be laid on the experimental proof of a spontaneous increase in surface in two-phase systems. Surface increases due to three positive surface tensions have long been noted in three-phase systems (as in the spreading of oil on water). Regarding the view that solution is a chemical process consisting of the formation of compounds of solvent and solute in indefinite propor- tions, we need only remark that this assumption, even if correct, does not explain the extraordinary increase in surface which occurs in the process of solution. But this increase in surface is by definition a physical process which like all other physical phenomena depends among others upon the chemical properties of both phases but also upon their electrical, thermal, etc., properties, all of which influence the extent of the surface increase. No chemical conception of the process of solution, whatever its nature, is able to explain why a given solid (say tannin) dissolves as a colloid in one solvent (water) and as a molecular dispersoid in another (alcohol). If we regard the extensive "division" of a dissolved substance as the characteristic of both colloid and molecular-dispersoid solution then every process of solution becomes physical. We can only speak of "chemical" solution (with the exceptions noted above) when free surface energy of the second order is derived exclusively or mainly from chemical energy. The solution of metals in acids is an example of this sort. For details see the book announced on p. 68. GENERAL ENERGETICS OF THE DISPERSOIDS 7 1 Regarding the remarkable fact that separate particles are formed immediately in the expansive increase of surface in the case of solid phases (with the exception of lead sponge) while a progressive increase is often observed in liquids, and for further details regarding the conditions for molecular subdivision, see P- 77- 4. The Relation of Sixrface Energy of the Second Order to Other Forms of Energy. — Since the concept of expansive surface energy is an unfamiliar one, it is necessary to disctiss briefly its relation to other physical and chemical factors. Theoretically, many properties of this surface energy of the second order may be predicted, and this on the basis of the fact that the two types of volume energy show in most respects a reciprocal behavior. Thus, positive surface tension decreases as a rule with increasing temperature; conversely, expansive surface tension should in- crease when the temperature increases. This requirement is satisfied by the general increase in solubility which substances show with rising temperature. Further, the positive surface tension of a system falls if a difference of potential is established at its surface; the negative surface tension should increase under such circumstances. That such is the case was repeatedly de- monstrated in the earlier paragraphs of this book. An increase in surface may be effected very generally and often strikingly by different electrical means, as in the production of colloid solutions from non-disperse phases^ (electric synthesis of the colloids). As already mentioned, but few quantitative relationships have been established between the surface tensions of different sub- stances. A similarly great variation should therefore exist in the values of the expansive surface tension. This requirement is satisfied in our lack of stoichiometrical generalizations regarding both the molecular-disperse and the colloid solubihty of sub- stances, etc. These remarks may suffice to demonstrate the justice of assum- ing the existence of a surface energy of the second order with the described properties. We shall accordingly make use of this concept in the special parts of this book. ' See Wo. Ostwald, Koll.-Zeitschr., 7, 132 (1910). 72 GENEKAL COLLOID-CHEMISTRY §14. Dependence of Surface Energies upon Specific Svuface I. General Considerations. — Relations exist between the surface energies and the shape of the phases at the boundaries of contact. These are extremely important. First, as regards surface energy of the first order: As is well known, its most striking effects appear in systems which have markedly curved surfaces or which, when possessed of plane boundaries enclose a relatively small volume. The so-called capillary phenomena in the strict sense of the word illustrate the influence of the mark- edly curved surface. The effect of the second factor is illustrated in the relation which exists between the height to which a Hquid Fig. 10. — Capillary rise and specific surface. ascends between two glass plates that are in contact with each other along one edge, and the thickness of the layer of the liquid and of the gas above it (see Fig. 10). The thinner the layer, the more definite the capillary phenomena, that is, the higher the ascent of the liquid. The general effect of the influence of the cur- vature as well as of the thickness of the layer of the liquid upon the magnitude of the surface energy of the first order is expressed in the relation between the surface energy and the specific surface of the phases. Thin or markedly curved layers of a liquid are manifestly possessed of a relatively greater absolute surface than equivalent volumes of thicker or less markedly curved layers.^ An increase in surface energy in any given volume is therefore etc.). ' The same is true of structures in which two dimensions are very small (threads, GENERAL ENERGETICS OF THE DISPERSOIDS 73 produced whenever more absolute surface is developed or the specific surface is increased. When we apply this conclusion to typical dispersoids we find that a given volume of the disperse phase, absolutely considered, contains more surface energy than the same volume of the same substance in a non-disperse state. But the total amount of sur- face energy of a single particle is also relatively increased. Thus, when volume and mass are decreased to Mooo by decimal sub- division of a cube (see Table i), the surface of one of the resulting cubic .particles is only decreased ^^loo- The greater the degree of dispersion, the more surface does the disperse phase "contain." In fact we may say that when the disperse phase is so finely sub- divided that the diameter of the individual particles is only twice that of the sphere of action of molecular forces, it " consists only of surface." Evidently the shifting in any system of the relation of the different kinds of energy to each other in favor -of those found in surfaces must have a fundamental influence upon the character of these systems. This growth of the surface energies with increasing subdivision, and their extraordinarily great importance in dispersoids having a high specific surface may be further illustrated as follows:^ If the "internal energy," that is, the total energy of a system minus the surface energy is designated by /, and the surface energy by 5, then the total energy of the system equals I + S. The quantities of energy comprised under the name "internal energy" (for example, kinetic energy, chemical energy, etc.), are proportional to the volume v, while the surface energies are proportional to the surface s, in other words, I = iv and S — ts when i is the internal energy of the unit of volume and / is its surface tension. The total energy T of a system is therefore T = iv -\- ts. If now we consider T the total energy of the unit volume - = T^, in other words, if we fs \ s divide the entire equation by v we obtam Tv = i +l--tj. It is small, that is, if the specific surface of the system is small, the second member is also small and may be neglected. This is the case in most of the physico-chemical reactions hitherto investi- 1 Wilh. Ostwald, Grundr. d. allg. Chem., 4 Aufl., 531, Leipzig, 1909. Tlie above is a somewhat modified presentation of the subject. 74 GENERAL COLLOID-CHEMISTEY gated in which interest has chiefly centered upon part i of the total energy. But if v is kept constant and ^ is increased, as in the subdivision of a given cube for example, the second member may grow tremendously in value. If the subdivision is very great, part i, which is proportional to the volume, may disappear alto- gether in comparison with the value of the second member. Under such circumstances the total energy of the system consists almost entirely of surface energy and all its activities are charac- terized by the properties of the latter.^ 2. Surface Energy of the First Order and Specific Surface. — Illustrations of the relations between surface energy of the first order and specific surface were given above. Another example of such a relation is the fact that the height of ascent of a liquid in a capillary tube is inversely proportional to the diameter of the tube ; in other words, the product of the height of ascent and the diame- ter of the tube is a constant. This means that if the diameter of the capillary tube is reduced by half, the height of ascent of the liquid is doubled, and if the former is decreased to one-tenth its value, the magnitude of the latter becomes ten times as great. If we write the surface energy of a cube with an edge i cm. long as i, the surface energy of the same cube colloidally subdivided (so as to have cubes with io//ju edges) amounts to 1,000,000 when we assume that the surface tension remains unchanged. 3 . Surface Energy of the Second Order and Specific Surface. — Since the surface energy of the second order contains the absolute surface of a system as its capacity factor, we would imagine that its effects should also increase with increase in curvature, decrease in thickness, or increase in degree of dispersion. Is there any experimental evidence for this? It is found in what is called the influence of the size of the particles of solid substances upon their solubility. As Wilhelm Ostwald,^ G. Hulett,^ and others have shown, substances in a finely dispersed state, as produced by tritu- ration for example, are more soluble than those in a coarsely dis- persed state. Hulett found finely triturated mercury oxide to be more than three times as soluble as coarser pieces. The solubility of a highly triturated powder as determined by conductivity • See p. 93 for the interesting conclusions deducible from this discussion. ^ Wilh. Ostwald, Z. f. physik. Chem., 34, 496 (1900). ' G. Hulett, Z. f. physik. Chem., 37, 385 (1901); see also Hulett and Allen, Journ. Am. Chem. Soc, 24, 667 (1902). GENERAL ENERGETICS OF THE DISPERSOIDS 75 measurements amounted to 0.694 millimols (150 mg. per liter). An especially interesting scries of experiments of this kind was carried out by Stas in the year 1870 regarding the solubility of the different "precipitates" of silver chloride.^ Stas found that, depending upon the experimental conditions u,nder which it is obtained, silver chloride assumes the forms: i. "gelatineux; 2. caseeux, flocconeux; 3. pulverulent; 4. grenu, ecailleux, crystallin fondu;" and that the solubilities of these modifications, the degree of dispersion of which undoubtedly decreases in the order given below, was as follows: 1. Flocculent silver chloride 0.0140 gram per liter at 20°. 2. Powdered silver chloride 0.0060 gram per liter at 17°. 3. Granular silver chloride o.oooi gram per liter at 15°. 4. Granular silver chloride 0.03 gram per liter at 100°. The solubility of the granular preparation had to be measured at 100° because it is too slight at room temperature to be determined analyticall}'. The solubility of the gelatinous chloride could not be determined because of the difficulty of separating it iji this con- dition from the fluid in which it is precipitated and on account of its instability. A still older observation of this kind was made by Thomas Graham.^ Graham found that silicic acid jellies of different concentrations have different (maximum) molecular solubilities. Thus, only two parts of the silicic acid of a i percent jelly formed a molecular-disperse solution in 10,000 parts of water, only one part of a 5 percent jelly, and even less of the more highly con- centrated jellies. But silicic acid is a typical emulsion colloid, that is, its degree of dispersion changes with variations in concentra- tion. Concentrated jellies are presumably less disperse than the more dilute and so have a lower molecular solubility. In harmony with the above-sketched conception of solution as a process of extreme increase in surface produced by a free expansive surface energy, it is evident that such influence must act by effecting an absolute increase in surface energy by increas- ing the specific surface. Such a relationship is rendered plausible by the fact that the "artificial" breaking up of a substance 1 See K. Drucker, Koll.-Zeitschr., 4, 216, (1909). * Thos. Graham, Journ. Chem. Soc, 1864; see also his collected papers, p. 618. 76 GENERAL COLLOID-CHEMISTRY preparatory for solution already represents surface work which is later saved in the process of that further surface increase which we call "solution." An interesting observation apparently contradicts this concep- tion of an increase in the surface energy of the second order of a system with its degree of dispersion. According to the concur- rent statements of R. Zsigmondy,^ J. Donau^ and The Sved- berg' colloid gold is only slightly amalgamated, if at all, by mer- cury. But this is really a question of solution velocity, not of maximum solubility. Besides, this case should not be compared with what was said above, for in the amalgamation of colloid gold by mercury we are dealing with a triphasic rather than a diphasic system; furthermore, an absolutely necessary preliminary condi- tion, namely, contact of the two phases is absent. This must first be produced by shaking, etc., and is presumably hindered by the fact that surfaces, especially when markedly curved, are sur- rounded by liquid films having special properties such as great tenacity, etc., which must be broken before direct contact of the phases and solution may take place (see later). 4. Dependence of Surface Tensions upon Specific Surface. — Besides this influence of the specific surface upon the absolute and relative amounts of the surface energies, there exists another be- tween the latter and the shape of phases encountered when equiva- lent but differently constituted surfaces are compared. This relation depends upon the circumstance, which has both an experi- mental and a theoretical basis, that the direct effects of the sur- face energies extend to a certain depth on both sides of the mathe- matical surfaces of contact. In curved surfaces such subsurface effects of the surface energies may weaken or strengthen these, depending upon the convexity or the concavity of the curvature as well as upon the nature of the phase. Thus, in a surface which is convexly curved with regard to one of the phases and which has a positive surface tension, the subsurface- effects may strengthen each other in the "convex" phase while they weaken each other in the "concave" phase. Since we must believe that these subsurface effects are produced by the surface energies or that, conversely, the latter are '■ R. Zsigmondy, Liebig's Ann., 301, 37 (iSpg). " J. Donau, Monatshefte f. Chem., 26, 525 (1905). ' The Svedberg, Koll.-Zeitschr., 5, 323 (1909). GENERAL ENERGETICS OF THE DISPERSOIDS 77 the result of changes in the constitution of one phase produced by contact with another, this mutual weakening or strengthening of the subsurface effects must have a reciprocal influence upon the surface energies, more particularly upon their intensity factors, the surface tensions. If the simultaneous and opposite strength- enings and weakenings of such subsurface effects produced through curvature of the surface do not completely neutralize each other, the surface tension of one and the same surface may assume different values, depending upon its curvature. Special relationships are encountered when the curvature is so great or when the particles are so small or when a layer of one of the phases is so thin that the layers in which the effects of the sur- face energies still manifest themselves come very close to each other or into actual contact. As is demonstrable through molecu- lar physics^ and on thermodynamic grounds,^ the intensity factors of the surface energies change much under such circum- stances. This variableness of the positive surface tension in sys- tems having small dimensions has been demonstrated experimen- tally by the work of Reynold and Rucker' on soap films. Since we have no direct method of measuring negative surface tension in systems having small dimensions, experimental demonstration of its variableness has not yet been possible. For its indirect de- termination molecular dispersoids, or better, ionic dispersoids might be used. Special attention might be directed to the prop- erties of very dilute or extremely ionized solutions of electrolytes and their conductivity or viscosity peculiarities, and these might be correlated with variations in expansive tension. §15. Reciprocal Effects of the Two Surface Energies (Theory of Dispersion and Condensation) I. General Considerations. — Ordinarily, only progressive varia- tions, that is to say, uninterrupted increases or diminutions in surface are considered when the phenomena of surface tension are » See Lord Rayleigh, Phil. Mag. (5), 30, 475 (1890). * W. Gibbs, Thermodynamische Studien, 274, Leipzig, 1892; van der Waals and Kohnstamm, Lehrb. d. Thermodynamik., i, 207, Leipzig, 1908. * Reynold and Rucker, Phil. Trans, Roy. Soc, London (2), 171, 447 (1881); 174, 64s (1883); 177, 627 (1886); 184, 505 (1893). See also P. Drude, Ann. d. Physik. (3). 43, 158 (1891); Johanott, Phil. Mag. (s), 47, 501 (1899); (6) 11, 746 (1906); Schiitt, Ann. d. Physik. (4), 13, 712 (1904), etc.; also A. Pockels, Nature, 43, 437 (1891); Lord Rayleigh, Phil. Mag (5), 48, 331 (1899). 78 GENERAL COLLOID-CHEMISTRY discussed. While the coalescence of liquid droplets when they come in close contact with each other is usually attributed to a sur- face tension effect, such processes are less satisfactorily explained on such a basis alone than is, for example, the contraction of a soap iilm. Conditions when droplets are in "close" contact are highly complex in character (see later). An analogous difficulty is encountered when a progressive increase in surface gives way to droplet formation. We have before us here the general problem: Under what conditions does a progressive variation in surface be- come discontinuous? It is evident that this question is of special importance in the dynamics of the dispersoids, more particularly in that of the colloids, for these are produced either by increasing the dispersion of slightly disperse or non-disperse systems, or by condensing maximally disperse (for example, molecular) systems. 2. Discontinuous Increase in Siurface. — The simplest case of a progressive increase in surface is encountered when we observe the Pig. II. — Spontaneous changes in shape of drops on a plane surface. form of different sized droplets of a non-wetting liquid such as mercury resting on some solid support like a glass plate (see Fig. ii). The smaller the droplet, the greater its relative (specific) surface and the more completely does it retain a spherical form {a). Larger droplets become flattened by their own weight {b), in other words, they increase their absolute surfaces. If we attempt to enlarge the volume of the droplet on the plate by adding more liquid to it, the droplet becomes progressively flatter until at a maximum volume, different with different liquids, it breaks up into several smaller droplets. Thus, with ordinary materials it is not possible to make a coherent droplet, that is, a continuous layer of more than about 25 cc. of mercury on a glass or porcelain surface.^ An analogous phenomenon is offered in the well-known fact that cylindrical deformation of a given volume of liquid can- not be produced after a certain maximum value has been attained, without having the liquid thread break. We need but call to mind ' It is not denied that thinner, continuous layers of mercury might be prepared by other methods or by using very pure materials. GENERAL ENERGETICS OF THE DISPERSOIDS 79 the difficulties which must be overcome in the preparation and progressive deformation of fine mercury threads in the making of thermometers. Subdivision of the droplet of mercury may be facilitated by increasing its absolute (and specific) surface "arti- ficially" through the introduction of energy from without as b}- pressing upon it with a glass .plate as shown in Fig. ii, c. This increase in surface, which is entirely analogous to that produced through gravity, leads to a dispersion of the drop into droplets which are at first irregular in size, but which approximate the spherical more and more as they become smaller. Analogous phenomena are observed when a drop of rancid oil is placed upon a very dilute alkaline solution in which it changes its shape "spontaneously" and finally emulsifies itself; or when, at a temperature of 4o°C., a crystal of cholesterin is introduced into a solution of bile salts.-' As soon as the progressive deforma- tion associated with increase in surface has attained a certain value it becomes discontinuous and the process called "dispersion" begins. This may also be clearly observed in the electric dis- persion of liquids. We need but recall the facts illustrated in Fig. 9. A weak electric charge produces only a deformation and enlargement of surface, which when a stronger charge is given becomes discontinuous and so gives rise to droplet forma- tion. That a progressive increase of surface may also take place in the "spontaneous" production of colloid and molecular dis- persoids is indicated by the appearance in them of "solution figures."^ One can also easily see that the greater the positive surface tension of a drop of liquid as compared with that of the medium in which it is placed, the more easily will its dispersion be accomplishable. Thus, on the same glass or porcelain plate a drop of water, or better yet a drop of ether, may be spread into a much thinner continuous layer than a drop of mercury. The corresponding surface tensions are: ether, 16.5 (at 20°), water, 70.6 (at 20°), mercury, 436 (at 15°). A somewhat simpler method of demonstrating this relation between the discontinuous en- largement of a surface and its surface tension is to deform liquids by causing them to flow through a capillary tip (see Fig. 12). ' H. Schade, KoUoidchem. Beihefte, i, 377 (1910). ^ See the earlier compilation in O. Lehmann, Molekularphysik., i, 481, Leipzig, 1888; where striking illustrations may also be seen. 8o GENERAL COLLOID-CHEMISTRY \/ V While ether and water may flow through such a tip in a fine stream (a); mercury passes through in the form of droplets (b). It must further be pointed out that, as far as known, all phenomena of dispersion are connected with movement of the resulting disperse particles. It is evident that such spatial rearrangements of the disperse particles, in other words, these "dispersion movements" are to be separated theoretically from „ J, the process of dispersion itself, in other words, the increase in surface. They are to be con- sidered as phenomena secondary to the transformations of energy which produce dispersion. Regarding the more intimate relationships between the intensity of these movements and the dispersive forces, only suppositions may be made, for no exact investigations of them exist at present. 3. Theory of Dispersion. — All increases in surface are regarded in this volume as ex- pressions of surface energy of the second order. The work necessary for such transformations is made up of the product of the magnitude and of the tension of the surface. It is therefore by definition sur- PiG. 12.— Appearance face work. From this point of view all of liquids o£ slight (a) increases in surface, whether produced and of great (o) surface ' ^ tension when issuingfrom through gravity, through pressure or com- pression, or by any other means, in other words, all processes of trituration, pulverization, comminution, etc., are only expressions of this surface energy of the second order and differ from each other only in the nature of the sources of the energy employed in bringing about the increase. As previously emphasized, not only mechanical energies but also heat and electrical energies may be transformed into surface energy of the second order, and by this means lead to an increase of surface. Since the increase in surface is always the same, inde- pendently of the nature of the energies employed to bring it about, it must remain the characterizing feature of these phenomena. If we consider one of the simpler effects of surface energy of the second order, as the deformation of a drop of liquid by its own GENERAL ENERGETICS OF THE DISPERSOIDS 8 1 weight, with regard to its possible effect upon the surface energy of the first order, we reach the important conclusion that the de- crease in the free surface energy of the second order when converted into an equivalent of other energies through the increase in surface, increases the amount of surface energy of the first order in the system,- for the quantity of surface energy of the first order in a system is proportional to the absolute surface when the intensity factor of tension remains constant. If tension is constant — which is cer- tainly the case in the non-disperse and coarsely disperse systems to be first considered — the amount of surface energy of the first order increases with every decrease of the other surface energies. This is true for example when the surface of a liquid drop is pro- gessively increased. But there is no reason for assuming that the increase in surface energy of the first order is always equivalent to the decrease of expansive surface energy. Experience shows (see the above-mentioned examples) that when certain increases in surface are brought about, the increase in surface energy of the first order is greater than the decrease in surface energy of the second order, for as soon as there exists an excess of contractile surface energy the surface of the given volume becomes discontinuous. The equilibrium between the two energies which is "dynamically" displaced by a slight deformation of the surface is destroyed as soon as the amount of surface energy of the first order produced, more than compensates for the decrease of expansive surface energy. EquiUbrium will not be reestablished until the hberated amount of contractile surface energy has been transformed (into heat for the most part) , a change which can be accomplished only by an accompanying diminution in surface. Since the expansile tension prevents a diminution of the volume as a whole this tend- ency toward diminution can only be satisfied by a subdivision of the volume into smaller parts, for then only can both require- ments be fulfilled at the same time, on the one hand the increase in absolute surface as demanded by the expansile tension, on the other the decrease in absolute surface as demanded by the contract- ile tension. Subdivision is the only possible result; or to put it in another way, the reciprocal effects of these surface energies must lead to subdivision. Dispersion, or the conversion of a progressive increase in surface into a discontinuous one is characterized energetically by a liberation 6 82 GENERAL COLLOID-CHEMISTRY of positive surface energy brought about by an excessive development of absolute surface through the effects of expansile surface energy.''- 4. Consequences of the Energetic Theory of Dispersion. — If the suggested conception of dispersion is correct, a number of deductions therefrom must be capable of practical support. It follows from what was said that, neglecting certain transi- tion phenomena, dispersion should set in suddenly as soon as a definite amount of deformation has been induced, for discontinu- ous increase in surface corresponds to an intersection point of two changes in energy. We should expect to encounter especially clear examples of such "critical" points when increases in surface are produced by the transformation ot other energies into surface energy of the second order, for then a better control of conditions is possible than in the spontaneous increases in surface. As a matter of fact, such "critical" points have long been recognized, especially in the electric dispersion of liquids and solids. There exists, for example, a so-called "disintegration tension" in all the known electric methods of making colloid solutions,^ at which the dispersion of the previously non-disperse electrodes suddenly begins. Further, the critical point should vary with the value of the positive surface tension, in other words, with the value of the free surface energy of the first order of the substance to be dis- persed. As a matter of fact, the greater the positive surface tension of the substance to be subdivided, the greater is the amount of surface energy of the second order consumed, in other words, the greater must be the amount of electrical energy, for example, that must be introduced into the system. These de- ductions are supported by the well-known fact that progressive increases in surface which do not immediately yield disperse systems are observed more commonly in liquids than in solids. As a rule, large quantities of energy are necessary to produce an increase of surface in liquids and then they do not usually yield disperse systems at once. As already mentioned, only certain solids like lead show progressive increases in surface. We may ex- plain this interesting difference between solids and liquids by the well-known fact that solid phases possess a greater positive surface tension than liquids as indicated by the progressive increase in ' A mathematical formulation of the conditions necessary for dispersion on the basis of surface energy will be given in the new book I have announced. ■• ■ ' See Wo. Ostwald, KoU.-Zeitschr., 7, r32 (1910). , GENERAL ENERGETICS OF THE DISPERSOIDS 83 surface tension of cooling, molten substances. The transitional behavior of substances like lead is also in harmony with this view. When we apply this to the question of the dispersive effects of equal quantities of surface energy of the second order upon substances having different positive surface tensions, we find that the greater the positive surface tension of the substance to he sub- divided, the greater the degree of dispersion of the system. This brings up the question: under what circumstances can we obtain the highest degree of dispersion in one and the same substance? The answer to this is not that we must have present the greatest possible amount of free surface energy of the second order. Were this the case then the degree of dispersion of a dispersoid would have to be proportional to its solubility and this is by no means the case. To produce a maximum degree of dispersion a maxi- mum of free surface energy of the first order must also exist in the system, either to begin with as in solids, or as the result of an es- pecially great increase produced through an increase in surface. Hence, molecular-disperse systems will be formed when the two surface energies acting between solvent and solute attain the phys- ical maximum. To the important consequences of this charac- terization of "molecules" in the terms of surface energies for our conceptions of the structure of matter we shall return later (see p. 96). Let it here be mentioned that F. G. Donnan^ following a suggestion of J. H. van't Hoff, has constructed a capillary theory of colloid solution in which he also uses the concept of "nega- tive" surface tension. He proceeds from the fact mentioned above that in very thin layers of a liquid the surface tension of a particle is no longer independent of the thickness of the layer. On the basis of theoretical considerations which originated with Gauss, he concludes that in layers of such critical thickness the thicker layers tend to spread and become thinner while the thinner layers tend to shrink and become thicker. The resultant constitutes an equilibrium yielding the stable "critical particle." It is evi- dent that this interesting theory^ differs fundamentally from that ' F. G. Donnan, Z. f. physik. Chem., 37, 735 (igoi); 46, 197 (1903). * It should also be noted that F. G. Donnan in his first paper {1901), outHneda , more kinetic theory of colloid solution in that the state of dispersion was regarded as the result of two opposed " molecular streams " occurring in the surface. These proc- esses also take place only "within the molecular spheres of action." 84 GENERAL COLLOID-CHEMISTRY outlined above in that the sphere of action of the expansile surface tension is assumed to lie only within the "layers of critical thick- ness" or the "spheres of molecular activities." According to our view one may observe the effects of expansile surface tension macroscopically, just as one may observe the effects of positive surface tension in coarsely disperse systems, and all independently of the thickness of the layers of particles involved. Moreover, Donnan's view compels him to assume a qualitative difference between colloid and molecular-disperse solutions which is unnec- essary in our conception.^ It is also hard to conceive of the in- creases in surface until the sphere of molecular activities is reached in Donnan's theory. "It is hard to conceive just what happens. Apparently the solid substance spreads into (the solvent) in extremely thin layers or in the form of thin branching threads. It should be noted that the solid colloid is not in an explosive state, for dispersion takes place only in the thin surface layers so that the process of 'solution' of the colloid need not be a rapid one, etc." (Donnan, I.e., 1901, p. 738). The progressive, macro- scopic, microscopic and uUramicroscopic increases in surface of diphasic systems discussed above show that the energetic theory is easily capable of filling this gap. 5. Discontinuous Diminutions in Surface. — When one dis- cusses discontinuous diminutions in surface one must bear in mind that we deal not with diminutions in the surface of the individual particles of a dispersoid but with a decrease in the sum of the surfaces of all the particles in the dispersion medium. As a rule, such decreases in total surface are produced by approximation or coalescence of the individual smaller particles into larger ones. It is important to note that such decreases need not take place only through condensation (agglutination, agglomeration, coales- cence, etc.). Slight decreases in surface may be accomplished when for any reason elongated or flattened particles become more spherical. Tendencies toward such progressive reductions in surface are encountered when dispersoids are cooled. So far as known, the positive surface tension between two (homogeneous) phases always increases with decrease in temperature. Under such conditions irregularly shaped particles would therefore tend to become more spherical with fall in temperature. Yet the amounts ' See p. 134 concerning the "saturation point" of colloids postulated by Donnan. GENERAL ENERGETICS OF THE DISPERSOIDS 85 of such internal diminutions in surface would at all times be small. The "condensation" type of diminution in surface is important in determining the properties of disperse, more particularly of col- loid systems. In coarsely disperse s)'stems it shows itself in the coalescence of emulsified particles, in the formation of threads and flakes from microscopic precipitates, etc. It may be observed ultramicroscopically in colloid systems as a union of ultramicrons to form crystalline or non-crystalline particles. In molecular- disperse systems the process of "crystallization" is encountered, attained at times only after passing through an intermediate stage (see above). Generally speaking, such condensations are produced by the same means which accomplish their dispersion, only different intensities, concentrations, etc., have to be used. Thus, while electric energy has a dispersive effect, removal of the charge leads to condensation, especially in colloids. On the other hand, under proper circumstances and with certain charges, condensing effects may be accomplished electrically, as in the coalescence of electrified droplets.^ Changes in temperature with- in certain limits and additions of foreign, especially ionized, sub- stances have similar effects. Mechanical treatment, like sudden one-sided pressure, may also bring about condensation, especially in coarsely disperse systems. When we study a simple case of condensation, as the coa- lescence of two Uquid droplets, we find it hard to follow the transi- tion changes from the original state to the final. Coalescence usually takes place very rapidly so that the two droplets suddenly become one, though it may still be possible to observe the con- tractile effects of the positive surface tension in the movements of the surface. To study such processes of condensation in detail it is best to use drops of viscid material^ which consume more time in the process. The intermediate phenomena, which we shall find of special theoretical importance, may then be studied to better advantage. Intimate contact of the droplets of a system with each other seems to be an absolute pre-requisite for coalescence (as well as for the union or clumping of solid particles). The droplets must 1 See Lord Rayleigh, Proc. Roy. Soc, London, 28, 406 (1879); 34, 130 (1882). 2 See J. Loeb, Koll.-Zeitschr., 3, 113 (igo8); L. Michaelis, ibid., 4, 55 (1909). 86 GENERAL COLLOID-CHEMISTRY be brought so close together that their surfaces have at least one "point" in common. To put it another way, condensation of two particles can occur only when their surfaces are continuous, even though such surface continuity be limited to a single point. It should be noted that we mean a "physical" and not a "mathe- matical" point, in other words, a structure at present unmeasur- able but nevertheless of finite dimensions. Such a point, greatly "magnified" and fixed at the beginning of its development shows itself as a cylindrical or doubly coniform neck, as illustrated schematically in Fig. 13 taken from L. Michaelis. This connecting piece broadens during coalescence until complete condensation is attained. Sometimes (especially in viscid and in solid disperse particles) another type of contact may be encountered which does not CO CO o Fig. 13.: — Diagram of coalescence of two fluid particles. (According to L. Michaelis.) correspond with the description just given. Here the particles also approach each other very closely but do not come in direct contact. In other words, while fixed to each other they are nevertheless still separated by a very thin layer of the dispersion medium. The adhesion of iron filings to magnets or of powders to hot objects, etc., are macroscopic illustrations of such contacts. The precipitation of coarse suspensions (of kaolin, quartz, etc.) illustrates the same phenomenon in a disperse system. It is probably characteristic of such "flakes" that the individual particles in them are separated from each other by a distance less than the diameter of the surface tension films. Figure 14 repre- sents the matter diagrammatically. Even though the individual particles in this type of contact are not in themselves continuous, the liquid membranes of their surfaces are (see Fig. 14). Because of this difference it seems well to distinguish between the two and to designate the former as condensation while the latter is better called "aggregation." Evidently aggregation may of ten lead to condensation, and conversely aggregation may be assumed to constitute a precursor of condensation. As is well known, special means have to be employed to GENERAL ENERGETICS OF THE DISPERSOIDS 87 bring about such intimate contact or continuity of surfaces. One of the chief factors which tends to prevent this is the fact that the dispersion medium (gas or liquid) exists at the phase surfaces in a denser state, has in other words a so-called surface viscosity. These envelopes act hke the vapor envelopes about the drops of liquid formed when water is poured on a hot surface; they cause a "repulsion" of the particles when they meet acci- dentally and so tend to prevent their coalescence. These phenom- ena are closely related to the processes of "wetting" touched upon above. Stress was laid upon the importance of these envelopes in phenomena of condensation early in the history of colloid- chemistry. Thus, J. M. van Bemmelen wrote in 1888: "I think Fig. 14. — Diagram illustrating con- Fig. 15. — Appearance preceding coagu- densation. lation in a concentrated gold solution. (According to H. Siedenlopf.) it possible that the formation of the flakes which are precipitated in a liquid is dependent upon a change in the surface tension of the liquid membranes surrounding the colloid particles, of such type that these membranes between the particles are torn at some point, thus per- mitting the particles to form aggregates."^ The condensation of disperse particles is connected with phenomena of movement just as is their dispersion. These "condensation movements" consist of a mutual approach of the particles and are also a necessary preliminary for their contact and coalescence. In fact these movements precede contact. The first demonstrable changes in a process of condensation are therefore kinetic in character. This fact is of importance for the theory of condensation. 'J. M. van Bemmelen, "Die Absorption" Gesaramelte Abhandl., 22, Dresden, 1910. The passage quoted is printed in italics in the original also. 88 GENERAL COLLOID-CHEMISTRY The appearance of such condensation movements is not a mere theoretical assumption but a necessary conclusion derived from the experimentally observed behavior of disperse systems before and after processes of condensation have occurred in them. Such condensation movements have actually been observed both microscopically and ultramicroscopically as illustrated in the ac- companying Fig. 15 taken from Siedentopf.^ 6. Theory of Condensation. — If we attempt to analyze the processes of condensation from an energetic standpoint as was done with the phenomena of dispersion, we discover that the former are more complex. In dispersion the phenomena of sur- face energy are the primary ones, while processes of movement, the formation of liquid films, etc., are secondary. But in the processes of condensation these different secondary phenomena must take place in reverse order before the surface phenomena proper come into play. Such considerations harmonize with the fact that phenomena of condensation and the means of initiating them are manifold in character as will appear later when we discuss the phenomena of coagulation. The theory of conden- sation therefore divides itself into two parts, first, to put it briefly, the means by which "intimate contact" of the particles is brought about, and second, the analysis of the processes taking place after contact has been established. Since a discussion of the dififerent means by which the intimate contact of the particles is assured belongs to the field of special dispersoid and colloid- chemistry, this must be postponed. Subject to general discussion here are the changes which begin when the particles of a dispersoid begin to aggregate. This process is characterized by the formation of a common liquid film about the particles. Since this surface film grows smaller in the process of aggregation,^ the whole seems to be produced through the action of surface energy of the first order. It is also clear that with any increase in the contractile surface energy the liquid film tends to push the particles closer and closer together until they come in actual contact. When we deal with liquids, coalescence of the particles then occurs as described above. In the case ' See H. Siedentopf, Verh. d. Dtsch. Physik. Ges., 1910, 25. ^ See in this connection the quotation from J. M. van Bemmelen, on p. 87. GENERAL ENERGETICS OF THE DISPERSOIDS 89 of solids (provided we are dealing with actual phenomena of condensation such as crystal formation) there is a coalescence of at least the solid surface layers. The action of the posi- tive surface energy in the latter case may be imagined as shown in Fig. 16. There results in all instances a decrease of the total sur- face separating the disperse phase from the dispersion medium. The process of condensation is therefore to be regarded as the con- sequence of a transformation of surface energy of the first order. The greater the condensation, that is, the smaller the resulting absolute surface separating disperse phase from dispersion medium, the greater the amount of surface energy of the first order that has been transformed. Processes of condensation do not always yield coarsely disperse or non-disperse systems but may stop when very different de- grees of dispersion have been ;"" '"; attained depending upon the | . i concentration of the reaction .■•/^^\~ — ' j mixture, as shown in the for- // ^^~-~\;--.. -~.^ mation of precipitates in chem- ""•-^^r^^..^^^ ^~"^~-0- , ical reactions (P. P. von Wei- """-■^^rr!^--.-^.,^^ // marn). This variety in degree "^^/ of condensation is analogous to Fig. i6.— Diagram illustrating the .11 J • J • , • theory of condensation. the above-discussed variety in degree of dispersion under different experimental conditions, and must therefore have an analogous energetic significance. The degree of dispersion in a condensing system depends upon the amount of expansive surface energy 'present in it. The smaller the surface becomes by condensation, the greater must become the tendency of the expansive surface energy to counteract the diminu- tion of surface. The system becomes stable when an intermediate degree of dispersion has been attained, in other words, when the surface energy of the second order balances the surface energy of the first order which is producing the condensation. The in- fluence of the introduction of other forms of energy upon the degree of condensation is analogous to the influence of these as discussed for dispersion on p. 82. A relation between condensation in colloids and surface ener- gies was first pointed out by G. Bredig^ in explaining a special form ' G. Bredig, Anorg. Fermente, 15, Leipzig, 1901. 90 GENERAL COLLOID-CHEMISTRY of coagulation. At an even earlier date P. Curie^ pointed out the role of surface energy of the first order in bringing about condensa- tion in molecular-disperse systems in processes of crystallization. It is remarkable that the important suggestions of this investigator have received but slight (or one-sided) development since they were first expressed. P. P. von Weimarn (whose numerous papers appear in the Kolloid Zeitschrift and in the Kolloid- Chemische Beihefte) has also developed theories of condensation and dispersion which he believes to be so universally applicable that he would explain through them all known processes of con- densation and dispersion. A detailed account of his views cannot be given here. It should, however, be emphasized that no theo- retical conception of such processes can be formulated comparable in universality with the energetic one which must by definition always remain the broadest form in which natural phenomena may be described. P. P. von Weimarn in his theories of condensation and dispersion often makes use of moleculo-kinetic conceptions, in other words, he employs special expedients in the elaboration of his views. ^ §i6. Influence of the Specific Stirface upon the Relations be- tween Surface Energies and Other Forms of Energy I. Specific Surface and Volume Energy; Capillary Pressure. — The relation between surface energies and volume energies plays an important role in the phenomena observed in dispersoids. If the surface energies are not confined to a plane surface, in other words, if we deal with structures having a spatially defined surface or one which is curved, than the two surface tensions exert pres- 1 P. Curie, BuU. Soc. Min., 8, 145 (1885). ^ Objections may be raised against certain details of the argument of this author. According to his theory, electrical methods of pulverization are explainable only as condensation processes, which is obviously wrong in view of the dispersing effects of electrical energy described and illustrated on p. 69. Furthermore, he formulates the basic idea of his theory thus: "When, for any reason, the intensity of the dis- solving forces increases on the surface of the dispersed particles, but does not exceed that value at which the velocity of crystallization or of solution becomes considerable, then the dispersed particles are peptized (dispersed) by the dispersion medium." [KoUoidchem. Beih., i, 398 (igro)]. I can see in this only a "translation," and not an analysis of the process of dispersion, for the assumption of "dissolving forces" and of a relation of these to other processes constitutes the problem of dispersion but does not solve it. These objections, however, are not valid if von Weimarn's theories are limited to condensation and dispersion phenomena produced by chemical means. As will become evident later, the theories of P. P. von Weimarn agree throughout with the phenomena observed in this particular field. GENERAL ENERGETICS OF THE DISPERSOIDS 91 sure. To put it more correctly, the surface energies in such bodies, more particularly in curved surfaces, readily change into volume energies when opportunity for such change offers. Thus, in the positive surface tension of a markedly curved system the centripetally directed capillary pressure may bring about a change in the pressure. If we assume the particle to be spherical, the value of this pressure is inversely proportional to the radius and directly proportional to the surface tension. Analogous phenom- ena are encountered when the curved surfaces have a negative surface tension. As indicated above^ these relations between surface and volume energies may be demonstrated experimentally and are of course of great importance in dispersoids. It will be shown later that an increase in density due to positive capillary pressure may be demonstrated experimentally. 2. Specific Surface andChanges of State. — The surface energies which dominate the behavior of disperse systems are also much influenced by temperature (and corresponding herewith, by pressure). Thus the vapor pressure of small droplets or particles is found to be greater, at a given temperature, than that of the same substance in larger masses. Smaller drops therefore tend to evaporate more easily than larger ones, wherefore, in a closed system these recondense^ upon the larger ones. A lowering of the melting point of sohd bodies occurs when their specific surface is increased, just as does a decrease in the evaporation temperature with increasing specific surface. Thus P. Pawlow' found dusts of salol, antipyrin, etc., to melt at a temperature some 7° lower than larger particles. He calculates that in the case of salol, a depres- sion of the melting point of 2.8° about corresponds to a hundred- fold increase in specific surface. A far-reaching influence of the specific surface or curvature is indicated also in the phenomena of solidification or freezing of homogeneous systems. According to Miiller-Thurgau,^ filter paper, moistened with distilled water, freezes at —0.1°, while a clay sphere, moistened with water, freezes, according to Bachmetjew,^ at —0.7°. These figures are 1 Wilh. Ostwald, Grundr. d. allg. Chem., 4 Aufl., Leipzig, 533, 1909. '^ Wilh. Ostwald, Lehrb. d. allgem. Chemie, 2 Aufl. II, 2, 362; Z. f. physik.Chem., 22, 289 (1897). ' P. Pawlow, Z. f. physik. Chem., 65, i, 545 (1909); 74, 562 (1910); KoU.-Zeitschr., 6, 37 (1910); 7, 265 (1910); P. P. von Weimarn, ibid., 6, 32 (1910); 7, 205 (1910). * Miiller-Thurgau, Landwirtschaftl. Jahrb., 9, 176 (1880). ' Bachmetjew, Z. f. wissensch. Zoologie, 66, 584 (1899). 92 GENERAL COLLOID-CHEMISTRY not simply so called under-cooled values for water, but indicate freezing temperatures after such under-cooling is eliminated. In these processes of evaporation, of melting and freezing, a number of energies change. Positive and negative changes in volume and density take place, solid bodies acquire, on melting, free surface energies of the first order, the optical properties change etc. For these reasons it is, as yet, not possible to show the relations which exist between single energy changes and the simultaneously appearing changes in the surface energies, the effect of which increases with increasing specific surface. 3. Specific Surface and Electrical Energy. — The relations between electrical energy and surface energies must also change when macro-heterogeneous are compared with disperse systems. Th. Des Coudres^ showed that in harmony with our theory, a difference of potential between curved and flat surfaces of mercury may not only be proved experimentally but its value be ap- proximately calculated. Of the influence of an electrical potential opposing the positive surface tension, O. Lodge^ states that in a drop this influence increases inversely as the fourth power of the diam- eter of the drop. In this connection should also be mentioned the important study of H.von Steinwehr' who found that finely ground calomel, as used in the preparation of normal electrodes, shows a greater difference of potential toward its saturated solution than does the same substance when less highly dispersed. Further relations between the value of the specific surface of electrodes and electrochemical phenomena may be found in the paper of G. Bredig and J. Teletow.* We would expect, on general principles, that the relations between surface energies and electrical energy would play an especially important part in the case of dispersoids. The majority of electrical phenomena take place on the surface since electrical energy, in contrast to heat, for example, tends to reside on the surface of a homogeneous body. The electrical capacity of a hollow metal condenser is therefore about as great as that of a correspondingly large solid body. Electrical energy will therefore ' Th. Des Coudres, Wiedem. Ann. d. Physik., 46, 292 (1892). ^Wm. C. McC. Lewis, KoU.-Zeitschr., 5, 91 (1909); also E. Hatschek, ibid. ,'7, 158 (1910). ^ H. von Steinwehr, Z. f. Instrumentenkunde, 25, 205 (1906). * G. Bredig and J. Teletow, Z. f. Elektroch., 12, 589 (1906). GENERAL ENERGETICS OF THE DISPERSOIDS 93 often enter easily into reciprocal action with the surface energies. The great importance of these electrical phenomena in colloid systems will become apparent later. 4. Specific Surface and Chemical Energy. — Since colloids belong to the heterogeneous systems, the general law of chemical kinetics governing such systems, may be applied to them. This states that the amount of chemical change in the unit of time is proportional to the absolute surface (Wenzel).^ This leads one to suspect, because of the extraordinarily large absolute surface in colloids, that many reactions will occur more rapidly in them than in coarse heterogeneous systems. Such is, in fact, true. M. Raffo and A. Pieroni^ found that colloid sulphur behaved toward silver salts Hke an energetic reducing agent; while non-colloid sulphur, even though finely divided and obtained by precipitation of a polysulphide, would not form silver sulphide in the cold. Even after prolonged boiling this occurred only partially. The reactions of precipitated metallic silver vary according to the size of its particles. The coarsely dispersed "gray" silver, obtained by reduction with oxalates, is less sensitive to mercuric chloride than is the highly dispersed "black" silver, precipitated by sulphites, etc. (R. Liesegang, Liippo- Cramer).' Analogous rela- tions exist in the decomposition of hydrogen peroxide by platinum. While smooth platinum foil decomposes this compound slowly, a "platinized" foil (one covered with finely divided metallic plati- num) does it more rapidly. When colloid platinum is used, the effect is still observable, if there is but i gram-atom of platinum in 70 million hters of the reaction mixture (or i gram-atom of colloid palladium in 26 million liters: or i gram -atom of colloid gold in one million hters).* Still greater surface effects are naturally to be expected when, as in the last example, we deal with phases having different specific surfaces, that is, having different surface concentrations in space. From the existence of capillary pressure and from the changes in density which result from this pressure we would expect an in- ' See Wilh. Ostwald, Grundr. d. allg. Chemie, 4 Aufl., 328, Leipzig, 1909. 2 M. Raffo and A. Pieroni, KoU.-Zeitschr., 7. 158 (1910). ' Liippo-Cramer, Koll.-Zeitschr., 3, 35 (igo8). * G. Bredig, Bioch. Zeitschr., 6, 315 (1907); G. Bredig and J. Teletow, Z. f. Elek- troch., 12, 581 (1906); J. Teletow (abstract), Chem. Centr., i, 793 (1908). 94 GENERAL COLLOID-CHEMISTRY fluence upon the velocity of chemical reactions, for the speed of a chemical reaction is primarily dependent on the density of con- centration of the reacting components. Therefore, we would expect that the phenomenon of catalysis would be especially marked in colloid- systems. The distinguishing characteristic of a catalyzer resides in the enormous change which it is capable of bringing about in the velocity of a chemical reaction. Thanks to the brilliant investigations of G. Bredig,^ his students and others, it has been shown that many catalytic effects may be brought about by highly dispersed surfaces of all kinds, and that the especially important catalytic reactions of the or gsmic ferments may be closely imitated by various inorganic materials in the colloid state, such as the colloid metals. We need in illustration but to recall the catalytic effects on gases of a trace of platinum sponge or platinum black as compared with the effects of a piece of smooth platinum foil. The great part played here by the specific surface, that is, the volume concentration of the surface, is also apparent. Closely connected with density changes of great surfaces are the so-called adsorption phenomena, which we shall consider in detail later. With these are also connected changes of a chemical nature and reaction accelerations. But since they cannot be discussed to advantage without a previous discussion of adsorp- tion itself, we must postpone the whole matter. Even now, however, we may point out that theoretically the amount of a reaction product ultimately obtained, in other words, the equilib- rium point in a chemical reaction, may be shifted under the influence of great spatial concentrations of the surface energies, 1 G. Bredig, Anorganische Fermente., Leipzig, 1901 ; further, the recent review of the author in Bioch. Z^itschr., 6, 283 (rgoy) ; here may also be found many references to the literature. The following according to Bredig, are the best connected pre- sentations of the field. Bodlaender, tjber langsame Verbrennung, Stuttgart, 1899. W. Ostwald, Grundr. d. allgem. Chem., 1909; Leitlinien der Chemie, r9o6; tJber Katalyse, Leipzig, 1902; Natur-philosophie, 1902. Sv. Arrhenius, Immunochemie, Leipzig, 1907; Theoriender Chemie, Leipzig, 1906. W. Nernst, Theoret. Chemie, r909. W. Herz, Lehre von der Reaktionsbeschleunigung., Stuttgart, 1906. R. Hoeber, Physikalische Chemie der Zelle u. d. Gewebe, Leipzig, 1906. E. Cohen, Physical Chemistry for Physicians and Biologists, Trans, by M. H. Fischer, New York, 1903; H. J. Hamburger, Osmotischer Druck u. lonenlehre i. d. mediz. Wiss., Wiesbaden, 1904. G. Bredig, Elemente der chemischen Kinetik, in Spiro u. Ashers Ergeb. d. Physiol., 1902. Schade, Bedeutung der Katalyse in der Medizin, Kiel, 1907. M. Bodenstein, Chem.-Zeitg., 26, ro75, 1902. J. W. Mellor, Chemical Statics and Dynamics, London, 1904; H. Frfeundlich, KapiUarchemie, Leipzig, 1909. Com- prehensive presentations by Bredig appear in Oppenheimer's Handb. d. Bioch. as well as in Bredig's Handb. d. angewandt. physik. Chemie. GENERAL ENERGETICS OF THE DISPERSOIDS 95 as obtain in dispersoids, for example.^ If a chemical reaction occurs in the zone of contact between two phases, in which, for example, a positive surface tension is present, either of two things may happen. The surface tension may be either raised or lowered by the chemical change occurring in the two phases. In the first instance, the "chemicalresistance," that is, the speed of the oppos- ing reaction, would be decreased through the consumption of energy necessary for the increase in the surface tension; in the second, wherein the surface tension diminishes, an acceleration of the reaction would occur, for the free surface energy produced would now tend to change into chemical energy. Besides the increase in rate, there would also be an increase in the product of the reaction, since the amount of chemical energy available for its formation is increased by the amount resulting from the transformation of surface energy into chemical energy. A great specific surface will therefore be able to shift the equihbrium point of a chemical reaction just as does a rise in temperature. Ostwald^ has given a practical illustration of this. If the the solution of a salt of a fatty acid is brought in contact with a large surface, the fatty acid set free by hydrolysis tends to collect in the surface, that is, it concentrates itself there more than does the base. The hydrolytic equilibrium of the remaining solu- tion is thereby disturbed, and to reestablish it, more of the salt must hydrolyze. Other phenomena of this class, especially as observed in colloid systems, will be discussed later. Finally, it should be noted that several exceptions have been observed to the general rule that substances with large specific surfaces react more rapidly than coarsely dispersed ones. Mc- intosh' states that colloid silver dissolves very slowly in acids. Its solution can be greatly accelerated by the addition of small amounts of permanganate. Occasionally in the literature of colloid-chemistry, we encounter the statement that colloid solu- tions react "sluggishly." In the light of our discussion, this ' J. J. Thomson, Applications of Dynamics to Physics and Chemistry, 203, 234, London (1888) ; see also the extensive discussion of this question but one not free from objection, by T. B. Robertson, Koll.-Zeitschr., 3, 49 (1908). That the osmotic equilibrium between two molecular dispersoids, and that the distribution of a molecularly dispersed substance between two phases depends on the specific surface of the phases has been proved theoretically by F. Kaufler, Zeitschr. f. phjsik. Chem., 43, 686 (1908). 'Wilh. Ostwald, Z. f. physik. Chem., 62, 512 (1908). 'Mcintosh, Amer. Journ. Physic. Chem., 6, 17 (1902). 96 GENEEA.L COLLOID-CHEMISTRY statement is not correct when comparison is made between col- loidally and coarsely dispersed systems. But when comparison is made with the reactivity of molecular and ionic dispersoids, it is. It has been proved with any two substances composing a dispersoid that the reactivity decreases progressively with decreasing degree of dispersion. In molecular and ionic dispersoids in which it might be said that the disperse particles consist "almost entirely of surface," one would therefore expect an enormous development of surface energies. In this connection, one is, as a matter of fact, reminded of the old chemical saying, "Corpora non agunt nisi soluta." But the part played by the chemical energy resulting from the conversion of surface energies during chemical reactions in disperse systems, must also decrease with increasing degree of dispersion. When we come to deal with maximum degrees of dispersion, in other words, with "indivisible" particles such as molecules, atoms or even electrons, one might develop a conception according to which chemical reactions, that is, the union and sepa- ration of molecules or atoms, etc., represent merely the results of decreases in the surfaces of the particles involved. The dynamics of molecules and atoms and especially the effects of chemical energy can in this sense come to be viewed as mere manifestations of the surface energies of maximally dispersed particles. The discontinixity of matter in which we have always beheved and which has been proved in various ways then becomes synonymous with the existence of an immensely great absolute, as well as specific surface; and all changes in this discontinuity become connected with changes in the amount of the surface, or of the degree of discontinuity of the substance, in other words, with changes in the capacity factors as well as the spatial concentrations of the surface energies.-' 5. Specific Surface and Radiant Energy. — The connection ' The history o£ science teaches that we have always held to the theory of the dis- continuity of matter, but that different kinds of energy were in turn made responsible for or associated with the elementary changes in the discontinuity. Distance energy (attracting and repelling forces) kinetic energy, and more recently, electrical energy have in turn been associated with the discontinuity. It is of interest to point out that this electrical theory of the structure of matter is closely allied with the concept that the surface energies are the forces responsible for the elementary changes in discontinuity, for, as pointed out above, electrical phenomena occur chiefly on surfaces. It seems, therefore, but a further step in the same direction, if we add surface tension and surface energies to the "forces" already considered, since both of them are as widely distributed and important as the discontinuity of matter itself. GENERAL ENERGETICS OF THE D^SPERSOIDS 97 between specific surface and another type of energy, namely, radiant energy, is closely related to the chemical phenomena discussed in the previous division. Stas^ found that the photo- chemical sensitiveness of silver chloride precipitates increased with their degree of dispersion. Corresponding to the series given on p. 75, the sensitiveness to light increased from the granular, through the powdered, up to the flocculent or cheesy. Interest- ingly enough, Stas emphasized that it is the latter type and not the "gelatinous" state of silver chloride which is most sensitive to light. Were we to assume, as does P. P. von Weimarn, that the gelatinous is only a continuation of the other varieties of pre- cipitates, in the sense that the precipitate in the gelatinous form represents merely a still finer division of the particles, but is otherwise of the same general character, is crystalline, for ex- ample, then the behavior observed by Stas would constitute a contradiction of Wenzel's law. But not only the improbability of such an exception but many other reasons indicate that in "gelatinous" silver chloride we are dealing with a system fun- damentally different from that characterizing the other solid precipitates. It is an emulsoid in contrast to the others which are suspensoids. These relations between photochemical sensitiveness and size of granules have often been observed since Stas's work and have attained great importance in the practice of photography and in the preparation of photographic films. ^ A more interesting, and perhaps more important discovery is the unusual one of E. Wedekind and H. Baumhauer' (together with Gockel) that the emanations of radio-active substances may be much increased if they are highly dispersed, as by being converted into colloid form. These authors succeeded in prepar- ing radio-active thorium in colloid form. A comparison of the radio-activity of this thoriumsol with that of the metallic (coarsely dispersed) element, measured by the volt decrease per hour, showed the surprising fact that the radio-activity of a sol contain- ing only 0.0235 gram was equal to that of a coarsely dispersed sus- pension containing o.iii gram. In other words, the radio-activity ' Stas, see K. Drucker, KoU.-Zeitschr., 4, 2j6 (1909). ^ See Liippo-Cramer, KoUoidchemie u Photographie (Dresden 1908) as well as the numerous papers of this author in the Kolloid Zeitschrift. ' E. Wedekind and H. Baumhauer, KoU.-Zeitschr., 5, 192 (1909). 7 gS GENERAL COLLOID-CHEMISTRY of the sol was 4.8 times as great as that of the coarsely dispersed element. The extraordinary significance of this discovery^ lies in the fact it has not as yet proved possible to influence markedly the emana- tion from a radio-active substance by any other means^ as by raising the temperature,' evacuation, electrolysis, etc. A more striking demonstration of the great effect of the surface energies which come into play with increase in dispersion could scarcely be found than this singular effect of degree of dispersion upon the radio-active disintegration of the elements. Furthermore, this fact seems to indicate that the surface energies will come to play not only an important, but, in comparison with the other kinds of energy, perhaps a dominant part in a general theory of matter.^ ' It was not recognized by the authors themselves. '^ See the Textbooks on Radio-activity. ' ' Recently an insignificant influence of temperature has been observed (Engler, etc.). * It would be a feat in colloid chemistry to carry out analogous experiments with colloid radium salts. Since colloid, especially suspensoid systems exhibit their characteristic properties with the most minute amounts of disperse phase, only small amounts of radium salts would be necessary. One might first test out their prepa- ration by using the physico-chemically similar barium salts, and after having dis- covered a suitable "micro-chemical" method apply it to radium. Gelatinous radium salts could perhaps be prepared by methods analogous to those used by C. Neuberg and his students (KoU.-Zeitschr., 2, 321, 354) on barium salts in alcoholic solvents. CHAPTER IV DISTRIBUTION OF THE COLLOID STATE AND THE CONCEPT OF COLLOID CHEMISTRY §17. The Fundamental Independence of the Colloid State of the Chemical Nature of the Phases I. Statistical and Experimental Development of the Idea of the Universality of the Colloid State. — In the forthcoming historical portion of this work it will be shown that the number of known colloid systems has steadily increased as colloid chemistry has developed. In Graham's time (1861) and even later, colloidality was generally held to be characteristic of certain substances, but with the discovery of general methods of preparing colloid systems, it soon became clear that this was too narrow a view. At the present time, we may say that practically all solid substances have been, or can be prepared in colloid form by some method or other. P. P. von Weimarn, for example, has by a single method "con- verted" over two hundred diiierent substances (salts, elements, etc.) into coUoids. Of course, different substances are changed into the colloid condition with different degrees of ease, but no decisive effect of the chemical nature of the substance whose dispersion is attempted has as yet been discoverable. Nor is the chemical nature of a dispersion medium of funda- mental significance in determining its ability to maintain a second substance in the colloid condition. Even Graham knew that dif- ferent dispersion media could mutually displace each other with- out destroying the colloid state. He was able to replace the water of a silicic acid gel with alcohol, with sulphuric acid, etc. And while the first known metallic colloids were hydrosols, many metal- lic organosols (metallic colloids in various organic dispersion media) have recently been prepared. Among these are the sols of the alkali metals which cannot even exist in water (The Svedberg). Neither is the suspensoid or emulsoid character of a colloid 99 lOO GENERAL COLLOID-CHEMISTRY determined by any perticular chemical properties of the disperse phase. There exist inorganic as well as organic suspensoids. Generally speaking, the emulsoid states are more common than the suspensoid, in the case of the albumins, for example; but suspens- oids are also found among these, as shown by their ready pre- cipitability through traces of electrolytes, by their low internal friction, etc. (see pp. 12, 13). As P. P. von. Weimarn has shown in his fundamental researches, the same substance may be ob- tained either in the suspensoid or emulsoid state (as a jelly) depending upon the conditions of its preparation. One and the same substance may also exhibit either a suspensoid or an emulsoid character depending upon the nature of the dispersion medium as Freundlich and Neumann have found in the case of dyes (see p. 56). Finally, one and the same substance may appear under dif- ferent circumstances either as a crystalloid or a colloid. We need but recall the crystallization of albumin or, on the other hand, the production in colloid form of materials usually known only as crys- talloids, such as common salt.' P. P. von Weimarn {I.e.) recently showed that mere change in the concentration of the components of a reaction mixture sufficed to precipitate them either in colloid or crystalloid form. These facts show clearly the fundamental independence of the colloid (and crystalloid) state, of the special chemical properties of the substances involved. Obviously, the growing acquaintance of investigators with new colloid materials could not but lead them gradually to rec- ognize that colloid properties were not confined to specific chemical substances. The attempts of P. Rohland^ in 1907 to tabulate colloid materials showed clearly how impossible was such a chemical viewpoint. The result was entirely unsatisfactory, for the table included not only a heterogeneous lot of chemical substances, but was incomplete. Conversely, however, it dem- onstrated the impossibility of coordinating satisfactorily chemical composition with colloid properties and brought home the fact that all materials may occur in the colloid state. But while this view was already beginning to be recognized in 1905 as a necessary conclusion to be drawn from the rapidly increasing list of colloid 1 C. Paal, Ber. d. D. chem. Ges., 39, 1436, 2859, 2863 (1906). ^ P. Rohland, Koll.-Zeitschr., i, 201, 289 (1907); 2, 53 (1907). DISTRIBUTION OF THE COLLOID STATE 1 01 materials^ it should be emphasized that P. P von Weimarn (1906) was the first to express clearly and emphatically on the basis of these findings that the colloid, like the crystalloid, is a universally possible state of matter. Although experiment shows the colloid state to be independent of the chemical composition of the phases, this does not of course mean that the properties of the dispersoids may not change with varying chemical composition of the phases. Examples have already been given which show that one and the same chemical substance may assume different types of dispersion with different kinds of dispersion media. The usual view of this behavior which holds the "chemical nature" of the phases responsible for the ob- served changes, may easily lead to error, for it is not the chemical properties, in other words, the analytical composition and the reactivity which determines whether a substance dissolves as a colloid or molecular dispersoid, but rather the different physical properties such as different "solubility" values, etc., in other words, the free surface energies which bring about the variations in degree of dispersion. Of course, these physical properties, like other properties are in good part dependent upon the chemical composition of the phases, and so change with chemical changes in these. Obviously the stability, reactivity, etc., of a colloid must therefore vary with changes in the chemical composition of the phases concerned. But the chemical relations between disperse phase and dispersion medium characterize the dispersoid just as little as the absorption or liberation of heat which always accompanies chemical processes completely characterize these, even though, as is well known, temperature influences them greatly. 2. Universality of the Colloid State as a Necessary Conse- quence of Characterizing Colloid Solutions as Disperse Systems. — If it is granted that colloid solutions are merely representatives of disperse systems and that their properties are determined through a degree of dispersion which has both an upper and a lower limiting value, it becomes self-evident that almost any de- sired material may be prepared in the colloid condition. For as certainly as all substances have not an iinlimilcd solubility in every solvent, equally certainly can these substances be gotten into a 1 In this connection see Wo. Ostwald, Koll.-Zeitschr.,6, 184 (i9io);R. Zsigmondy, zur Erkenntnis der Kolloide, pp. 170, 171, 175, Jena, 1905. I02 GENEEAL COLLOID-CHEMISTRY disperse form provided a proper dispersion medij/ / i/ ^ //7 l^ ? aVv io / ^^^^ X 1 ^^ >/ 10 •*r^ JJ :^ A^}^ 1 ^^^^ Qjy^ < /-^^^^^ ^^^^ — o o o # Gelatin (^t3WV o 1 1 — Minutes 10 20 Time 30 40 50 60 Fig. 25. — Increase in viscosity of emulsoids with time. (According to the experiments of P. von Schroeder, S. J. Levites and W. Biilz.) of suspensoids indicated that these too suffer viscosity changes, but the variations occur more slowly. " Spontaneous "changes in viscosity in pure suspensoids are usually to be reckoned in days (see p. 152), whereas in typical emulsoids they often occur from minute to minute. Another very interesting difference (which may possibly serve to differentiate suspension from emulsion colloids) is the fact that the viscosity of emulsoid solutions usually increases with time, whereas that of suspensoids decreases. ' H. Chick, Biochem. Journ., 8, 261 (1914). ^ F. Baker, Journ. Chem. Soc, 108, 1653 (1913). ^ R. Gaunt, Journ. Soc. Chem. Ind., 33, No. 9 (1914). * F. Kirchof, Koll.-Zeitschr., 15, 30 (1914)- 158 SPECIAL COLLOID-CHEMISTRY Table 16 and Fig. 25 give a picture of these changes. Table 16. — Increase in Viscosity of EiroLsoiDS with Time (After P. von Schroeder, S. J. Levites, and W. Biltz) Gelatine solution (P. von Schroeder) viscosity Gelatine solution (S. J. Levites) viscosity Benzopurpurin (W. Biltz) U.4 per cent. (25°) viscosity Time At 21.0° At 24.8° At 31.0° Time At 2S° Time Time of outflow, seconds After 5 min. After 10 min. After 15 min. After 30 min. After 60 min. 1.83 2. 10 2.45 4-13 13-76 1.65 1 .69 1-74 1.80 1.90 1. 41 1. 41 1.42 1.42 1.42 After 15 min. After 30 min. After 45 min. After 60 min. After 75 min. After 90 min. After II hr. gelatinized. 2.19 2-39 2 -59 2.80 3.00 3.20 3 -40 After 4 min. After 7 min. After 9 min. After 13 min. After 31 min. After 34 min. After 37 min. 75-4 75-8- 77. u 81.2 106.0 109.0 no. ^ The usual way in which viscosity changes with time is prob- ably best represented by the S-shaped curve found by W. Biltz and A. von Vegesack (I.e.) in their experiments on benzopurpurin solutions. In other words, on standing, the viscosity of an emul- soid first rises. This part of the curve corresponds with the first portion of the curves obtained for gelatine by P. von Schroeder (I.e.). Then follows an almost uniform increase in viscosity as shown by the straight middle part of the curve. This straight line was also found by S. J. Levites (I.e.). Finally, there follows a decrease in viscosity which is represented graphically by the turn of the curve toward the abscissa. This late behavior is found not only in the case of benzopurpurin but also (slightly marked) in the gelatine curve of P. von Schroeder at 24.8°. The pro- gressive variations in the viscosity observed under different ex- perimental conditions and discussed below can all be interpreted theoretically in the terms of this S curve. As shown by the experiments of P. von Schroeder (I.e.) on gelatine, the changes in viscosity with time decrease absolutely as well as relatively with rise in temperature. It is also an important fact that the changes with time are the more distinct the more concentrated the solution. Under certain conditions dilute emulsoids behave like molecular solutions, that is, they show a constant or perhaps a decreasing viscosity (see below, p. 160). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 159 Thus, W. Biltz and H. Steiner {I.e.) could observe well marked changes in state in night-blue solutions at 50° only if they con- tained more than 3 percent of the dye. The effect of the addi- tion of various substances, especially salts, upon the time-varia- tions in viscosity was studied by Gokun {I.e.), who found that when small amounts (of ammonium nitrate) were added, the vis- cosity of gelatine solutions increased more quickly than without such, while if larger amounts were added (say 0.32 to 1.4 normal NH4NO3) the viscosity remained almost constant. In very high concentrations (5.6 to 6.4 normal NH4NO3) the viscosity Jecrea^e^ with time as in the case of suspensoids. The latter is doubtless due to the fact that in such high concentrations "precipitation effects," in other words, coagulatory effects begin to manifest themselves. The following table gives some illustrative figures which are, however, not very exact. Table 17. — Effect of the Addition of NH4NO3 on the Change in Viscosity OF A Gelatine Solution (0.28 percent) with Time (After Gokun) Time Concentration of the added salt in normality o < 0.175 0.3s 0.7 I. OS 1.4 2.1 2.8 3.S 4-2 5.6 After 44 hours After 44 hours After 67 hours After 91 hours After IIS hours 1.36 1.42 1.62 1.77 2 .02 1.77 1.80 i.os(?) 1.90 2.06 .15 1.09 .70 I. 41 1.77 1.54 I.S0(?) 1.77 .07 I.OSll.os .35 1.12 I. OS .32ll.15jl.07 .32 I.l7jl.07 .37!l.I7 l.OS 1.07 1 .06 .13 :.o8 :.o8 :.o8 .07 .30 .18 LIS 1. 14 For the influence of non-electrolytes the reader is referred to the experiments of J. Simon {I.e., 1907-08) on the effect of alcohol on the increase of viscosity with time in albumin solutions. This author found that the viscosity increased as he added more alcohol. Acetone had a similar influence while the higher alcohols were less effective. Fig. 26 gives a picture of the results. A case differing from those hitherto mentioned in that it concerns an emulsoid which spontaneously grows less viscous with time has been described by H. W. Woudstra^ in his work on the toluene sols of rubber. As he has made only a preliminary state- ment we cannot be sure that this case is a true exception. Woudstra iH. W. Woudstra: KoUoid-Zeitschrift, 5, 33 (1909). i6o SPECIAL COLLOID-CHEMISTRY found his carbon tetrachloride sols to become cloudy with iJme.^ It seems possible, therefore, that under the experimental conditions chosen by this investigator his solutions coagulated, in which case their behavior would naturally be irregular. The method of preparation (swelling, trituration, filtration through glass wool) may also have influenced his findings. His solutions may have contained "undissolved" particles which at first caused a high viscosity, but which, later, after their "solution," led to decrease in the viscosity. Further experiments are needed on this point. 12 Hours Pig. 26. — Effect of time upon the viscosity of serum albumin to which alcohol has been added. (According to J . Simon.) In accord with Woudstra's observations are those of K. Schorr and H. Handovsky {I.e., 1910) who found that albumin solutions first show a gradual increase in viscosity but later a slow de- crease on the addition of alkali. Chemical changes (hydrolytic cleavage, etc.) which produce secondarily a decrease in viscosity, somewhat analogous to the hydrolytic action of ferments, are undoubtedly active here (see p. 164). ' Dr. Brauer of Leipzig has also observed that filtered solutions o£ purified rubber which are originally entirely clear show flocculi after standing some weeks. Since then I have been able to make analogous observations on benzene rubber sols. MECHANICAL PROPERTIES OF COLLOID SYSTEMS l6l 7. Effect of Mechanical Treatment of Viscosity of Emulsoids. — It is a remarkable fact that the viscosity of emulsoids is affected by mechanical treatment. If they are shaken for a period or simply pressed several times through a capillary, as in a viscosimeter, their viscosity decreases. Such phenomena have been observed by Gokun (I.e.) and W. Biltz (I.e.). They show that even in such apparently perfect Hquids there is present a kind of "structure" which is destroyed by mechanical treatment. This structure seems closely allied with the oft-mentioned hquid membranes of the dispersion medium which surround the disperse particles. One may imagine that in higher concentrations these membranes unite, somewhat as represented in Fig. 14 in p. 87, and that mechan- ical treatment pulls the individual envelopes apart again. In favor of this view is the fact that, according to W. Biltz and H. Steiner, this phenomenon is particularly marked in concentrated solutions; and that the viscosities of solutions of different ages may be reduced to the same value by sufficient shaking (see Table i8).i Table 18. — Influence of Shaking on the Viscosity of a 2.7 Per Cent. Solution of Night-blue (According to W. Biltz and H. Steiner) Without Shaking After Shaking a b 151.5 118. 2 117. O 143.4 n8.o 117. 139.9 118. 4 117. 4 8. Influence of "Inoculation" on Internal Friction of Emul- soids. — A remarkable phenomenon has been observed by H. Garrett (I.e.) in solutions of gelatine and by W. Biltz and H. Steiner (I.e.) in solutions of night-blue. They found their spontaneous increases in viscosity to be markedly accelerated through the addition of small quantities of aged or gelatinized solutions. As the table shows, this behavior is best observed only in colloid solutions of high concentration. It should be emphasized, as P. von Schroeder (I.e.) has shown, that this phenomenon depends upon a chemical change in the ' In passing it may be mentioned tliat H._Zangger observed ordinary milk to show this behavior. l62 SPECIAI COLLOID-CHEMISTRY Table 19. — Influence of Inoculation on the Internal Friction or a Solution of Technical Night-blue at 25° (According to Biltz and Steiner) Time of outflow Without Inoculation Alter Inoculation Per cent. At once After I day After 6 days At once After Hhr. After I hr. After 2 hr. After I day 0.90 ^■35 1.80 2.25 77.2" 79-3 77.6 85.2 78. 5" 82.0 85.6 91-3 78. s" 81.6 102.6 79.2" 82.2 83.. 88.9 85.2 88.9 86.1 91.6 85.6 103 -3 78.8" 82.0" 85-9 gelatine, probably upon its hydrolytic cleavage. This is proved not only by the fact that the decrease in viscosity, with prolonged heating, is irreversible, but also by the fact that it follows the laws of chemical mass action. Furthermore, after prolonged heating, precipitates appear in the solution, which I hold to be the products of this chemical reaction.^ Analogous considerations apply to the changes in viscosity which silicic acid, etc., show when heated or otherwise treated (W. Flemming, I.e.). This view also supported by the fact that long heating decreases the vis- cosity of many emulsoids, though by no means all. W. Biltz and H. Steiner (I.e.), for example, found that emulsoids of night- blue do not alter their viscosity even after being heated 7 hours. What has been said under headings 3 to 8 must always be borne in mind when making viscosity determinations on emulsoids. For this reason the discussion entered into here needed to pre- cede a consideration of the relations between internal friction, concentration, temperature, etc. 9. Rate of Shear and Viscosity of Emiilsoids.^ — As in the case of suspensions (Humphrey and Hatschek^) but in a much more marked degree, the viscosity of emulsoids is a function of the rate of shear. This could already be concluded from the experiments on gelatine sols by H. Garrett' in which the viscosity was measured by the oscillating disc method. The results are, however, not easy to interpret, since the rate of shear varies in ^Wo. Ostwald, Pfluger's Arch., 109, 277 (1905). ^ E. Humphrey and E. Hatschek, Proc. Physical Soc. Lond., 28, Pt. V, 274 (1916). ' H. Garrett, Diss. Heidelberg, 1903; Phil. Mag. (6), 6, 374 (1903). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 163 every experiment from zero at the center of the disc to a maximum at its periphery. Measurements carried out by E. Hatschek' with the Couette apparatus already described do not suffer from these complications, and two curves thus obtained, in which again the rate of shear is plotted as abscissa and the viscosity as ordinate are shown in Fig. 27. Both curves were obtained with the same (0.5 percent gelatine) sol, the lower when the sol was 24 hours, and the upper (double one) when it was 72 hours old. The lower Pig. 27. curve shows that the viscosity decreases with increasing rate of shear, although the difference is not very great. With the aged sol the viscosity is much higher throughout (see p. 154) and in addition the dependence on the rate of shear is extremely marked, the viscosity at the highest rate of shear being less than one-half of that measured at the lowest. To make sure that this variation is really a function of the rate of shear, and not due to a permanent destruction of structure^ ' E. Hatschek, Koll.-Zeitschr., 13, 88, 1913. 164 SPECIAL COLLOID-CHEMISTRY the series of measurements on the aged sol was repeated in the opposite sense, i.e., with increasing velocities. Marked hysteresis is shown, since the curve thus obtained Hes entirely above the one found with decreasing velocities, but this shows conclusively that no permanent alteration in the system has taken place. 10. Influence of Thermal History on Viscosity of Emulsoids. — When such typical emulsoids as gelatine, agar-agar, etc., are sub- jected to the influence of heat their viscosity is affected in the same way and as markedly as when they are treated mechanically. Prolonged heating decreases the internal friction of these solu- tions. By prolonged boiling, it is possible so to change a solution Fig. 28.- 5 10 Number of hours heated -Effect of prolonged heating on the viscosity of a gelatine solution. (According to P. von Schroeder.) of gelatine or glue that it will no longer solidify when cooled. When alcohol is added to a gelatine solution thus altered by pro- longed boihng, a yellow precipitate is thrown down, which is easily soluble in water. A precipitate similarly produced in nor- mal gelatine only "swells" when thrown into cold water. This was observed as early as 1867 by Moritz Traube.^ Traube called the modification which would no longer gelatinize, /3 gelatine or /3 glue in contrast to the normal, gelatinizing a form. Table 20 and Fig. 28 copied from P. von Schroeder (I.e.) illustrate what has been said. S. J. Levites^ has made further experiments on ^ M. Traube, Reichert und Du Bois Raymond's Arch., 87 (1867). ^ S. J. Levites, Koll.-Zeitschr., 2,239 (1907). MECHANICAL PROPERTIES OF COLLOID SYSTEMS i6s Table 20.— Influence or Heating on the Viscosity or Gelatine Solutions (According to P. von Schroeder) Internal friction of gelatine Hours of heating to about 100" ipe r cent. 2 per cent. 3 per cent. 0-5 I 29 I-7S I.O I 23 I ss ■i-S I 20 I 49 2.0 1 17 I 47 1.76 2.5 I IS 3.0 I 14 I 37 1.68 3-S I 13 40 1 I 13 I 32 i.S6 4-5 I II 5-° 1 30 I-S4 5-S I II 6.0 I 28 1-5° 7.0 I 26 1.47 8.0 I 25 1-47 9.0 1.44 10. I 24 1.42 12.0 I 23 1 .40 H.o I 22 i-39 16.0 I 22 1-39 purified gelatine (gluten), agar-agar and on the sodium salt of thymonucleic acid with entirely analogous results. II. Influence of Concentration on Internal Friction of Emul- soids. — The influence of concentration upon the viscosity of Table 21. — Ineluence of Concentration on Viscosity of Gelatine Solutions (According to S. J. Levites) a Gelatine, at 3S° a Gelatine, at 35" Per cent. Viscosity Per cent. Viscosity 0.2s I. 10 "■S i.i86 o-S I . 22 1 .0 1.262 0.7s J--32 i-S i-332 ' . 1.0 1.46 2.0 1-432 i-S 1-75 3'0 1-603 2.0 2.0s 4.0 1.856 3.0 2. 96. i66 SPECIAL COLLOID-CHEMISTRY emulsoids simulates its effect upon suspensoids. This is clearly evident on comparing Figs. 29 and 22, in doing which it is well to hmit oneself to comparisons involving the same temperatures. Examples of the effects of concentration are given in Tables 21 and 22. Emphasis should be laid on the fact that the above measure- ments refer either to low colloid concentrations or were obtained at higher temperatures. As every one who has experimented with gelatine or agar-agar well knows, there is, for every typical emul- soid, an optimum concentration and an optimum temperature at 3.00 - J ■ V •2. -A Concentration 1 1 1 1 Pig. 29.- 250 ZW 1^0 1.00 I 2 3 4% -Influence of concentration on viscosity of gelatine solutions at 35° (According to 5. J. Levites.) which the solution gelatinizes. Thus solutions of night-blue above 1.575 percent are so thick at 0° that they no longer flow through a viscosimeter. We wish here merely to point out that the influence of concentration on viscosity in typical emulsoids is very great. Thus the viscosity of an agar-agar solution (at room temperature) varies within the first 2 percent from that of pure water to that of a solid. If one compares molar instead of per- centage concentrations, the great absolute increases in the value as well as the abruptness of the viscosity changes appear still more striking. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 167 The effect of temperature on the concentration influence is such that decreasing the temperature makes the ascent of the curve steeper, while increasing the temperature flattens it (see Fig. 30) . This behavior is analogous to that observed in molecular dis- persoids and probably to that observed in suspensoids. Added substances Uke salts increase or decrease the slope of the curve as do temperature changes. Purification of the technical night-blue Table 22. — Influence op Concentration on Viscosity op Night-blue Solutions (According to W. Biltz and H. Steiner) Technical night-blue Purified night-blue At 50° At 25° At 0° At 25° Concen- tration, percent Internal friction Concen- tration, percent Internal friction after 6 days Concen- tration, percent Internal friction Concen- tration, percent Internal friction 0.225 1.007 0.02s 0-985 0.225 I.oog 0.25 1.008 0.4s 1. 019 0.04s o.ggo 0.45 1.026 0.50 1.027 0.67s 1.027 o.ogo 0.994 0.675 1.042 0-75 1.058 0.90 1.041 "-I45 0.997 0.90 1.068 1 .00 1.068 1. 125 I -054 0.180 0.996 I. 125 I.IOI 1-25 1. 091 I-3S I. 071 0.22s 1.006 1-35 1. 132 1-50 1 . 106 i-S7S 1.090 0.270 1.006 1-575 1.176 1-75 i-145 1.80 1.097 0-315 1.006 1.80 1. 180 2.00 I. 171 2.02s I.I2S 0.360 1.008 2.25 I. 221 2.2s I. 142 0-405 1 .014 2.50 1.263 2-475 I-IS7 0.450 1. 019 2-75 •■ - 334 2.70 1. 178 0-49S 1.020 3-00 1-403 3-15 1.240 0-540 1-033 3.60 1.298 0.607s 1-037 4-OS 1-393 0.675 1.042 4-50 I -455 0-7875 0.900 1.0125 I.I2S 1-237 1-35 I -575 1.80 2.02s 2-^5 2-475 2. 70 I -054 1.022 1.065 1.080 I. no I- 105 1. 139 1. 182 1.272 1.390 1.480 1.525 i68 SPECIAL COLLOID-CHEMISTRY (which is ordinarily contaminated by about 43 percent sodium sulphate) decreases the slope of the curve, that is, has the same effect as raising the temperature. It is not impossible, however, that the addition of other salts, such as the chlorides, nitrates, etc., might have an opposite effect. Chemical changes in the colloid itself also change the character of the concentration curve, as is evident in the tables and curves referring to a. and ^ gelatine. 12. Influence of Temperature on Viscosity of Emulsoids. — Besides the influence of concentration on the viscosity of emul- •5 - V - 70 1. ~ J ^ 25° 1 G 50° - (25°) / / — ^ ^ / in - ."0 Concent-ration — 1 1 ^ 1 1 7 2 3 '^ 5% Fig. 30. — Influence of concentration on the viscosity of night-blue solutions. (According to W. Biliz and H. Steiner.) The curve marked " G " shows the behavior of the purified night-blue. soids, described in the previous paragraphs, there exists also a re- lation between temperature and viscosity which is observed when all other factors are kept constant. But systematic investigations of this type over a larger temperature range have not as yet been made. For reasons already given, only dilute emulsoids can be used for such study. Some approximate determinations of the average temperature coefficients of the viscosity of emulsoids are, however, at hand. Thus the internal friction of pure water changes^ ub uuL 18 pex c ent -between- 2T^"and "31 °C.-^ In- contrast MECHANICAL PROPERTIES OF COLLOID SYSTEMS 169 to this, the viscosity of a 3 percent gelatine solution, within the same temperature range, changes from 1.42 to 13.76, in other words, almost 1000 percent (P. von Schroeder, l.c). According to Biltz and Steiner {I.e.), the absolute viscosity of a 1.8 percent solution of night-blue rises from 10.5 to 32 between 25° and 0°, in other words, triples, while the viscosity of water merely doubles under the same conditions. With higher concentrations the changes in viscosity within very narrow ranges of temperature become extraordinary, for the existence of gelatination and melting points means nothing else but that, within a temperature change of a degree or less, the viscosity of such systems changes from that of a fluid to that of a solid. 13. Influence of Added Substances on Viscosity of Emul- soids. — The influence of added substances on viscosity, when all other external factors have been kept constant, has also been thoroughly investigated. Of the mass of facts available in this field we shall mention only a few. For details the original papers should be consulted. So far as the important effect of salts upon emulsoids is con- cerned, the accuracy of most of the earlier measurements is vitiated because impure preparations, contaminated with electrolytes, were used. Only recently have Wo. Pauli and his co-workers {I.e.), in a careful and searching series of investigations, shown what minute amounts of electrolytes suffice to cause substantial changes in the viscosity of organic emulsoids. Nevertheless older experi- ments with commercial preparations and those purified by ordi- nary laboratory methods are not valueless, for such colloids are used in many of the arts and for some scientific purposes. We must distinguish between the effects of salts on the vis- cosity of emulsoids which with time are stable and those which are unstable. When of the latter class, as with gelatine, a distinction must be made between the initial value of the viscosity as observed immediately after the addition of a salt and the final value which is approached only asymptotically. According to the experi- ments of P. von Schroeder {I.e.), S. J. Levitcs {I.e.) and Gokun I.e.), the first of these values follows the general rule of mixtures: salts which raise the internal friction of water affect colloid solu- tions similarly, and vice versa . The final value exhibited by gela- tine solutions after the addition of salts is very different from this 170 SPECIAL COLLOID-CHEMISTRY MECHANICAL PROPERTIES OF COLLOID SYSTEMS 171 first. In Table 23 and Fig. 31, taken from P. von Schroeder (/ c), are collected a series of such viscosity values in gelatine solutions Table 23. — A. Influence of Salts on Internal Friction of Gelatine (After standing i hour) Salt Concentration Salt Concentration M norm. ^*j norm. ^i norm. norm. norm. y2 norm. I norm. Pure gelat. 1.78 1.73 1.78 Pure gelat. . 1.88 1.70 1.83 I. 71 NaSO, KjSO, (NH4)2S04,. i!.II . . . . 1.97.... I.9S..-. 2.72 2. 21 9.41 332 NaCl KCI NH4CI ..76 1.80 1-73 L.71 1.67 1.69 1-74 1.60 1.60 1-59 I-5I I-5I Pure gelat. . . 1.68.... 1.68 1.68 Pure gelat. . 1.65 1.68 1.76 1.70 Concentration NaNOj KNO3 NH4NO3... 1.63 X.65 1. 61 I.S7 ^■53 1.52 1.56 1.52 1.49 1.48 He norm. norm. norm. norm. 1-45 LiCl 1.73 1.76 1.78 2 .00 215 2. 42 1.66 1.88 MgClj LizSO, MgS04 1 .85 1.90 I.C 2. ] )2 2 B. Differences between Internal Friction of Salt-gelatine and Pure I Percent Gelatine Na NH. Mg Li SO4 J^6 norm. SO4 M norm. . SO4 J^ norm.T SO4 }i norm. . CI J^ norm. . . CI }4, norm. . . CI }^ norm. . . CI I norm. ... NO3 J^ norm. NO3 34 norm.. NO 3 J^ norm., NO J I norm.. . . +0-33 + 1 .01 + 7.63 — o. 12 +o.or —0.09 — o. 12 — o 02 — O. II — o. 20 +0.09 -0.08 -0.03 — o. 20 0.00 -o IS — 0.24 — 0.32 +0.17 +o.t8 + 1.64 -o.is —0.01 — 0.23 — o. 20 0.04 — o 16 — o. 27 -0.2s + 0. 22 +0.44 + 0.94 + 0. 10 + 0. 20 +0.32 +0.17 +0.24 +0.47 + 0.05 — 0.02 +0.20 The plus sign means that the internal friction of the salt- gelatine is greater than that of the pure gelatine, and the minus sign the reverse. 172 SPECIAL COLLOID-CHEMISTRY which have stood for an hour. Table 23, B, details the difference in viscosity between salt-gelatine and pure gelatine. If the dif- ference is positive it means that the viscosity of the gelatine has been increased by adding the salt, while if it is negative it means that the viscosity has been reduced to below that of pure gelatine. It appears that sulphates in all concentrations increase the internal friction of gelatine, while chlorides and nitrates decrease it, with the exception of MgCh and LiCl in higher concentrations. The exact concentration of the salt, however, plays an important part, especially in the chlorides which in medium concentrations (about Yi. normal) show a maximum of viscosity which sometimes exceeds that of pure gelatine. Further details may be found in the tables and curves.-^ If the anions of the added salts are arranged according to their effect we obtain the series: S04>C1>N03 In the case of the cations variations occur with different concentrations. If we choose the values found for 3^^ normal solutions we find that the sulphates and the chlorides arrange themselves as follows: Mg>Na>Li>NH4>K Ample opportunity will be found later, to return to these "ionic series," which in honor of the investigator who discovered them are now known as the Hofmeister series. There we shall also find that the complicated influence of the concentration of a salt is not an accidental or an exceptional one, but an expression of general characteristics of the relation between any salt and a change in the state of the colloid system. P. von Schroeder (Z.c.) has investigated the important influence of acids and alkalies on the viscosity of gelatine solutions. His findings are detailed in Table 24 and Fig. 32. The influence of concentration is again complex, for at certain low concentrations (M56 normal for HCl and J-f 28 normal for NaOH) a maximum ' It should again be emphasized that pure gelatine would, perhaps, show totally different results. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 173 viscosity is attained, while at concentrations above I32 normal a viscosity below that of pure gelatine is observed. Concentration 1/2 normal Pig. 32. — Effect of HCl and NaOH upon the viscosity of gelatine. (According to P. von Schroeder.) Table 24. — Influence op HCl and or NaOH on Viscosity of Gelatine (According to P. von Schroeder) HCl NaOH Concentration Viscosity Concentration Viscosity K12 norm. Kse H28 Hi M2 Me H H 1 .40 iSS 1.76 1.68 1.58 1.42 ^•25 1. 17 1 . 12 H12 norm. J^56 K28 Hi >3 2 H H 1.40 1-52 1 .60 r.79 1.62 1.38 I -25 1 .10 1 . 10 Similar effects of concentration, more especially of the alkalies, on the viscosity of soap solutions have been observed by F. Bot- tazzi and C. Victorow (I.e.). [^ 14. Effect of Added Substances on Internal Friction of Emul- soids; Behavior of Protein Solutions. — Through the work of E. Laqueur and 0. Sackur (I.e.), W. B. Hardy (I.e.) and others, and especially through that of Wo. Pauliand his coworkers (I.e.), we 174 SPECIAL COLLOID-CHEMISTRY have become better acquainted with the behavior of various pro- tein solutions such as those of serum albumin, egg albumin, globulin and casein in the matter of their viscosity when subjected to the effects of added chemical substances. These solutions be- long to the emulsoids. Time alone changes their internal friction, yet these changes take place so rapidly that the final viscosity value is reached within a few minutes. Because of this and be- /250 Monochloracetic Acid Oxalic Acid atric Add HudrcchloriC ' Acid . ■ AceNc Acid ■Suipiiuiic Add ■JTrichigracetic ;V^ Acid DicfiloracGl-ic Acid t.040C aoosaor o.oz ao3 0.04 aosn Pig. 33. — Influence of acids upon the viscosity of serum albumin. (According to Wo. Pauli and H. Handovsky.) 7] means viscosity. cause the proteins can be isolated and better purified than gelatine, for example, they adapt themselves especially well to a study of this important problem. The most striking fact that the study of the influence of elec- trolytes on the viscosity of purified proteins has brought out is the enormous change in viscosity which is produced by traces of electrolytes. This is especially true of acids^ and alkahes which '■ Regarding the effect of acids, more especially of acetic acid on protein, see the paper of L. Zoja, KoU.-Zeitschr., 3, 249 (ig MECHANICAL PROPERTIES OF COLLOID SYSTEMS 175 Table 25. — A. Influence of Acids on Viscosity of Serum Albumin (According to Wo Pauli and H. Handovsky)' Internal friction Concentration HCl Citric acid Oxalic acid Sulphuric acid Trichloracetic acid Acetic acid 0.00 norm. 0.005 0.01 0.015 0.017 0.02 0.03 0.04 0.05 I . 0409 1.0832 I. 1660 I -243-' 1.2432 1.2323 I. 1647 1-1356 i .1206 I . 0409 I . 0442 I 0661 I. 1002 I. III2 J. . 1408 I . 0409 r . 0688 I 1337 1. 1634. 1.1852 i . 1 700 I . 0409 I. 0613 I. 0613 I . 0604 1.0638 1.0656 I . 0409 I.0511 1.0725 I . 0409 1.0456 I -0504 1-0525 1.0564 I 0603 I .0518 1.0658 I. 0751 1 . 0906 B. Influence of Bases on Viscosity of Serum Albumin (According to Wo. Pauli and H. Handovsky) Base Concentration Friction inc- ;ase in per cent. Concentration. OH'.io-' O.OI norm. 0.02 0.03 0.01 norm. 0.03 0.05 O.OI 0.03 0.05- O.OI u.03 0.05 0.01 0.05 O.OI 0.03 0.05 0.01 0.03 0.05 O.OI 0.03 0.05 78 151 195 19 23 28 20 28 33 37 65 83 40 76 52 103 146 53 109 151 116 221 230 960 1900 2805 49 82 108 85 148 196 214 Ammonia Methylamine Diethylamine 39° 465 204 442 308 564 800 334 627 825 922 Tetraethylammonium 2718 4490 ' See H. Handovsky, Koll.-Zeitschr., 7, 268 (1910). 176 SPECIAL COLLOID-CHEMISTRY show a behavior entirely analogous to that discussed in connection with gelatine on p. 173. Thus Wo. Pauli and H. Handovsky (I.e.) found that the addition of 0.015 normal HCl suffices to raise the viscosity of a serum albumin solution from 1.0623 to 1.2937, in other words, more than 20 percent. With alkalies, a concentra- tion of 3'^oo normal tetraethylammonium hydroxide is enough to increase the viscosity by 230 per cent. Table 25 and Figs. 33 and 34 may serve to illustrate these facts. PipendinK-lSB'W-^ DieHij/hminK-aSxlO-S O.Oln 0.03n Concentration of the Base etttykminK-SS'-m-* MehyhminK-SIO-* TriirxHiyhminK-71'70-^ ■NH, K-S-ixJO-S Nicofm R/ridmK-Z2e'M-3 0-olfcgonin Fig. 34.- -Influence of bases upon the viscosity of serum albumin. (T'Fo. Pauli and, H. Handovsky.) (According to So far as the effect of salts is concerned, it is found that this is different depending upon whether neutral, acid or alkahne albumin is used (Wo. Pauli). The relations are compHcated especially when the effects of different concentrations of acids and alkahes as well as of salts are considered. It remains for future investigators to give us a clear and comprehensive presentation of this subject. The following features deserve emphasis: Neutral salts always lower the viscosity of neutral protein (Wo. Pauli). This MECHANICAL PROPERTIES OF COLLOID SYSTEMS 1 77 behavior is analogous to the effects of salts on the viscosity of sus- pensoids (see p. 152). When we deal with acid albumin it is found that the anions of the neutral salts play a greater role than do the cations. Salts usually lower the viscosity, though complicated concentration relations appear. With a common cation the anions decrease the viscosity in the following order: C2H302>S04>SCN>N03>C1 [E. Laqueur and O. Sackur (I.e.), W Frey (I.e.), H. Procter,^ L. Zoja {I.e.), Wo. Pauli (I.e.) and others.] The reverse is true with alkali albumin, where the cations play the chief part. From a qualitative point of view all the salts bring about a decrease in viscosity, but when the effects of equal amounts of salts are compared a greater decrease is noted in alkali albumin than in acid albumin. The salts of the alkali earth metals exert a stronger influence than those of the alkali metals. 15. Influence of Added Substances on Viscosity of Emul- soids. Effects of Non-electrolytes and Mixture of Dispersing Media. — Non-electrolytes in low concentrations usually change the viscosity of emulsiods only to the extent in which they increase the viscosity of the pure dispersion medium (S. J. Levites, Wo. Pauli, etc). Yet it is not impossible for non-electrolytes even in low concentrations to influence the viscosity somewhat. Thus Handovsky found that caffeine causes a very perceptible increase in the viscosity of acid albumin.^ We need more experiments in this field. In greater concentrations the addition of non-electrolytes causes very perceptible non-additive changes in viscosity. J. Simon (/.c), for example, found alcohols, acetone, etc., to increase markedly the viscosity of albumin solutions. In future studies of these phenomena it might be well to subtract from the observed changes in viscosity those increases which result from mere mixing of the alcohol with water. Only then will the true changes in visocsity due to the change in the colloids themselves be clearly evidenced. Several albumins, such as the zein of Indian corn, are remark- 'H. Procter, Koll.-Zeitschr.j 3, 307 (1908). * Morphine, alcohol in low concentration, etc., probably produce similar effects. 178 SPECIAL COLLOID-CHEMISTRY able in that they dissolve neither in water nor alcohol, but in a mixture of the two.^ It would be interesting to study the viscosity behavior of such systems. The same is true of many dyes which although soluble in each of the pure solvents show different de- grees of dispersion and even different types of colloidality in the two.^ 16. Theory of Viscosity of Emtilsoids. — The only attempt to deduce a rational formula for the viscosity of a system of two liquid phases has been made by E. Hatschek.^ By considering a system in which the disperse phase is present in such amount that the structure is necessarily polyhedral, and making certain as- sumptions regarding the behavior of such a system when sheared, he arrives at the following expression: Va where n^ is the viscosity of the system, 17 that of the continuous phase (dispersion medium) and A the ratio : total volume/ volume of disperse phase. The curve defined by this equation is hyperbolic in appearance, and 7]^ increases rapidly as A approaches unity, in other words as the volume of disperse phase approaches the total volume. The formula can be applied directly only to emulsions, in which A is known, but has hardly been tested in that respect. To apply it to emulsoids, it is necessary first to make assumptions regarding the volume of disperse phase, since in general only the weight of dispersed matter is given. The simplest assumption is obviously that the disperse phase "swells" by taking up a definite amount of dispersion medium (at constant temperature), in other words that the volume of disperse phase is a constant multiple of the weight of dispersed matter. This assumption has proved to give consistent results over a more or less extensive range of concentrations for sols of glycogen, casein,* and various serum proteins.^ On the other hand, the formula fails in the case of various ' See the detailed paper of G. Galeotti and G. Giampalmo, Koll.-Zeitschr., 3, 118 (1908), where references to the literature may also be found. ^ H. Freundlich and W. Neumann, KoU.-Zeitschr., 3, 80 (1908). ^ E. Hatschek, Koll.-Zeitschr., 8, 34, 1911. * E. Hatschek, Trans. Faraday Soc, 57, 56 (1913). ' H. Chick, Biochem. Journ., 7, 261 (1914). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 1 79 colloids showing very high viscosities, such as rubber or nitrocel- lulose in organic solvents, where the assumption of a constant as- sociation factor proves quite unworkable. On theoretical grounds M. von Smoluchowski considers that the method used in deduc- ing the formula cannot lead to a universally valid expression. S. Arrhenius' has shown on a large number of examples that his well-known logarithmic formula: log ri = 8 C where is a constant and C the "molecular concentration," can be used for expressing the viscosity of emulsoid sols with re- markable accuracy. As the molecular weights of the dispersed phases are unknown, C can only be calculated by making simpli- fying assumptions, and the introduction of "molecular concen- trations" appears unjustified in the case of systems which we have throughout considered to be sub-molecularly dispersed. The formula can therefore hardly be anything but an interpolation formula; but for colloids in aqueous dispersion it leads to remark- ably constant values both for 6 and for the hydration factors, which latter are of the same order of magnitude as for hydrated salts. The formula however also fails entirely for the above men- tioned sols in organic solvents, the association factor becoming variable, and even negative in some cases. A generally valid formula — either rational or empirical — for representing the viscosity of emulsoid sols is therefore still lacking, and it may be worth mentioning that M. von Smoluch- owski (I.e.) does not consider the prospect of deducing the former very hopeful. 17. Viscosity and Electrical Charge of Disperse Phase. — E. Laqueur and O. Sackur {I.e.), W. B. Hardy {I.e.) and especially Wo. Pauli {I.e.) pointed out long ago that the electric charge of protein particles greatly affects the viscosity of their solutions. These investigators hold the electrically or electrochemically charged particles in these solutions to spring from an electrolytic dissociation similar to that observed in molecularly dispersed, slightly dissociated systems. As will become more evident in the chapter on the electrical properties of colloid systems, this '■ S. Arrhenius, Meddelanden fran K. Vetenskapak. Nobelinstitut, 3, 1916. l8o SPECIAL COLLOID-CHEMISTRY assumption has proved both satisfactory and fruitful in explana- tion, for example, of the variations in viscosity caused by added substances. It may be said that when the viscosity of a neutral emulsoid rises on the addition of some substance, this is due chiefly to an increase in the number of dissociated (electrically charged) colloid particles. The correctness of this view is at once evidenced when we recall to mind the striking increase in the viscosity of gelatine, soap, or protein solutions when small amounts of acids or alkalies are added to them. The decrease in viscosity observed in higher concentrations of the acids and alkalies follows the decrease in dissociation. The effect of salts in lowering the vis- cosity of acid- and alkali-colloids corresponds with the effect of salts in depressing ionization when a common ion is introduced.^ Table 25 (p. 175) may serve to show the general parallelism be- tween concentration of OH ions and viscosity. Certain exceptions to the general rule are, however, to be noted, as in the case of piperidine. The well-grounded fact that ions are more strongly hydrated than electrically neutral undissociated molecules explains why increase in dissociation and increase in viscosity go hand in hand. As a result of the magnetic field about the charged particles, or at least through its increase, we may imagine the solvent to be held more closely in the solvent envelopes about the separate particles. Thus also will the internal friction be increased and the separate particles become less mobile for now the charged particles have larger envelopes of the dispersion medium about them. But let us not fail to point out that it does not seem safe to say that this direct application of electrochemical laws will, in the futiure, show itself to be entirely adequate. Nevertheless the ability of these laws to elucidate at least some of the complicated relations ob- served shows them to be at least partly active. Future investigators may reveal great discrepancies between the laws governing the behavior of colloid systems and the electro- chemical laws which apply to molecular and supermolecular dis- perse systems. Notwithstanding isolated analogies, colloid sys- tems may be found to be governed by electrochemical laws which are not subordinate to those governing molecular systems but ^ For a discussion of the electrochemical side of these views see the textbooks of physical chemistry. . MECHANICAL PROPERTIES OF CALLOID SYSTEMS l8l coordinated with them. Great variations from norrnal electro- chemical behavior are already known in the case of suspensoids.^ We can discuss these questions to greater advantage when we come to consider the electrical properties of colloid systems. i8. Viscosity and Degree of Dispersion; Viscosity of Coarse and Complex Dispersions. — Only few observations are available on the theoretically important relation between degree of disper- sion and viscosity, and no systematic study has as yet been made of any number of systems with progressively varying degrees of dispersion. A priori one would expect the viscosity of a dis- persoid to grow with every increase in the amount of contact sur- face, in other words, with the degree of dispersion. It is here as- sumed that the particles of the disperse phase move about with greater difficulty than do the particles of the dispersion medium itself. The dispersion medium, held in the of ten-mentioned sur- face membranes, must have in addition to its usual character- istics a decreased mobility. Experimental evidence can be cited to support this view. The experiments described on p. 152, deal- ing with the decrease in the viscosity on ageing or the addition of salts, show a distinct parallelism between decrease in degree of dispersion and decrease in viscosity. K. Beck and K. Ebbing- haus" found that coarse emulsions of castor oil in water did not greatly change the viscosity of the water, but after gum arable or similar substances had been added which permitted the attainment of higher dispersion, the viscosity rose considerably above that of the oil or the pure gum solution. The increase amounted to 44 per- cent. The fact that cellulose becomes slimy and viscous with long grinding indicates the same thing. G. Buglia^ found milk to show a distinct increase in viscosity after being "homogenized," that is to say, after having its fat finely divided by being squirted against an agate plate. A. Martici* has studied the viscosity of oil emulsions in soap water and found that their viscosity increases as the oil droplets become smaller. But observations can also be cited to support the opposite view. Cases are known in which the viscosity increases as the 1 See Wo. Ostwald, KoU.-Zeitschr., 7, 132 (1910). ^ K. Beck, Zeitschr. Physik. Chem., 58, 409 (1907); K. Ebbinghaus, Diss., Leipzig, 1907. ' G. Buglia, KoU.-Zeitschr., 2, 353 (1908). ■• A. Martici, Arch. di. Fisiol., 4, 133 (1907). 1 I02 SPECIAL COLLOID-CHEMISTRY degree of dispersion decreases. In the case of molecular disper- soids, it is the rule that when the substances have a high molecular weight that they show a greater viscosity. We need but consider the salts (soaps) of the homologous fatty acids in water (see p. 143). While the lower members (acetates) change the viscosity of water but little, aqueous solutions of the higher members are solid. The association of changes in molecular weight with changes in the viscosity of colloid night-blue solutions under the influence of changes in temperature has been observed by W. Biltz and A. von Vegesack {I.e.). They calculated from direct osmotic measurements the molecular weight of technical night-blue at 0°, 25° and 50° to be, respectively, 11550, 5260 and 3550. A glance at the viscosity curves of Fig. 30 shows that the greatest viscosity coincides with the greatest molecular weight. The phenomena in critical fluid mixtures may also be used to show direct parallelism between viscosity and degree of dispersion. As first observed by J. Friedlander,^ a mixture of butyric acid in water, which is completely miscible at higher temperatures, shows a great increase in internal friction when cooled, and this increase occurs in the region where the system begins to become turbid, in other words, where the components begin to separate. This separation must of necessity be highly dispersed at the beginning as evidenced by the fact that a bluish opalescence first appears when the solution is still perfectly transparent. The degree of turbidity and the viscosity at first increase steadily as the separa- tion proceeds. It is not impossible that in this case there occurs not only an increase in the number of droplets but also an increase in their size, for ultimately a coarse separation of the acid and the water is obtained which of course cannot have occurred suddenly. We can also cite an example which shows the opposite of what was said above in discussing cellulose. Highly "masticized," that is to say, mechanically treated rubber, yields much less viscous solutions than the untreated. In an analogous manner the increase in the viscosity of emul- soids with time, during their gelation, indicates a decrease in their degree of dispersion. There is much practical as well as theoretical interest attached ' J. Friedlander, Zeitschr. f. physik. Chem., 38, 430 (1901); V. Rothmund, ibid., 63, 54 (1908). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 183 to a cornparison of the viscosity of coarse dispersions with those of colloid systems. While observations on the viscosity of coarse suspensions^ are few, much more is known regarding the behavior of coarse emulsions.- Of course in many of these experiments we deal with complex emulsions consisting of more than two phases. Still a comparison of the viscosity relations of these systems with those of the emulsoids shows so many and at times such surprising analogies that a short discussion seems valuable especially since it serves to support the belief that emulsion colloids are systems hav- ing the composition liquid + liquid. An excellent example of the increase in the viscosity of a liquid when a second insoluble one is 10 20% Concen traHon Fig. zS- — Influence of concentration upon the viscosity of a castor oil-water emulsion. (According to K. Beck.) emulsified in it, is offered by the so-called solid lubricants (engine grease). Even 0.75 percent of water when thoroughly mixed into liquid solutions of soaps in mineral oil will convert these into salve- like bodies of such high viscosity that they may be spooned out in coherent masses (D. Holde, I.e.). The same example serves to ' Besides the well-known behavior of sand we may point out that M. Franken- heim [Journ. f. prakt. Chem., 54, 433 (1851)] details some observations on increase in viscosity caused by the taking up of solid particles. ^ Besides the works of K. Beck, K. Ebbinghaus, J. Friedlander, V. Rothmund, G. Buglia and J. Simon we may also cite M. Bose, Physik. Zeitschr., 83, 47 (1907); Z. f. Elektroch., 13, 499 (1907); R. Schenk, Kristall. Flussigkeiten, 32, Leipzig, 1905; Eichwald, Diss. Marburg, 1905; D. Holde, KoU.-Zeitschr., 4, 270 (1908) Emulsions of Water in Mineral Oils, etc.; Wo. Ostwald, Koll.-Zeitschr., 6, 103 (1910); E. Hatschek, ibid., 6, 254 (1910); 7. n (191°); T. B. Robertson, ibid., 7, 7 (1910); S. U. Pickering, ibid., 7, n (1910) where references to the older literature may be found; M. W. Beyerinck, ibid., 7, 16 (1910), Emulsions Consisting of Two Colloids; F. G. Donnan, Zeitschr. f. physik. Chem., 31, 42 (1899); KoU.-Zeitschr., 7, 208 (1910) with H. E. Potts.] 184 SPECIAL COLLOID-CHEMISTRY demonstrate the influence of concentration on the viscosity of coarse emulsions, for this varies within the concentration limits of o to 0.75 percent water from that of a liquid soap to that of a "solid" lubricant. Another illustration of the latter has been found by K. Beck (I.e.) and his co-workers in their work on emul- sions of acacia water and castor oil. While small amounts of emulsified castor oil but slightly increased the viscosity of the gum arable solutions certain higher concentrations caused sharp increases. Fig. 35 illustrates this behavior which is fully analogous to that observed in emulsoids. Excellent analogies for the great effect of temperature on the viscosity of lyophilic colloids can also be found in the case of the coarser emulsions. J. Friedlander (I.e.) and V. Rothmund (I.e.) found the viscosity of critical fluid mixtures to be very sensitive to temperature. The temperature coefficient of viscosity in these ranges is three to five times as great as in those in which the system has lost its emulsion nature. The machine oils already mentioned may serve as further illustra- tive material. Their decrease in viscosity with increase in tem- perature is so great that one may distinguish a softening point and a dropping point, the two at times lying but one degree apart. This indicates that their viscosity may fall from that of a solid to that of a liquid within the scope of a few degrees, a suddenness of change which is similar to that observed in the melting points of solids. Finally, attention should be called to a third system, namely, that of an alcoholic solution of rosin containing a little water, investigated by J. Friedlander. This also possesses a relatively large temperature coefficient, namely, one of 5 to 6 per- cent per degree of temperature against that of about 2 percent for water. 19. Viscosity and Type of Disperse Phase. — We have thus far considered the viscosity relations of only the more common and important dispersoids, namely, those having the composition liquid -{- solid and liquid + liquid. It should, however, be re- membered that remarkable increases in viscosity of a liquid disper- sion medium may be caused by finely dividing a gaseous phase in it as illustrated by the mechanical properties of foams which often have many of the characteristics of a solid. We have of course to take into account that strictly two-phase systems of the type liquid -f- gas are hardly known and that the stability of most MECHANICAL PROPERTIES OF COLLOID SYSTEMS 185 foams is closely associated with their so-called adsorption phe- nomena by virtue of which the gas bubbles condense dissolved sub- stances upon their surfaces with consequent formation of solid films. Yet such adsorption processes are, in many cases, com- pletely reversible and the fluid nature of the membranes is pre- served throughout. Thus saponin foam melts down to a homo- geneous fluid perfectly free from coagula, and egg-white may be freed of the threads and coagula present in it in its natural state by beating it to a foam. The greater part of the foam subse- quently melts down to a solution perfectly free from flocculi. This is evidence for the fluid nature of the walls of the foam. The preparation and detailed investigation of colloid foams would evidently be of great interest to general colloid chemistry. ^ If one compares the internal friction of the three typical dis- persoids having a fluid dispersion medium, it is found that a low initial viscosity of disperse phase by no means precludes the at- tainment of high viscosity values for the whole system. In fact, if colloid dispersoids are compared with each other, it is found that emulsoids usually exhibit a higher viscosity than the suspensoids having the same concentration and, in view of the great stability of highly dispersed foams, it even seems as though such when in a colloid degree of dispersion might show still higher viscosity values. We must of course distinguish between high viscosity and the value of other physical properties such as hardness. Paradox- ical as it may seem, it even appears as though viscosity of the dis- persion medium and viscosity of the disperse phase may be only of indirect significance, for it seems probable that the properties of the different surfaces (liquid-solid, liquid-liquid, and liquid- gaseous) and not the low viscosity value of the disperse phase itself are primarily responsible for the viscosity of the dispersoid as a whole. §26. Surface Tension of Colloid Solutions I. General Remarks. — A closed two-phase dispersoid has a series of surfaces. The most important is the one between the 1 For Some observations on fine foams see Wo. Ostwald, KoU.-Zeitschr., i, 333 (1907). Systems belonging to this class are also described by Schroeder, Poggen- dorf's Ann., 137, 76 (1869); see also the patent of J. Weinmayr, described in Chem. Centralbl., 586 (1910). 1 86 SPECIAL COLLOID-CHEMISTRY disperse phase and the dispersion medium. There is, in addition, the surface between the whole dispersoid and its surroundings, in considering which we must distinguish between the surface bound- ing the dispersoid and its vapor and that between the dispersoid and the walls of the vessel, etc. At the present time, however, the sense and value of only a few of these tensions are known; in fact quantitative measurements are available of but a single surface tension, namely, that of the positive tension in the surface between the dispersoid and its vapor. 2. Experimental Facts. — Investigations show that the positive surface tension^ of a colloid solution at its free surfaces may be more, or less, or equal to that of the pure dispersion medium (Rayleigh,^ A. Pockels,^ W. Ramsden,* G. Quincke,^ H. Picton and S. E. Linder," L. Zlobicki,' W. Frei,^ G. Buglia,^ F. Bottazzi and C. Victorow^"). Usually the tension is less. The surface tension of water is increased by gum arable, starch and plum gum. It is lowered by gelatine, glue, egg-albumin, dex- trin, cherry and sweet cherry gum. It is greatly lowered by fats, fatty acids, soaps, resins, tannic acid, etc. Tables 26 and 27 taken from G. Quincke and L. Zlobicki may serve in illustration. Both the increase or the decrease in surface tension follows the concentration of the colloid. Traces of fatty acids, of soaps, etc., suffice to lower greatly the surface tension of water as seen in Table 26. The surface tension of colloid solutions as of liquids in general decreases as the temperature rises but, as Table 27 shows, the decrease is much more marked than in the case of the pure dis- persion medium alone. The type of the disperse phase is of particular importance in determining the change in the surface tension of the pure disper- ' The textbooks of physics and physical chemistry should be consulted for methods of measuring the positive surface tension. ^ Rayleigh, Proc. Roy. Soc, 47, 364 (rSgo). 'A. Pockels, Nature, 46, 418 (1892); Drude's Ann. d. Physik., 8, (r902). * W. Ramsden, Engelmann's Arch. f. Anat. und Physiol. Abt. f. Physiol., 517 (1894); Z. f. physik. Chem., 47, 34r (r902); Proc. Roy. Soc, 72, 156 (1904). « G. Quincke, Wiedemann's Ann., 35, 582 (1888) Ber. d. Berl. Akad. d. Wissensch., 38, 493, 858 (1901); Drude's Ann. d. Physik., 7, 631 (igoi); ibid., 9, 969 {igo2); ibid., 10,507 (1903); ibid., II (1904). " H. Picton and S. E. Linder, Journ. Chem. Soc, 87, 1924 (1905). 'L. Zlobicki, Bull. Acad. Sc. Cracovie, Juli, 488 (1906). ' W. Frei, Zur Theorie der Hamolyse, Diss., Zurich, 1907; Transvaal Medic. Journ., August, 1908. ' G. Buglia, Biochem. Zeitschr., 11, 311 (1908). "F. Bottazzi and C. Victorow, Rend. R. Ac. Line, 19, 659 (1910). MECHANICAL PEOPERTEIS OF COLLOID SYSTEMS 187 Table 26. — Surface Tensions op Colloid Solutions at About 20° (According to G. Quincke) Substance AVater Egg-albumin Aqueous bile solution (9 percent) Venetian Soap 3^0 00 percent J4oo percent Jio percent Tannic acid, 10 per cent Gum arable, 20 per cent Isinglass 1 Gelatine [ very dilute Agar J Specific gravity I . 0000 I 0365 1.0384 1 1.0384 J 1.10133 0.9983 0.9992 I . 0009 1-0352 1 . 0708 I . 0000 I . 0000 I . 0000 Surface tension against "air" Table 27. — Surface Tensions of Colloid Solutions (According to L. Zlobicki) 2 Grams gelatine in lOO cc. solution 2 Grams gum arable in roo cc. solution Temp. Surface tension in mg./mm. Temp. Surface tension in mg./mm. Solution Water Solution Wtter 0.0 II-3 17. u 24-5 6.62 6. 21 5.98 S-70 7.69 i 0.0 7.52 6.6 7-43 17 -o 7.32 1 24.0 8.66 8.47 8.16 7 -75 7.69 7-S9 7.42 7-33 sion medium. This is indicated by the fact that all the above- mentioned examples are emulsoids. Coarse suspensions and suspensoids hardly alter the surface tension of the dispersion medium. H. Picton and S. E. Linder {I.e.) found suspensoid arsenious trisulphide, even in concentrations of 2 percent, and dilute iron hydroxide to produce so slight a decrease in the surface tension of pure water that it scarcely exceeds the experimental error. N. Sahlbom^ obtained analogous results. L. Zlobicki {I.e.) found coarse aqueous suspensions of emery, mastic and gamboge and colloid suspensions of silver and platinum to have the same 'N. Sahlbom, KoUoidchem. Beih., 2, No. 3 (r9io). 18(5 SPECIAL COLLOID-CHEMISTRY surface tension as the pure dispersion medium. The temperature coefficient of the surface tension of these systems was also the same as that of the pure dispersion medium. These results seem to show that only emulsoids decrease the surface tension of their dispersion media. This difference can, in fact, be used as a means for distinguishing the two classes of col- loids from each other. Here again is evidenced the close connec- tion between emulsoids and molecular-dispersoids in that the latter also always exhibit a surface tension different from that of their pure solvents. As already indicated (p. 55), the same chemical substance may assume either emulsoid or suspensoid properties in dif- ferent dispersion media. Thus soaps, many dyes, etc., form emul- soids in water while they form suspensoids in alcohol. One would expect this distinction to show itself in the surface tension behavior of the different solutions when compared with that of their pure dispersion media, which, in fact, it does, as H. Freund- lich and W. Neumann^ have found. Table 28 illustrates this in- teresting fact in that it shows that the surface tensions of the aqueous dispersion media are noticeably decreased while those of the alcoholic solutions show no change, or if anything, a slight in- crease. Table 28. — Surface Tensions oe Colloid Dyes in Water and in Alcohol (According to H. Freundlich and W. Neumann) * Water Alcohol Substance Surface tension Substance Surface tension Water . 7=^ .0 Alcohol Night-blue 21.9 22.3 Xight-blue 68 67 74 72 75 74 74 3 4 8 Congo red Crystal violet 22.2 New fuchsin Diamond fuchsin 3 6 New fuchsin 22 .2 22 I 21.9] 22.9 ti * The surface tensions were measured by the rise in capillary tubes. **This dye is probably molecularly dispersed. 1 H. Freundlich and W. Neumann, Koll.-Zeitschr., 3, 80 (1908). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 1 89 The surface tension of emulsoids is changed by the addition of dispersed substances. This was to be expected from their effect on viscosity. The addition of small quantities of hydroxyl ions raises, while the addition of hydrogen ions decreases the surface tension of neutral gelatine or neutral blood serum (G. Buglia, W. Frei). F. Bottazzi and C. Victorow observed NaOH to affect greatly the surface tension of soap solutions, a behavior which is the image of the corresponding one regarding viscosity. Very low concentrations caused a great decrease in tension; higher concen- trations led to an increase which soon reached a maximum to give way to a second more gradual decrease. W. Frei found the anions SO4, CI, NO3 and the cations Na, K, Mg and Ca to increase the surface tension of neutral gelatine solutions in almost the same order in which they increase that of pure water. It is an interesting fact that the order of the anions is reversed depend- ing upon whether the gelatine is acid or alkaline. Several interesting investigations show how varied and compli- cated are the relations when we also consider the changes in the other surface tensions of the dispersoid, especially if it is of the type fluid -\- fluid in which we are able to measure experimentally the interfacial tension between the disperse phase and dispersion medium. According to G. N. Antonow^ and W. C. McC.Lewis^ we know of two cases, namely, water-ether and aqueous glyco- colate solution-mineral oil, in which the tension at the interfaces increases with rising temperature instead of decreasing as is usually the case. Even though this was observed in the case of but slightly dispersed systems, there is no reason for not believing that temperature exerts a like influence in highly dispersed systems of the same composition. The decrease in capillary rise, with increas- ing temperature, observed by R. Schenck,^ in cholesteryl-benzoate (a so-called crystalline liquid) may come under this head. We have now to call attention to a factor which must be considered in measuring the surface tension of molecularly dis- persed systems but which assumes a still greater importance in colloid systems. Willard Gibbs has formulated a theorem which has been confirmed at least qualitatively by other investigators. It states that substances which lower the surface tension of the ' G. N. Antonow, Journ. Chim. physique, S, 372 (1907). 2 W. C. McC. Lewis, Pliilos. Mag., 15, 506 (1908). 'R. Schenck, Kristallinische Fliissiglceiten, 1129, Leipzig, 1901. 190 SPECIAL COLLOID-CHEMISTRY pure dispersion medium, tend to collect in its surface. Because of this rise in concentration the surface tension must, with time, be- come progressively lower wherefore it is conceivable that it may, under certain circumstances, attain a value different from the original present in the surface immediately after its formation. The latter surface tension, which can be measured only on freshly formed or constantly renewed surfaces is called the dynamic surface tension; that which is present after some time, the static. The distinction between these two surface tensions is of especial importance in colloid solutions because such very small amounts of many colloid substances are able to reduce so greatly the surface tension of the pure dispersion medium.' ' See the recent work of Wm. C. McC. Lewis, Z. f. physik. Chem., 74, 619 (1910) in wliich are detailed the surface tensions of colloid solutions against their own vapors and against the surface of various liquids. CHAPTER VI MECHANICAL PROPERTIES OF COLLOID SYSTEMS m. MOVEMENT IN COLLOID SYSTEMS AND ITS RESULTS §27. Brownian Movement I. General Remarks. — ^The Fundamental Phenomenon. — Literature. — All dispersoids of a sufficiently great degree of dis- persion and having a fluid or gaseous dispersion medium, show under the microscope and ultramicroscope a characteristic move- ment. This was discovered by the English botanist, R. Brown, ^ and has been named for him. Brownian movement is also known as "spontaneous" or "molecular" movement though the latter term should be used cautiously. The separate particles of the disperse phase exhibit a trembling and rotary movement and when the particles are very small, as in colloid solutions, the movement has been described by Zsigmondy as "dancing, hopping and skipping" in nature and also as " translatory " and "progressive." The movement of the smaller particles differs from that of the larger (microscopic) ones in that the former travel along straight lines and suddenly change their direction while the latter follow a more curved path. The movements do not occur in one plane only but in all direc- tions. As one observes the "optical cross section" of a prepara- tion either microscopically or ultramicroscopically, the individual particles disappear and reappear as they move downwards and upwards. Many pictures of this characteristic movement have been published. In Figs. 36, 37, 38, 39, and 40 are reproduced some particularly characteristic types of the movement according to V. Henri,^ R. Zsigmondy^ and 0. Lehmann.^ In picturing such 1 R. Brown, Philos. Mag. (i), 4, loi (1828); 6, 161 (1829); 8, 41 (1830); and also Poggendorf's Ann. d. Physik., 14, 29 (1828). ^ V. Henri, Compt. rend., 147, 62 (1908). A review of the subject of Brownian movement, particularly as illustrated in the movements of the spherules of liquid rubber, may be found in his Le Caoutchouc et la Guta-percha, 2405, 1906 and 1908. ' R. Zsigmondy, Z. Erkenntnis d. KoUoide, 106, Jena, 1905. ^ O. Lehmann, Molekularphysik., i, 264, Leipzig, 1888. 191 192 SPECIAL COLLOID-CHEMISTRY movement in one plane we can, of course, show only the projections of the paths of the particles. To a discussion of more exact methods of determining and measuring these movements we sbaH shall return later (p. 197). Examples of dispersions exhibiting Brownian movement are suspensions of gutta-percha, mastic, etc., prepared by adding water to very dilute alcoholic solutions ;^ suspensions of ultramarine, cinnabar, carmine, etc., in which the disperse phase is amorphous or cryp to-crystalline; the contents of the chalk sacs to be found on either side of the spine in the frog, in which the disperse phase Fig. 36. — Brownian movement in milk. (According to O. Lehmann.) consists of definite prismatic crystals; metal hydrosols; metal sulphide hydrosols and other suspensoids. Milk and latex [0. Lehmann, V. Henri, {l-c.)] are examples of systems having a liquid disperse phase and showing Brownian movement. Brownian movement may also be observed in gas-solid dispersoids such as tobacco smoke, cooling ammonium chloride vapors and con- densing metal vapors. It may also be observed in gas-liquid dispersoids as in fog. Strong magnification is usually necessary to observe Brownian movement. Dark ground illumination together with ultramicro- scopic methods are especially suited for the examination of colloids. H. Molisch^ has shown how, under favorable conditions, this movement may be detected with the naked eye. The latex of 1 See J. Perrin, Die Brownsche Bewegung und die wahre Existenz der Molekiile (Dresden, 1910) for methods of preparing suitable suspensions for the observation of the movement. ^ H. Molisch, KoU.-Zeitschr., 2, Suppl. I, g (1907); Zeitschr. f. wissensch. Mikros., 23, 97 (1907); Sitz. Ak. Wiss. Wien, 116, Abt. 1, Marz., 1907. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 193 A Pic. 37. — In a neutral medium. ^4L ^^ Fig. 38. — In an alkaline medium. Fig. 39. — In an acid medium. Fig. 40. Figs. 37 to 40. — Brownian movement. A, B and C are drawn from Idnemato- graphic photographs of gutta-percha particles. D represents the translatory Brownian movement of a gold particle having a diameter of about iomm- (Accord- ing to R. Zsigmondy.) Figures .4 , B and C are enlargements of 34,000 to i ; figure D one of 5000 to I. 13 194 SPECIAL COLLOID-CHEMISTRY the spurge (Euphorbia) is especially adapted for this. A drop of the material is placed upon a slide and held at a good visual distance in a vertical or almost vertical position while sunlight or the concentrated light from an arc is allowed to fall upon it at a slight angle. When properly placed "the molecular movement (Brownian movement) of the resin particles appears in the form of a peculiar trembling, a lively dance, and a swarming of the microscopic particles giving rise to a beautiful play of colors. Finely ground India ink in water may also be recommended for this experiment" (H. Molisch, I.e.). Directions for observing Brownian movement with the aid of a projection-apparatus have been suggested by J. Perrin (I.e.). Since its discovery in 1827, Brownian movement has been much investigated both experimentally and theoretically. The rather comprehensive literature, for it comprises more than 100 articles, cannot be quoted here. We shall refer to some specific papers only; for reviews of the subject the reader must look elsewhere.^ 2. The Independence of Brownian Movement of External Sources of Energy. — When trying to account for the forces re- sponsible for these remarkable movements one is at first inclined to think them due to the effect of external agencies such as vibra- tions, differences in temperature, etc., due to unequal irradiation, evaporation, surface tension movements, chemical changes, etc. Chr. Wiener^ and G. Guoy^ are to be especially mentioned of those who have made critical investigations to show that none of these factors is responsible for Brownian movement. We cannot go into a detailed restatement of the many experiments that prove the fundamental independenee of Brownian movement of external sources of energy. Only the following points are mentioned as of particular importance. In connection with them it must be kept in mind that all the various factors mentioned of course influence 1 See, for example, O. Lehmann, Molekularphysik., i, 264, Leipzig, 1888, where'a detailed review of the older papers up to 1888 maybe found; The Svedberg, Nov. Act. Soc. Sc. Upsaliensis, Ser, IV, 2, 125 (1907) where 50 articles are referred to; KoU.-Zeitschr., 7, i (1910), where references to newer work and methods of observa- tion are found; J. Perrin, KoUoidch. Beih., i, Heft 6-7 (1910) also available inmono- graph form and dealing particularly with French workers; W. Mecklenburg Die experimentelle Grundlegung der Atomistik, Jena, igio, etc. * Chr. Wiener, Poggendorf's Ann., 118, 79 (1863). ' G. Gouy, Journ. de Physique, 2 Ser., 7, 561 (1888); Compt. rend., 109, 102 (1889); Revue generale des Sciences, i, 1895. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 1 95 the extent of Brownian movement but neither positive nor negative variations in them are capable of suppressing entirely the move- ment inherent in the particles themselves. The effects of vibration, changes in temperature, evaporation, etc., in modifying Brownian movement, may be excluded by working in basements, mines and fields (G. Gouy, Chr. Wiener), with water baths and with sealed containers, etc. The independence of Brownian movement of light effects may be proved by working with different kinds of light from which the heat waves have been carefully excluded [G. Gouy, R. Zsigmondy {I.e., 1905)]. That mutual attractions and repulsions of oscillating particles, dependent, for example, upon differences in electrostatic charge, are not the cause of Brownian movement has been shown by Chr. Wiener {I.e.) and C. Fuchs.^ The Svedberg {I.e.) has shown on silver hydrosols that neither neutralization of the charge nor its reversal, as may be brought about through the addition of traces of electrolytes, have any effect upon the velocity of the particles. The amount and sign of the electric charge of such particles may be determined from their migration in an electric field. The following table illustrates the independence of Brownian movement of the sign and amount of the electric charge as determined by measuring the value of 2A, of the significance of which more will be said later (see page 198). Table 29. — Independence of Brownian JIovement of the Sign and Amount OF THE Electric Charge of Colloid Silver Particles (According to The Svedberg) Sign and amount of electric charge determined and Intensity of Brownian move- measured from the speed of migration of the ment (value of 2 A) particles, /^/seconds: volt/cm. -I-2.IO 2.$ -I-O.26 2.S — 0.42 2.4 — 1.76 2.4 J. Perrin {I.e., 1910, 273) has also found that the addition of traces of acids to gutta-percha suspensions, which first neutralize and then reverse the sign of their original charge, ht^s "no appre- ciable" influence on their Brownian movement. 1 C. Fuchs, Rep. d. Physik., 25, 735 (1889). 196 SPECIAL COLLOID-CHEMISTRY The movement is not caused or markedly influenced by any chemical reactions occurring between disperse phase and dispersion medium. This is proved not only by the fact that all chemically heterogeneous substances thus far investigated exhibit the move- ment when sufficiently dispersed, but by the further fact that the intensity of the Brownian movement in a given dispersoid is always the same, in other words, does not change with ageing. If chemical reactions were responsible for the movements, say indi- rectly through changes in capillarity (as in the case of mercury droplets in contact with sulphuric acid and potassium bichromate) then the movements would cease after a time. As a matter of fact, solid particles and gas bubbles, suspended in liquid occluded in many minerals, and therefore of course, very old, show Brown- ian movement. (See for example G. Gouy, I.e.). Like considera- tions exclude all the capillary theories^ of Brownian movement which at first glance are so plausible. At present, it is inconceiva- ble why in a closed system an equilibrium should not ultimately become established between the participating surface tensions for example. Finally, it should be mentioned that the type of the disperse phase or of the dispersion medium is not of decisive importance in bringing about Brownian movement provided the system is from the outset of a kind to permit it, in other words, is either liquid or gaseous. Not only solid and liquid particles but gaseous ones as well show Brownian movement in fluids. But solid and liquid particles show Brownian movement in gases also, as in smoke, in condensing metallic vapors, in fog, etc.^ (L. J. Bodaszewski,' H. Molisch, (I.e.), F. Ehrenhaft,^ M. de BrogHe^). That Brown- ian movement is independent of the type of the disperse phase also proves that density is not of fundamental importance in its causation as already emphasized by the earlier writers, Chr. Wiener, G. Gouy, etc. These investigations, often of a most painstaking nature, show ' Such capillary theories of Brownian movement have been suggested by G. van der Mensbrugghe, Poggendorf's Ann., 138, 323 (1869); C. Maltezos, Ann. chim. phys. (7), I) SS9 (i8g4); Compt. rend., 121, 303 (1895); G. Quincke, Verb. d. Naturforscher usw., 26, Diisseldorf, 1898; Beibl. Ann. d. Physik., 23, 934 (1899) etc., and others. ^ O. Lehmann, Molekularphysik., 2, 5, Leipzig, 1888. 'L. J. Bodaszewski, Dingler's Polytechn. Journ., 239, 325 (1881). * F. Ehrenhaft, Sitz. Ak. Wiss. Wien., 116, 1139, (1907), ibid Marz., 1909; Physik. Zeitschr., 12, 308 (1909), etc. ' M. de Broglie, 148, 1165, 1315 (1906); Le Radium 203, (1909). MECHANICAL PROPERTIES OF COLLOID SYSTEMS I97 that the source of energy for Brownian movement Hes within the disperse system itself and is obviously of a very general nature for it evidences its effects under the most varied external condi- tions. Brownian movement is, however, observed only in disperse systems, more particularly only in such as have a high degree of dispersion. The kinetic hypothesis according to which gases and liquids are regarded as conglomerates of rapidly moving molecularly dispersed particles, has recently been applied to Brownian movement. In fact, some have seen in this direct evidence for the correctness of the kinetic theory as applied, say, to the movement of liquid particles. We return to this question on page 210. While really marvelling at the successful applica- tions that have been made of this kinetic hypothesis, it seems to me not impossible that future investigations may yield another more universal and less hypothetical explanation of this sponta- neous movement. 3. More Exact Determination and Measurement of Browm'an Movement. — Various methods have been devised for the exact quantitative study of this very irregular movement.^ Evidently graphic representations in one plane can only show a part of the movement, viz., one projection of it. Itmaybe assumed, however, that the movement in all directions is of the same nature. The paths of the Brownian movement of isolated particles have been traced by F. M. Exner ( I.e.). He equipped his microscope with a drawing apparatus and followed the movements of the particles on a smoked glass with a needle. If the time required for a par- ticle to traverse a certain path is noted with a stop watch and the path is then measured one obtains, by division, the average velocity of the particle. Another ingenious method, devised by The Svedberg,^ is based on the following principle. When a fluid dispersoid is allowed to flow at constant velocity and with sufficient speed through the field of a microscope or ultramicroscope one observes a whole series of light curves.^ These are the optical after-images of the individual dispersed particles which themselves move too rapidly to be seen. The curves have a wave-like or zigzag form ' See especially the critical presentation of The Svedberg, KoU.-Zeitschr., 7, i (igio); J. Perrin, I.e.; St. Jahn, Jahrb. f. Radioakt., 16, 23s (1909). ^ The Svedberg, I.e., also Z. f. Elektroch., 12, 853, 909 (1906). ' Dark ground illumination, must, of course, be used. igS SPECIAL COLLOID-CHEMISTRY as shown in Fig. 41 . Their deviation from the horizontal evidently is a measure of the intensity of oscillation occasioned by the spontaneous motion of the particles. The height of the crests or the amplitude { — A) may be measured directly with a microme- ter, or be estimated. When the rate of flow is known, the average absolute velocity of a particle may be calculated' after the constant of the apparatus itself has been determined.^ With the fluid at rest, 2A corresponds to the total deviation of the particle from its initial position. p^ Still more exact measurements may be obtained by photo- graphic and especially by kinematographic means. With these, which M. Seddig,' V. Henri (I.e.), H. Siedentopf* and The Svedberg n±i Fig. 41. — Diagram illustrating the measurement of Brownian movement. (I.e., 1910) have used in different ways, the change in position of the particles with time, in other words, their oscillations, may be accurately measured. Seddig, Henri, Chaudesaigues, etc., deter- mined the change of position of a particle by photographing a preparation at short intervals. Seddig photographed it twice every }{o second, Henri, with kinematographic apparatus, on a moving film several times every ^io second. The interesting ap- paratus of the last-named investigator is shown in Fig. 42. To the left is the kinematographic camera, in the middle, the ultra- microscope, to the right, the source of light, as an arc lamp. As the time is known, the average velocity of the particles may be ^ For details regarding these measurements see especially The Svedberg, Nov. Ac. Soc. Sc. Upsaliensis, I.e., 143. ^ By an analogous method, M. de Broglie has measured the Brownian movement in gas suspensions. ' M. Seddig, Sitz. Marburger Ges., Nov., 1907; Physik. Zeitschr., 9, 465 (1908); Habilitationsschrift, Frankfurt a. M., 1909. * H. Siedentopf, Zeitschr. f. wiss. Mikrosk., 26, 407 (1909). MECHANICAL PRdPERTlKS OF C(JLLL)ID SYSTEMS 199 determined \er\' eloseh" 1>\' nieasurin.ij: (he ])lates and the clianges in the position of the particles. Mdst of tlie patlis of J-trownian movement reproiUiced in JMgs. ,:;7 to 40 were olitained in this manner. H. Siedenttipf uses a falhnL,' photo.L^r.iplde plate. In tliis wise he obtains eur\es as shown in .I'^ig. 43, wliicli correspond to those tirst obser\'ed b\' Svedljerg. S\-edberg (I.e.) has more re( ently used a photographic metliod in which tlie position changes are registered as points on a rotating lilni. For details his original cincniali p^'raphicatK". paper must be consulted. Finall\' it should 1)e mentioned that ]'. Chaude,-^aigues' and later J. I'errin in the e.\ten(k'(| work alread}' referrefl to, also calculated the \ cliK.at \' b\- fnllowing the position changes of the particles at dermiti.' interxals witli (he aid of a drawing apparatus. The ruliirv moticjn whi( h main' partit les show has been ^ludied by J. IVTrin.'-' Tn this vni\ he measured the r()lar\- niii\emeiit of small excrescences, such as air hubbies, eecentricall y attaehed to the larger globules "f a mastic liydrosol in unit time. 1 P. Ctiaudesaif,'uc-s, CompC rrnil., 149, 1044 (ii)O.S). ^ J. I'crrin, CompC rc-ml., 149, 549 (nioy). :oo SPECIAL COLLOID-CHEMISTRY By these methods thai most characteristic propert}- of Rrown- ian mo^-emcllt, nameh- its A'elocit\-, can l)e accurateh- measured and the intluence of different external conditions on it be studieiL 4. Uniformity of Brownian Movement. — A huv of fundamental imjiortance to the tlieory of Brownian nun'cment has been stated by SvecUjcrg.' The ampliliidc of movement is direelly [>roporlioiuiI to llie period of vibration, in other wor V it is seen that the square of the path length is directly proportional to the ratio of the absolute temperature and the viscosity. If the influence of the latter is eliminated by using Svedberg's law {Ai) = k), the result is: AiT) = A.Ar] = kiT;Ar] = const. ;74 = k2.T, in other words, the path length is directly proportional to the absolute temperature when the viscosity is constant. ' The figures used in this table were collected by W. Mecklenburg (Lc, page 77). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 205 9. Influence of Added Substances. — Even the earlier authors knew that small amounts of electrolytes greatly reduce or even stop Brownian movement. As a rule, this cessation is closely connected with a clumping into larger complexes, a process usually ending in a precipitation or coagulation of the si'stem. Hence it has been assumed [see Svedberg {I.e., 1907)] that the addition of electrolytes retards Brownian movement only because it causes an increase in- the size of the vibrating particles. Though this view may be largely correct, recent observations have shown that retardation of Brownian movement may occur even when there is no clumping, and what is more important, it may even be ae- celerated on adding electrolytes. Thus V. Henri (I.e.) found the movement of the rubber globules in latex to be reduced by half on adding N/io NaOH, and to one-ninth the original rate when N/32 HCl was added, even though no clumping could be detected. While the path length normally averaged o.62yu per i-io second, it was reduced to 0.3 iju in the alkaline medium and to but o.oyyu in the acid. The normal path of these globules is shown in Fig. 34. The path in an alkaline mediu'm is shown in Fig. 35 ; that in an acid one in Fig. 36. Lecoq^ observed the addition of electrolytes distinctly to increase the movement of colloid metallic arsenic, but unfortunately he gives no details as to the substances and concentrations used. The retardation might be explained by assuming that the added ions are absorbed by the latex globules causing an enlarge- ment of the particles and a slowing of their movement through the hydrate envelopes added in this way. But this explanation does not harmonize with Lecoq's results, who found the rate to increase on adding electrolytes. Perhaps we need to consider other factors, such as electrical ones. The retarding influence of certain non-electrolytes such as urea (Perrin, I.e.) on Brownian movement can easily be explained through the increase in viscosity of the dispersion medium which they bring about. 10. Influence of the Electrical Charge.- — The investigations of The Svedberg and J. Perrin (discussed on p. 195) proved con- clusively that the degree of movement of vibrating particles was independent of their electrical charges. These are the only 'Lecoq, Compt. Rend., 150, 700 (1910). 206 SPECIAL COLLOID-CHEMISTRY investigations available on this point. Their rej5etition and ex- tension to other dispersed particles is greatly needed. The follow- ing theoretical considerations make this complete independence appear strange. As familiarly known from the study of gaseous ions/ an electrically charged particle induces in its surroundings an electromagnetic field which opposes its movement. One would therefore assume, if any effect of the particles upon each other were excluded, that the spontaneous movement would de- crease as the electric charge increased and that when the charge is zero, in other words, at the iso-electric point, motion would be greatest. As a matter of fact, traces of electrolytes when ad- sorbed by the vibrating particles may retard their movement through changes in their charges in either a positive or a negative sense and it is therefore not impossible that the phenomena ob- served by Henri and Lecoq may be associated with such charging and discharging. Perhaps future investigators, using more exact methods than could Svedberg, will prove the fundamental though not the func- tional independence of Brownian movement of the size of the charge of the particles. 1 1. Influence of Gravity on the Distribution of Oscillatory Particles. — J. Perrin {I.e., 1910), in part with Chaudesaigues and Dabrowski has studied the distribution of the dispersed particles in fine mastic dispersoids when left to the influence of gravity. The problem is not one of simple sedimentation in the ordinary sense of the word, for these systems are so highly disperse (the particles having a diameter of 0.5-0.7/14) that a settling out of the disperse phase can occur only after a very long time, if at all. The investigation dealt rather with the stratification of particles showing Brownian movement. Such stratification is evidently the result of a force (gravity) acting in one direction on the Brownian movement.^ On a priori grounds we would expect that the greater density^ or "excess weight" possessed by some of the particles would add to the previously irregular movement a component directed downwards, thus changing the previously 1 See J. J. Thomson, Conduction of Electricity through Gases, Cambridge, 1903. '^ Analogous stratifications are to be expected under the influence of other uni- directional forces, as electrical or magnetic. ^ In the case of specifically lighter particles we would except similar differences in distribution to result in the formation of a "scum." MECHANICAL PROPERTIES OF COLLOID SYSTEMS 207 uniform distribution of the particles into an uneven one in which a more concentrated layer appears at the bottom. Perrin's investigations were not carried out in tall cylinders as one might be inclined to expect, but in small microscopic columns of liquid not more than loo/i high. The arrangement of the apparatus is shown in Fig. 44. The different levels in the liquid columns were reached and studied by simply raising or lowering the objective. The particles in the different optical sections were counted either photographically or by a special process^ and their number in the different sections compared.^ Figs. 45 Objective Cover Class Emulsion I Slide Fig. 44. — Arrangement for determining the distribution of particles in a mastic dispersoid. (According to /. Perrin.) and 46 are photographs obtained by Perrin showing the distribu- tion of gutta-percha particles when such methods are used. These measurements yielded the important law: The con- centration of the disperse phase increases in geometric progression with the algebraic decrease in the height of the level. Symbolically expressed this would be: ' This consisted of so narrowing the field of vision by a diaphragm that only a few (5-6) particles were visible at a time. They could then be easily counted. The average of a large number of such readings (Perrin made thousands in some cases) yields with sufi&cient exactness, the number of particles in each of the layers. ^ Two other methods which seem to offer advantages and which might be used are the following: Metallic colloids are produced in molten paraffin and poured into heated metallic rings or tubes. After standing some time, the paraffin is soUdified as quickly as possible, as by plunging into liquid air. Sections arc then cut with an ordinary microtome and the particles in each counted ultramicroscopically. A yet simpler method would consist in filling a burette with a suitable dispersoid and keeping this for some time at constant temperature. Different layers could then be carefully drawn off and their content of the disperse phase be determined either gravimetrically or by titration. 208 SPECIAL COLLOID-CHEMISTRY 1 ^o 7 2-303 log — = S-^i wherein »„ is the concentration (number of particles per unit volume) in the initial level 0, Jih the concentration in the level h, g • • • • • . • • • ; • '0 . • • •.. • • . ° . ^ . . ■ > • • • • ■ - '•••"f ..•; •••■:;•:•.•-■ •.. •..••.^•' •• • •: •. • .1 • • .•"••• " • • •.'. .*. V-:;v:Vv /?•..•..•.? . y-.—'i: •• Vx-.v-. '.5 0. 29 0.212 0.12 0-4SSM 0.30 Whether this law holds for systems of still higher degrees of dispersion has not yet been determined, though it is already ap- plied with remarkable results, not only to the theory of molecularly dispersed solutions (see for example W. R. Bousfield^) but to the migration phenomena of gaseous ions.'' 13. Kinetic Theory of Brownian Movement. — As mentioned before, the sources of energy for Brownian movement must be sought in some very general mechanical forces resident within fluid or gaseous dispersoids. In ha,rmony with the old accepted and widespread kinetic views it was to be expected that Brownian movement would sooner or later be regarded as a direct result of the supposed collisions between the molecules of the dispersion ' J. Perrin, Compt. rend., 146, 967 (1908); Kolloidch. Beih., I.e., igio. 2 J. Duclaux, Compt. rend., 147, 131 (1908). ^ W. R. Bousfield, Z. £. phys. Chem., 53, 270 (1905). * See J. J. Thomson, Conduction of Electricity through Gases, Cambridge, 1903. = ^K. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 211 medium. As a matter of fact, the early authors (Chr. Wiener, G. Gouy, etc.), saw in this its only possible explanation. Recently, A. Einstein^ and M. von Smoluchowski,^ in some exceedingly important papers on molecular physics have developed by some- what different methods a theory of Brownian movement resulting in two almost identical formulae. Their fundamental equation governing the kinetics of disperse systems reads : RT t N Tjr In this A is the average path length of the particle, K a constant, R the gas constant, T the absolute temperature, N the number of particles in a gram molecule of the disperse phase (Avogadro's constant), t the period of vibration, rj the viscosity of the dispersion medium and r the radius of the spherical particle. The formula of M. von Smoluchowski differs from that given above only in having the factor ^ =2.37 preceding the root on 27 the right side. The derivation of the formula cannot be detailed here.' It will only be shown how well this equation, deduced theoretically, agrees with the experimental results of The Svedberg and J. Perrin. It should be emphasized that the two laws formulated by Svedberg concerning the uniformity of Brownian movement and its depend- ence on viscosity were discovered before he had any knowledge of the Einstein-Smoluchowski formula. Discussion of the equation leads to the following conclusions. If we assume all the factors in the equation to be constant, except the path length, period of vibration and viscosity, the equation becomes A = kJ^- or A^ = Ki. ^ \ V V The latter form states that not the path length but its square is directly proportional to the period of vibration and inversely proportional to the viscosity of the dispersion medium. Svedberg, however, found (see p. 202) the first power of the path length ' A. Einstein, Ann. d. Physik. (4), 21, 17, 549 (1905); (4), iPi 37i (1906); Z. f. Elektrochem., 13, 41 (1907). * M. von Smoluchowski, Ann. d. Physik. (4), 21, 756 (1906). ' See the original papers as well as the excellent pamphlet of W. Mecklenburg, Die experimentelle Grundlegung der Atomistik, Jena, 1910. 212 SPECIAL COLLOID-CHEMISTRY to be proportional to the period of vibration and inversely pro- portional to the viscosity. As a matter of fact, the above equation can be separated into two of the form : A^ = k.- = ^Av = k If one of the factors, say — > is constant, as demanded by the kinetic t gas theory (which assumes uniformity of average velocity of the gas molecules) and as Svedberg found it to be experimentally, then the other factor At] must also be constant. Conversely, assuming the validity of Stokes' law, A-q becomes a constant, and therefore -J also. The formulas deduced from the kinetic theory therefore t really cover the case in which both Svedberg laws are simultane- ously active. Experience therefore confirms the moleculo-kinetic deductions of the authors named. With this equation it now becomes possible, conversely, to calculate the absolute value of Brownian movement when vis- cosity, size of particles, etc., are known. Svedberg (I.e.) and V. Henri (I.e., 1908) have done this. Their calculated and observed results do not agree absolutely, but they are of the same order of magnitude and the deviations are all of about the same proportion. Undoubtedly the arbitrariness or inexactness of some of the con- stants used may therefore be held responsible. Table 37 shows the more important of these calculations. Table 37. — Calculation of Path Length of Colloid Platinum Par- ticles Exhibiting Brownian Movement, after the Einstein- Smoluchowski Formula (According to The Svedberg) /7JI03 (in sec.) A observed (inn) A calculated (in m) A found Dispersion medium A calculated 0.032 2.3 0.028 4.6 0.026 5.9 0.013 10.2 0.009 2 2.6 31 2 .0 I-S I .1 0.7 0.71 o.,44 0.38 0.20 I 4-4 4-5 .4.0 S-S 6.4 Ethyl acetate Water n Propyl alcohol With due allowance for the large experimental error, the value of the rotational movement of particles of disperse phase also MECHANICAL PROPERTIES OF COLLOID SYSTEMS 213 agrees, as J. Perrin (I.e.) has shown, with that derived from the formula of A. Einstein. The law developed by J. Perrin governing the changes in concentration of a suspension at different levels (as discussed in §ii) has also been deduced from considerations of the kinetics of gases. Only the constants of the formulas are different in the two cases. Thus while the density of the earth's atmosphere does not decrease by half until a height of about 6 kilometers is attained, the concentration of the dispersoids in- vestigated by Perrin often fell off this amount when the difference between levels was only about lo/x. It should also be pointed out that the constant N of the Ein- stein-Smoluchowski equation, in other words, the number of particles in a gram-molecule, which is of such great importance in various fields in physics and physical chemistry, can be calcu- lated in different ways. The values thus obtained agree sur- prisingly well with those calculated from observations of the Brownian movement. Indeed it seems as though these methods as applied to submolecular dispersed systems yield the most exact figures of this fundamental value now obtainable. As this con- stitutes one of the brilliant achievements of colloid or dispersoid chemistry the following table taken from J. Perrin {I.e., 1910) is given in full. Table 38. — Determination of the Number of Particles in a Gram-molecdxe (AvoGADRo's Constant N) by Different Methods (According to J. Perrin) Phenomenon Studied N. io~^^ Viscosity ( Average of volume in liquid state > 45 of ] From tlie dielectric force of gases < 200 gases [ By using Van der Waal's equation 60 From distribution of a uniform suspension 70 . S From the average displacement in a given time 7i-5 From the average rotation in a given time 65 Diffusion of dissolved substances 40-90 Mobility of ions in water 60-150 Radiance of the sky 3°~i5o Direct measurements 1 Of droplets condensed upon ions 60-90 of atomic charge J Of ions attached to dust particles 64 Total charge emitted 62 Emissions of ■ Time constant of radium 7° -5 a corpuscles Helium produced by radium 71 Energy of the Infra-red spectrum 60-80' 'See Perrin (1910) for details regarding other phenomena. Brownian movement 214 SPECIAL COLLOID-CHEMISTRY These brilliant results fill one with admiration for the remark- able fertility of the Einstein-Smoluchowski equation, especially when it is remembered how many still purely hypothetical factors enter into its composition. Nothing better illustrates the daring, we might say, of this train of thought than the remark of Perrin, to whom, with Svedberg, science owes most in this field, anent the theorem of the equality of the distribution of energy which is the nucleus of all kinetic deductions. "The word theorem should deceive no one, for it is full of hypotheses as is almost every theory of mathematical physics." It is safest, perhaps, to hold that the future will preserve but a part of our present kinetic notions and embody it into a more general, less conjectural theory. As a matter of fact, several of the laws governing Brownian movement may be deduced even without recourse to kinetic assumptions, as for example, the inverse proportionality of velocity to viscosity, from Stokes' law.^ Possibly this purely inductive method will some day discover these same laws; in fact, consideration of the methods of science demands it, but when the day will come must remain a matter of opinion. 14. Determination of the "Molecular Weight" of Dispersed Particles from their Brownian Movement. — Since N can be cal- culated, the so-called "molecular weight" of dispersed particles may also be determined from the formula of Einstein and Smo- luchowski. This can also be done from the logarithmic distribu- tion equation governing concentration in different levels. J. Perrin^ made such calculations and by this method found his gutta-percha particles to have a molecular weight of about 30,000,- 000,000. It must again be emphasized that these values cannot be compared with the molecular weights of molecularly dispersed particles. In the former, the diameter of the particles (or their volume) is under discussion, and this "molecular weight" becomes progressively less as the size of the particles decreases. The normal concept of molecular weight does not consider the size of the particles as at all variable, but deals simply with that single 1 The influence o£ electrical energy upon Brownian movement as postulated on p. 205 cannot be deduced from kinetic considerations, but is an inductive conclu- sion. It appears in the Einstein-Smoluchowski equation as a factor analogous to the viscosity factor since the velocity would be approximately inversely propor- tional to the intensity of the induced field of force. Judging from the experi- . mental results of Svedberg and Perrin these proportionality constants would, in many cases, have a very small value. 'J. Perrin, Compt. rend., 147, 475 (rgoS). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 215 value which is observed at the maximum degree of dispersion. This obviously constitutes a fundamental distinction between the "molecular weights" of differently dispersed systems. §28. Diffusibility of CoUoids I. General Remarks. — When a quantity of the pure dispersion medium is poured on a molecular dispersion, with due precautions so as to avoid mixing, the dispersed phase wanders over into the dis- persion medium until uniform distribution throughout both phases is attained. This phenomenon is known as diffusion. In trying to explain what has happened it is natural to think of the influence of Brownian movement. In the irregular, particularly in the forward, movements of small particles, as observed, for example, by Zsig- mondy in colloid solutions, it is to be expected that an accidental wandering of the particles over into the pure dispersion medium must take place. But such accidental migration cannot wholly explain all diffusion, the laws of which A. Fick formulated in 1855. In order that Brownian movement may lead to diffusion, it must become directive in character toward the pure dispersion medium or toward the "more dilute" parts of any continuous system. As a matter of fact, the existence of such a directive movement in diffusion until uniform distribution of the dispersed phase throughout the whole system is attained can be foreseen, when the relation between degree of movement and concentration of dispersed particles is called to mind. As noted above (p. 202), R. Zsig- mondy observed less movement in dilute systems than in concen- trated ones. Because of this, equilibrium cannot exist, so far as average velocity of particles is concerned, in a system consisting, say, of a colloid solution covered by a layer of the pure dispersion medium. In places of greater concentration, the particles will be moving faster than in those of a lower one. The sources of energy for Brownian movement, whatever they be, must there- fore have different values in different parts of the system at the beginning of diffusion. But following the general laws of energy, equilibrium cannot be attained in a closed system until the energy intensities have the same value e\'erywhere.^ We need but call to mind the electrostatic charge on the surface of a metallic sphere. If the energy intensities in a "diffusion field" 1 Wilh. Ostwald, Lehrb. d. allgem. Chem., 2 Aufl., 2, 35 (1903) 2l6 SPECIAL COLLOID-CHEMISTRY are not everywhere the same the system is unstable and changes must occur of a directive character leading, in the end, to an equali- zation of the intensities in the whole system. Thus, if only a local charge were present on the metallic sphere, currents would ema- nate from this to all other points on its surface. In the case of diffusion, a movement of the dispersed particles toward regions of lower concentration would have to occur until the average velocity of the particles was the same in all portions of the system. The average absolute value of the movement would therefore become progressively less until the minimum is attained at the end of diffusion. It is in keeping with this general notion that in the molecular dispersoids the diffusion coefficients (distances traversed in centimeters per day) are greater at higher concentrations than at lower ones.^ The influence of concentration on the diffusion of a colloid has not yet been studied. The experimental diffi- culties besetting such a study will be- come clear in the following paragraphs. Diffusion experiments have a special interest in colloid chemistry because were built upon them by Th. Graham Fig. 47. — Apparatus for the study of difEusion as ar- ranged by Thomas Graham. its very , foundations (1850-1862). 2. Experimental Study of Diffusion of Colloids. — The common method of determining the diffusion coefficient was originated by Th. Graham.^ A wide-necked bottle is filled with the solution to be investigated and placed in a second vessel; the pure dispersion medium is then poured with special care^ into the second vessel until it covers the inner bottle to a depth of several centimeters. Fig. 47 is an exact copy of the sketch from Graham's original work. After a given time, the amount of dissolved substance which has escaped from the inner vessel is determined. The rela- tion of this to the time (at constant diffusion surface, tempera- ' The complicated diffusion phenomena observed in certain ionic dispersoids, as in hydrochloric acid, form exceptions to the general rule because electrochemical processes come into play. See Wilh. Ostwald, Lehrb. d. allg. Chem., z Aufl., i, 686 (1903)- ^Th. Graham, Philos. Trans., 1-46, 805-836 (1850); 483-494 (1851), etc.; Liebig's Ann., 77, 56, 129 (1851); 121, s, 29 (1862). ' To prevent mixing of the two liquids at the critical moment Graham used a pointed sponge from which to express the second liquid. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 217 ture, etc.) is a measure of the velocity of diffusion. For a discus- sion of the more modern methods of using Graham's principle, as well as for the methods of calculating the absolute diffusion coefficients from the experimental data, the text-books of physics and physical chemistry should be consulted. ^ It is difficult in Graham's method to bring the two liquids into contact with each other without disturbing their surfaces. Slight differences in temperature, vibrations, etc., may, moreover, in- troduce great experimental errors. But Graham already knew a remedy for this. He found that the velocity of diffusion was not much influenced if the experiment was carried out in a not too highly concentrated agar-agar, gelatin or starch paste, instead of in pure water. Thus, when he placed in the diffusion cell a 2 per- cent agar solution containing 10 percent salt, and a pure agar solution of the same concentration in the outer vessel and allowed both to solidify, he found after 15 to 16 days that the latter con- tained 9.992 grams of diffused salt. Normal diffusion into pure water, after 14 days showed 9.999 grams, all other conditions, in eluding temperature (10°), being constant. These findings have often been verified. Thus F. Voightlander^ observed 0.72 per- cent sulphuric acid to diffuse the following distances into agar jellies of different concentrations after i hour. Agar jelly, i percent = 8.5 mm. 2 percent = 7.8 4 percent =7.0 The amounts that diffused were as follows: Into agar jelly, i percent = i .08 mg. SO 3 2 percent = i . 10 4 percent = i . 09 The absolute values for NaCl of the diffusion coefficients, amounts diffused in grams . „ i ) are as follows: days Agar jelly, i percent = i . 04 2 percent = i . 03 3 percent = i . 03 1 See, for example, Wilh. Ostwald, Grundr. d. allg. Chem., 4 AuQ., 194, Leipzig, 1909; Wilh. Ostwald-Luther-Drucker, Hand- und Hilfsbuch, 3 Aufl. 2 F. Voightlander, Z. f. physik. Chem., 3, 329 (i8" ' 2l8 SPECIAL COLLOID-CHEMISTRY G. Hiifner^ and others obtained similar results. But it should again be emphasized that diffusion is thus independent of the presence of gels only when these are there in low concentrations. Marked retardations appear at higher concentrations as even H. de Vries^ knew. Diffusion is also influenced, of course, when chem- ical or colloid-chemical changes, as precipitations, are produced in the gels by the diffusing substances. A convenient arrangement for demonstrating diffusion has been described in the practical introduction on p. 9. Test tubes are half filled with colloid gels and the diffusing solution poured upon them. Figs. 2, 48 and 49 illustrate the results. Disturbance of the diffusion surfaces may also be avoided by stretching over the inner vessel a suitable membrane through which the dissolved substances pass freely. Hydrophane plates (G. Hiifner, l.c), filter papers (S. Exner, see below), parchment papers (The Svedberg, see below), etc., have been used for this purpose. Or, the diffusing substance may be placed directly in cells entirely made of such substances. But the membranes used must be completely permeable to the diffusing substance and must not affect it, as through adsorption, etc. §29 on dialysis should be studied in this connection. 3. Experimental Facts Regarding Diffusion of Colloids. — It follows from the relation between velocity of Brownian movement and size of particles discussed above that the velocities of colloid particles must be considerably less than those of molecularly or ionically dispersed ones. The compilation in Table 39 shows this clearly; additional facts regarding diffusion velocities are given below. As is clearly evident, the diffusion coefficients of typical col- loids average 3'lo that of the slowly diffusing cane sugar and only J-ioo that of the rapidly diffusing electrolytes such as acids and alkalies. The highly dispersed goldsol of The Svedberg which, for a colloid, diffuses exceptionally fast, takes an intermediate posi- tion. It should be remembered that the particles of the latter have a diameter of about i^m; in other words, this goldsol is on the boundary between molecular and colloid dispersoids. Figs. 48 and 49 illustrate quantitatively the diffusion velocities ^ G. Hufner, Z. f. physik. Chem., 27, 227 (18 ^ H. de Vries, Fittica's Jahresber. d. Chem., i, 144 (ig MECHANICAL PROPERTIES OF COLLOID SYSTEMS Table 39. — Diffusion Coefficients of Dispersoids 219 Molecular and ionic dispersoids. Specific area > 6 X lO' Nitric acid (Voightlander)... . 2.10 (20°) Sodium chloride (Voight- lander) 1 .04 (20°) Magnesium chloride (Voight- lander) 0.77 (20°) Copper sulphate (Landolt- Bornstein) o 47 (17°) Urea (Scheffer') 0.81 (7.5°) Cane sugar (Graham- Stefan^) 0.31 (9°) Mannite (Scheflfer) 0.38 (10°) Gold hydrosol (The Sved- berg') o. 27(11 . 7°N Colloids. Specific area about 6 X 10' to 6 X lo^ Clupeinsulphate (Plerzog) 0.074 (18°) Pepsin (Herzog') 0.070 (18°) Rennin (Herzog) u.o66 (18°) Egg-albumin (Herzog).. . 0.059 (18°) Albumin (Graham-Stefan) 0.063 (13°) Caramel (Graham-SteIan)o.o47 (10°) Ovomucoid (Herzog). . . . 0.044 (18°) Emulsin (Herzog) 0.036 (18°) Invertin (Herzog) 0-033 (18°) Diphtheria-toxin (Arrhe- nius and Madsen') u. 014 (12°) Diphtheria-antitoxin (.Vr- rhenius and (Madsen): 0.0015 (12°) Tetanolysin (Arrhenius Madsen) 0.037(12°) Antitetanolysin (Arrhe- nius and Madsen) 0.0021(12°) of various dispersoids. They show what has happened after about 3 day's diffusion into solid 1.5 percent agar at 20°. In Fig. 49 the supernatant liquids out of which diffusion has occurred have been poured off so that the diffusion phenomena may show up more clearly. To the left in this figure are found molecular dispersoids, to the right, typical colloids. The tubes are arranged, from left to right, according to the lengths of the diffusion paths.® The picric acid, cobalt nitrate and eosin of tubes 1,2, and 3 have wandered almost to the bottom of the agar column; benzo-pur- purin and congo red on the extreme right have scarcely moved. The dyes lying between these, show intermediate degrees of diffusi- bility.'' Fig. 48 shows the results, after 8 days, of experiments on the diffusion of typical colloids (hydrosols of silver, gold, anti- 1 G. Scheffer, Z. f. physik. Chem., 2, 390 (1888). 2 Graham-Stefan, Sitz. Ber. Ak. Wien, 77, II, 161 (1879). ' R. O. Herzog (and H, Kasarnowski) Koll.-Zeitschr., 2, i (1907); 3, 83 (1908); Bioch. Zeitschr., 11, 172 (1908). ■• S. Arrhenius and Th. Madsen, Immunochemie, 16, Leipzig, 1907. 5 The Svedberg Z. f. physik. Chem., 67, 107 (1909). « The gradation is not as clearly shown in the photograph as it actually appears since the different (mostly o.i percent) solutions have different colors. ,\ photo- graphic plate does not bring this out. ' Regarding the diffusibility of dyes seeL. Vignon, Compt. rend., 150, 690 (1910). 220 SPECIAL COLLOID-CHEMISTRY mon_\- sulphide, arsenic sulphicie and iron h)-(lroxide). The sharp- ness of llic boundary line between the diffusing substance and the gel should be noted. It is sharp in the case of typical colloids; but badly marked in that of the molecular dispersoids and systems of intermediate degrees of dispersion. 4. Influence of Degree of Dispersion on Diffusion Velocity. — The diifusibility of a disperse phase is intimately connected with its degree of dispersion as shown in Table 39. Among molecularly Fig. 48. — Diffusion of colloids into 2 percent agar-agar at the end of a week. I. Gold hydrosol; 2, silver solution (Crede); 3. antimony sulphide solution; 4. arsenic trisulphide solution: 5, iron hydro.xide solution. dispersed substances, ions or electrolytes migrate most rapidly. Substances of higher molecular weight, or, more correctly, of greater atomic aggregation, follow. Last in the list, stand the colloids. This dependence of diffusion velocity on the size of the particles is of great interest. Of special importance is the possi- bility of procuring one and the same suhstance in different degrees of dispersion and therefore possessed of different degrees of diffu- MECHANICAL PROPERTIES OF COLLOID SYSTEMS 221 sibilit)'. S. E. Linder ami H. Ticlon' were able to prepare the following four systems of arsenic trisulphide in water: aAsjSa; particles microscoj^ically \'isible, non-dilTusible (coarse suspensions), /SAs-jSa; microscopicall)- homogeneous, non-diffusible, 7AS2S3; diffusible, Init unfilterable through porcelain cups, 5AS2S3; diffusible and tilterable. Alter \\\>. Ostwald- had repeatedly eni])hasized the great theo- retical interest attaching itself to a systematic and Cjuantitatix'e Fig. 49. — Diffusion into a 2 jiLTLunt a^jar-a^'ar at the end 'A three da>'s. I. Pienc acid; 2, cobalt nitrate; 3, o.r percent cosin; 4, o.l percent jionceau R. R. R.; 5. 0.1 percent new fuchsin O; 6, o.i percent vestivin; 7, o.i percent safranin G.; 8, 0.1 percent benzopurpnrin; 9, 0.1 percent conge, red. investigation of the relations between diffusibility (and other properties) and degree of dispersion. The Svedbcrg,' in the prose- cution of his experimental study of the liinstein-Smoluchowski formula (see above), attacked the problem. He determined the diffusion vehjcities of different gold hyilrosols l)_\- pouring these into parchment cells having different porosities. His results arc given in Table 40: ' S. E. Linder and IL Pii ton, Trans. C'hem. Soc., Lund., 6i, 1 14, 137, 14S (1S92); 67. 63 (1895); 71, S(j.S (i«97); 87, iQO(j (1905). 2 See, for example. Wo. Ostwald, Koll.-Zcitselir., i, 298 (1907). 2 The Svedberg, Z. f. pliysik. Chem., 67, 105 (1909). 222 SPECIAL COLLOID-CHEMISTRY Table 40. — Dependence op Diffusion Velocity on Size of Particles in goldsols (According to The Svedberg) Size of particles in mm Concentration of in- ner liquid in normality Concentration of outer liquid in normality Relation of con- centration to each other 14 20-30 1.5 10-^ I . 5 10-^ 1.5 io-» 1.3 IO-' 100 1200 The gold content was determined colorimetrically. The reciprocal values of the concentration ratios are measures of the diffusion velocity. In other words, Di = ^1 -Hoo, when D is the diffusion coefficient and ki the proportionality constant. Simi- larly, 2)2 = ^2^200- When the ratio of these diffusion coefficients is compared with the size of the particles (taking the latter to 2.5 ^^ 100 average respectively 2.5 and 2 5;u) we observe, since 25 1200 that the diffusion velocity is approximately inversely proportional to the size of the particles, or D.r = constant. True it is, that we are basing these conclusions on studies in- volving but two degrees of dispersion. An investigation cover- ing a wider range would be of great interest. Finally, it should be mentioned that S. Exner^ found coarse suspensions, such as clay silt, to show a distinct though slow diffu- sion. But whether pure dispersions made up of particles larger than 5/i and therefore free from Brownian movements are really capable of true diffusion appears doubtful (see below, p. 224). 5, Theory of Colloid Diffusion. — The close relation between Brownian movement and diffusion was mentioned at the beginning of this paragraph. It seems natural, therefore, that the moleculo- kinetic considerations of A. Einstein and M. von Smoluchowski,^ which proved so fruitful in the mathematical discussion of Brown- ian movement, should lead to similarly important results when ap- plied to diffusion. For example, the inverse proportion between size of particles and diffusion velocity is deducible from the equa- tions of Einstein and von Smoluchowski. For the diffusion co- efficient they developed the equation: RT I N 6ivqr 1 S. Exner, Sitz. Ak. Wiss., Wien, 56, 116 (1867). ^ According to von Smoluchowski the right side of the equation contains the factor 2.03. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 223 The symbols have again the meaning indicated on p. 211, r repre- senting the radius of the particles. If dispersion medium, tempera- ture, internal friction, etc., are constant, the diffusion coefficients of two dispersed phases bear the following relation to each other : Z)i ^ r2 in other words, they correspond to Svedberg's experimental find- ings. This relation has much in common with the equation which expresses the connection between the diffusion of molecular dis- persoids and their molecular weight. The relation: D.\/m = constant has been established by S. Exner, for gases, and by L. L. Oholm^ for (theoretically) infinitely dilute solutions of non-electrolytes. In this equation m is the molecular weight. Conversely, with the laws of Exner-Oholm and Einstein- Smoluchowski, we may calculate the size of the particles as well as their molecular weight. In this way R. O. Herzog (I.e.) found an approximate agreement between the "molecular weights" of ov- albumin, hemoglobin, etc., thus calculated; and the iigures ob- tained by other methods. For the values for toxins, etc., as calculated by Svante Arrhenius and Th. Madsen (I.e.), control measurements are not yet available. The same objections may be raised against all these calculations which were raised in discussing the determination of the molecular weight of colloid systems by freezing point, boiling point and vapor pressure methods. The calculations by R. O. Herzog and The Svedberg (I.e.) of the size of the particles by the formula of Einstein-Smoluchowski are less open to objection. Herzog, on this basis, calculated the size of the particles of ovalbumin to be 2.86fifi. This figure about corresponds to the higher dispersion values obtaining within col- loid systems, and therefore agrees well with the general fact that the colloid properties of the albumins place them near the mo- lecularly dispersed systems. Svedberg calculated, in a rexersc manner, the size of the particles of the highly dispersed gold solu- 'L. L. Oholm, Z. f. physik. Chem., 70, 378 (1910); this also includes the earlier literature. 224 SPECIAL COLLOID-CHEMISTRY tion of R. Zsigmondy, the particles of which, according to Zsig- mondy, had a diameter of i to 4mm- He obtained, by Einstein's formula, 0.94^^, and by Smoluchowski's 2.i6mm- The calculated diameter of the particles of molecularly dispersed systems also agrees well with the values obtained by other methods. 6. Effect of Added Substances on Colloid Diffusion. Spurious Diffusion of Colloids. — The effects upon diffusion of adding differ- ent substances are so complicated, even in molecular dispersoids, that general laws governing them have not been formulated.^ It is to be expected that these relations will be still more complicated when phases having different degrees of dispersion are mixed. The more important phenomena observed when colloid systems are mixed with molecularly dispersed ones are the following: The effect of electrolytes on the diffusion velocity of colloids may be discussed under two headings — the electrolyte may he added to the disusing substance, or the diffusion of the colloid may be permitted to occur into the solution of an electrolyte. In either case, different results may be expected, depending on whether the electrolyte does not affect the degree of dispersion of the colloid (which is exceptional) or whether it increases or decreases it. Both an increase and a decrease in the degree of dispersion on add- ing substances from without have been described in the literature. An illustration of the latter is found in the common and well- known effects of electrolytes on colloids (aggregation, coagulation) ; an illustration of the former, in the phenomena of peptization. The inhibiting ejfect of added substances on diffusion has been studied by E. von Regeczy.^ He found pure albumin when placed in parchment-paper tubes to diffuse out of these in the course of 12 hours in sufficient amount to impart a decided albumin re- action to the outer liquid. But when some solid NaCl was pre- viously added to the albumin, no trace came out. S. E. Linder and H. Picton {I.e., 1905) noted a similar behavior in an inorganic colloid, arsenic trisulphide. They allowed a highly dispersed arsenic trisulphidesol to diffuse, on the one hand, into water, on the other, into an NH4CI solution, so dilute that it caused no visible coagulation. Their results are given in the following table: 1 See, for example, Wilh. Ostwald, Lehrb. d. allg. Chem., 2 Aufl., 674, Leipzig, 1903. 2 E. von Reg^czy, Pfliiger's Arch., 34, 431 (1884). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 225 Table 41. — Diffusion of As^Sj Sol into Pure Water and into NH4CI Solution (According to S. E. Linder and H. Picton) Time Diffused amounts in percent of the inner fluid Into pure water Into NHiCl 24 hours 48 96 10 percent 23 I percent 3 An antimony sulphidesol gave similar results when permitted to diffuse into water and into a solution of tartar emetic. An example of how the addition of an electrolyte may favor diffusion of a colloid is found in Th. Graham's paper (I.e.). He observed egg albumin, which in its natural state is slightly alkaline and diffuses but slowly, to diffuse more rapidly if it is carefully neutraKzed with acetic acid. While after a week but 0.63 gm. of native (alkaline) albumin dift'used out, 0.94 gm., in other words 30 percent more, came out when the albumin was neutralized. The neutralization increases the degree of dispersion, as proved by the observations of Wo. Pauli and others (see p. 173), who found neutral albumin to increase the viscosity of water less than acidified albumin. H. Picton (I.e., 1892) made similar observations on suspen- soids of antimony trisulphide. He found this to diffuse rapidly when still contaminated with the tartar emetic from which it was prepared. Whether the electrolyte serves to increase the degree of dispersion in this case remains a matter of question, though such an influence on suspensoids has been observed. It is more reasonable to assume that the electrolytes in their rapid diffusion simply drag the colloid particles along with them, a view held by H. Picton himself; or that the movements of the liquid, caused by the diffusion of the electrolytes, set up currents which bring about the observed results. The interesting experiments of W. R. Whitney and J. Blake^ on the great veloeity of diffusion of goldsols, produced by reducing ether solutions of gold chloride by means of acetylene, must, no ' W. R. Whitney and J. Blake, Journ. Amer. Chem. See, 26, 1339 (1904). IS 226 SPECIAL COLLOID-CHEMISTRY doubt, be similarly explained. When they concentrated their colloid gold at the lower end of a vertically placed cylinder by electrophoresis and then carefully poured pure water upon it, they observed an unusually rapid and spontaneous upward movement of the gold which increased with the increase in the concentration of the gold. The observed velocities varied between o.oi cm. and 0.24 cm. per hour. When it is recalled that F. Voightlander (p. 217) found the rapidly diffusing sulphuric acid to cover only 0.85 cm. per hour in i percent agar while the finest goldsols of The Svedberg have a diffusion coefficient of only 0.27 (as compared with one of 2.0 for sulphuric acid on the same scale), it becomes impossible to believe that the experiments of Whitney and Blake deal with true diffusion of a colloid phase. The diffusion movements of the molecular dispersoids present in their preparations may have led to the high (apparent) diffusion of the colloid particles, as in the experiments of H. Picton. More probably still, the gold particles became coated with gas through the electrical treatment to which the gold was subjected and this then led to their rapid rise. Suitable experiments could easily ■ be arranged to test the validity of such an explanation. The favorable effect of electrolytes upon the diffusion of col- loids has again been observed when they are permitted to diffuse into solutions of electrolytes. Thus von Wittich^ found, as far back as 1856, that albumin diffuses more easily into a salt solution than into pure water. Within certain limits, the diffusion is the more rapid the greater the concentration of the salt. E. von Regeczy {I.e.), M. Oker-Blom^ and others have since studied this phenom- enon. The paper of M. Oker-Blom is the source of Table 42. It is readily apparent that the amounts of diffused albumin in- crease with increase in the concentrations of NaCl, but in the in- termediate concentrations, from 0.56 to 1.30 percent, a region of minimum diffusion is observed. What follows will show that this need by no means be due to experimental error. To explain these phenomena,^ we need but remember that albumin solutions are more strongly hydrated, in other words, swell more in many salt solutions than in pure water. We may assume that in this process the free, dispersed albumin particles 1 von Wittich, J. MuUer's Arch. f. Physiol, 286 (1856). ^ M. Oker-Blom, Skandinav, Arch. f. Physiol., 20, 102 (1904). 8 Wo. Pauli, KoU.-Zeitschr., 3, 11 (1908). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 227 wander into the strongly hydrating dispersion medium just as the Uquid wanders into the solid colloid to make it "swell." A suffi- ciently marked hydration of the dispersed particles must separate them from one another. Table 42. — Diffusion of Serum: Albumin into NaCl Solutions (According to M. Oker-Blom) Concentration of NaCl in the outer liquid Amount of albumin, in grams, after 24 hours diffused ° 0-053 about 0.28 percent "•053 0.56 0.052 0.74 0.052 0-93 . 050 1-3° 0.052 1.48 0.058 1.86 0.060 2.38 0.079 At the present time, we can only guess at what may be the influence of several colloids upon each other when they are mixed, and how they must affect each other's diffusion velocity. The influence of concentration and of temperature on the diffu- sion of colloids has not yet been studied. Judging from the find- ings of Th. Graham {I.e.), the rate of increase in diffusion velocity of egg albumin with the temperature is about as great as that of molecularly dispersed systems under the same circumstances, but exact figures on the subject are still wanting. §29. Dialysis of Colloid Systems I. General Remarks. — The impeding effect of concentrated gels or membranes upon free diffusion was touched upon above. While ordinary electrolytes pass through parchment-paper mem- branes almost as rapidly as though they were not there, albumin and gum arabic cannot penetrate them. Th. Graham, who first investigated this phenomenon, called it dialysis (i86r). He noted that all substances which, when allowed to diffuse in the open, do so only slowly or not at all are also restrained by parchment mem- 2 28 SPECIAL COLLOID-CHEMISTRY branes. On the other hand, those which diffuse rapidly are not markedly checked in their movement through the presence of membranes. This difference in behavior of "dissolved" sub- stances toward parchment paper formed the basis of the whole concept of the colloids. Substances which do not dialyze (or pass through parchment paper) Graham called colloids, those which do, crystalloids. The latter systems are today known as "molecular dispersoids." One can readily accomplish a separation of the different classes of dispersed systems by dialysis. As a matter of fact, Graham called his fundamental work "Liquid Diffusion Applied to Analy- sis." By using a constant type of membrane, systems of unknown degrees of dispersion may be classified into such as dialyze and such as do not (see the practical introduction). When, by any method whatsoever, coarsely dispersed systems have been ex- cluded, dialysis offers a convenient method of distinguishing be- tween the colloid and molecularly dispersed systems. It must be emphasized that comparable results may be ob- tained only by use of one and the same kind of membrane. The precipitation membranes of copper ferrocyanide and tannic acid- protein, for example, are impermeable even to many molecular dispersoids and may, therefore, give rise to the phenomena of os- motic pressure (see the following paragraphs). 2. Methods of Dialysis. — Parchment tubes, parchment diffu- sion thimbles, reed tubes, fish bladders, urinary bladders, egg membranes and amniotic membranes are most used in the dialysis of colloids.^ Membranes of collodion, as first used in colloid studies by G. Malfitano,^ are especially convenient in many respects. Their preparation is discussed in the practical introduc- ion (p. lo). Several forms of dialyzers were illustrated on page 11.^ Because of their historical interest. Figs. 50, 51 and 52 are introduced, which are copies of the two types of apparatus which Graham used in the great work upon which colloid chemistry is built. In dialyzing non-aqueous liquids, the effect of the dispersion 1 A detailed discussion of dialysis and its methods may be found in R. P. von Calcar, Dialyse, Eiweisscliemie und Immunitat, Leipzig-Leiden, 1908. ^ G. Malfitano, Compt. rend., 139, 1221 (1904). ' For a new form see R. Zsigmondy and R. Heyer, Z. f. anorg. Chem., 68, 916 (1910). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 229 medium upon the membrane must be considered. A possible chemical effect of the substances subjected to dialysis must also be kept in mind, though such is rarely met with among the colloids. Fig. so. — Thomas Graham's disc dialyzer. Fig. 51.- -Thomas Graham's bell dialyzer. 3. Experimental Facts Regarding Dialysis of Colloids. — Since the days of Graham, almost every student of the general proper- ties of colloid systems has made use of dialysis. It is, therefore, not possible to review all the work that has been done in this field. Generally speaking, dialysis teaches the same facts as diffusion. Fig. 52. — A second method of using Grahatn's bell dialyzer. Thus, S. E. Linder and H. Picton (I.e.) were able to distinguish between dialyzing and non-dialyzing metaUic sulphides. Of the many groups of compounds studied, only one will be discussed here, that of the technically and theoretically important water soluble dyes. F. Krafft and G. Preuner,' 0. Teague and B. H. Buxton,^ 1 F. Kra£ft and G. Preuner, Ber. d. Dtsch. chem. Ges., 32, 1620 (1899). 2 O. Teague and B. H. Buxton, Z. f. physik. Cham., 60, 469 (1907). 230 SPECIAL COLLOID-CHEMISTRY H. Freundlich and W. Neumann/ R. Hober,^ L. Vignon/ W. Biltz and F. Pfenning,* have all studied these. In Table 43 are reproduced some of the findings tabulated by Biltz. In connec- tion with this table it should be noted that Krafft and Preuner used parchment tubes; Teague, Buxton, Hober and Vignon, parch- ment-paper thimbles, manufactured by Schleicher and Schiill; Biltz and Pfenning, collodion membranes. The solutions used were usually o.i percent; Teague and Buxton used 0.02 percent; Biltz and Pfenning 0.5 percent. The abbreviations in parenthe- ses after the names of the dyes mark their origin. Table 43. — Dialysis of Dyes Typical Molecular Dispersoids Name Atomic number Molecular weight Dialyzes Observer Picric acid Toluidin blue (Hoechst) Naphthol yellow g (Bayer. Hoechst) Chrysoidin Methylene blue Eosin Erythrosin Bengal rose Quinolin yellow (Akt.) True acid fuchsin B (Bayer), Auramin O (Akt.) Saf ranin Wool violet S (Bad.) Brilliant crocein 3 B Acid fuchsin S (Akt.) ....... Methyl violet Patent blue V (Hoechst) Guinea green B Erioglaucin 19 19 27 37 37 37 37 40 41 43 44 46 SI 52 56 to 66 84 86 95 229.0 143 -S 35S-0 2r4.o 317-5 692.0 1050 477 467 303 350 445 SS6 572 393 to 469 804 730 782 Quickly . Quickly. Quickly. Quickly. Quickly. Quickly. Quickly. Quickly Quickly Quickly Quickly Quickly. Quickly, Quickly, Quickly, Quickly, Quickly. Quickly. Quickly. Vignon. Biltz. Hober, Vignon, Biltz. Teague and Buxton. Krafft and Preuner, Teague and Bux- ton, Biltz. Teague and Bu.xton, Vignon, Biltz. Hober, Biltz. Hober. Biltz. Biltz. Biltz. Teague and Buxton, Vignon, Biltz. Hober. Hober. Vignon. Biltz. Hober, Biltz. Hober. Hober. ' H. Freundlich and W. Neumann, Koll.-Zeitschr., 3, 80 (1908). ^ R. Hober, Koll.-Zeitschr., 3, 76 (1908); Bioch. Zeitschr., 20, 80 (1909). ^L. Vignon, Compt. rend., 150, 619 (1910). * W. Biltz (with F. Pfenning), van Bemmelen-Gedenkboek, 108, 1910. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 23 1 Transition Systems between Molecular Dispersoids and Colloid Solutions Name ' Atomic number Molecular weight Dialyzes Observer 37 41 45 51 58 58 59 74 86 91 288. s 400.0 480.0 556.0 622.0 443-4 407-5 565. 5 960.0 733-° Slowly Slowly Rather quickly Rather (Quickly Only in traces Slowly Rather quickly Very slowly Slowly Some- what Teague and Buxton. Hober, Biltz. True red A (Akt. Bayer) .... Ponceau 2 R (Akt.) Ponceau B extra (Akt.) . . . Victoria black B (Bayer) .... Nile blue Biltz. BUtz. Teague and Buxton. Freundlich and Neu- Crystal violet. . . . Aniline hlne. . . . mann, Vignon. ' Teague and Buxton. Hober. Hober. Hober, Biltz. Benzo-blue 3 B (Bayer) Acid violet 6 B (.Vkt.) Typical Colloid Solutions Name Atomic num,ber Cloth red 6 A (Akt.) Congo brown 9 (Akt.) Congo red (Akt.) Azo-blue (Akt.) Benzopurpurin Congo blue B X 2B (Akt.).. Night-blue Heliotrope B B (Bayer) Chicago blue 6 B R W (.\kt.) 53 68 70 74 76 80 Molecular weight 4»2 .0 682.0 606.0 726.0 724.0 860.0 575-5 010. o 992 .0 Dialyzes Obserrer Not at all Biltz. Not at all Hober, Biltz, Teague and Buxton. Not at all Vignon, Biltz. Not at all Krafft and Preuner, I Teague and Buxton, Hober, Biltz. Not at all Krafft and Preuner, ' Hober, Biltz. Not at all I Biltz. Not at alli Teague and Buxton, Freundlich and Neumann, Biltz. Not at all Hober. Not at all Biltz. The table shows that, in general, dialyzability decreases with rising atomic number and increasing molecular weight. That the rule is only approximately true can be seen by comparing the tables horizontally. (The vertical rows are arranged according to in- 232 SPECIAL COLLOID-CHEMISTRY creasing atomic numbers.) In each of the three classes, some of the dyes have a low, while some have a high, atomic number. Even substances with high molecular weights, as Bengal Rose may be found in the rapidly dialyzing class. The degree of dis- persion of the dye is therefore dependent not alone on the atomic number or the molecular weight, but on other factors as well. It seems natural to try to explain the lack of parallelism through the chemical constitution of the dyes, as W. Biltz and others have done with a fair degree of success. A review of Blitz's results is beyond the limits of this book, but it should be noted that even so, a quantitative relation between chemical constitution and degree of dispersion does not appear even when only simple compounds in homologous series are considered. The absence of parallelism between size of particles and molecular weight demonstrates also the danger of trying to determine molecular weight from diffusion constants as discussed on p. 223. When the dialysis of non-aqueous colloids is discussed it must first be remembered that many dyes "dissolve" to form colloid solutions in water, but molecularly dispersed ones in other sol- vents, such as alcohol (F. Krafft, I.e., and others). Corresponding herewith, the alcoholic solutions dialyze better than the aqueous ones. Especially interesting results have been obtained with iodine dissolved in different organic solvents. J. Amann^ has shown that iodine dissolves in benzene as a molecular dispersoid, in petroleum as a colloid. Corresponding to this fact, it dialyzes through a parchment thimble out of its solution in benzene but not out of that in petroleum.^ 4. Special Observations Regarding the Dialysis of Colloids. — Colloids frequently pass through a dialyzing membrane for a short time immediately following their preparation. This is especially true of freshly prepared silicic acid as observed by Th. Graham and more recently confirmed by F. Mylius and E. Groschuff.^ The explanation of this interesting fact is to be found in the instability of the degree of dispersion in colloid systems. When a colloid solution is prepared by condensation of a molecularly dispersed system, the desired product is not obtained at once, but only after hours or days. Sometimes, moreover, the condensation occurs ' J. Amann, Koll.-Zeitschr., 7, 235 (1910); 7, 67 (1910). ^ According to the unpublished results of Prof. S. Suzuki and the author. ' F. Mylius and E. Groschuff, Ber. d. Dtsch. chem. Ges., 39, 119 (1906). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 233 unequally, in other words, a few colloid particles are first produced but their number gradually increases with time, at the expense of the molecularly dispersed. It is to such changes that the behavior of silicic acid, of many albumin solutions, of humic acid, etc., must be referred. Another phenomenon of both practical and theoretical impor- tance is the clicmical decomposilioii through dialysis of molecularly dispersed substances with formation of a colloid phase. It was known to Graham and belongs to the earhest methods of preparing colloid systems. It is essential that the original material suffer hydrolj'sis in water, yielding an insoluble, or but slightly soluble, component. This is true of the chlorides, nitrates, acetates, etc., of the metals. Since the molecularly soluble product of the hy- drolysis passes through the dialyzing membrane while the "insol- uble" component remains behind in colloid form, a continual displacement of the hydrolysis takes place, favoring the forma- tion of the colloid. To obtain the corresponding colloid hydrate it is only necessary, therefore, to place the proper salt solutions in the dialyzer. From the abundant literature describing these phenomena we may cite the following example of the chemical changes exhibited by iron hydroxide-iron chloride solutions, during dialysis, as ob- served by S. E. Linder and H. Picton.^ Table 44 shows the changes in composition of the outer liquid during the process. Table 44. — Change in Composition of Outer Liquid Dueixg Dialysis OF Iron Hydroxide-ieon Chloride Solutions (According to S. E. Linder and H. Picton) Time of dialysis in hours Relation of Fe to HCl in outer liquid 24 48 120 168 56:109.5 56:137-° 56:609.0 56: 1086.0 Not demonstrable: evident Toward the end of the experiment, as can be seen, only HCl passed through the dialyzing membrane. The changes in composition of the inner liquid during the di- alysis is shown in Table 45. ' S. E. Linder and H. Picton, Trans. Chem. Soc, Lond., 1909 (1905). 234 specia:[^ colloid chemistry Table 45. — Changes in Composition op Inner Liquid During Dialysis OE Iron Hydroxide-iron Chloride Solutions (According to S. E. Linder and H. Picton) Composition in grams per lOO cc. Time of dialysis in Calculated molecu- days lar weight Fe CI Formula s 1-2303 0.1410 i3Fe(OH)3,FeCU 1767 9 ± . 2300 O.IIIO 2oFe(OH)3,FeCl3 2302 10 1.7200 0.1250 25Fe(OH)3,Fea3 2837 17 I . 5000 0.0773 36Fe(OH)3,FeCl3 4014 30 I . 2400 U.0S50 42Fe(CH)3,FeCl3 4656 37 I. 1800 0.0490 45 Fe(0H)3, FeCh 4977 44 I .1400 0.0460 46Fe(OH)3,FeCl3 5084 61 1 . 0400 . 0430 4sFe(OH)3,Fea3 4977 210 "■6550 U.OISO 82Fe(OH)s,FeCl3 8936 Gels separated out per O.OI2C1 i62Fe(OH)3,FeCl3 17496 after 120 days. gram Fe The table shows plainly the relative increase in iron hydroxide content at the cost of the hydrochloric acid. The formulas of the iron compounds produced and their respective molecular weights, as calculated by Linder and Picton, are also given. The impossi- bility of isolating the compounds, coupled with the fact that they show a progressive change makes the chemical significance of the numbers assigned as molecular weights rather fanciful. On the other hand, they demonstrate very well the progressive trans- formation into' the colloid. The progressive decomposition during dialysis, with formation of colloid in such solutions can be shown in a striking manner, according to N. Sahlbom,^ by " capillarizing " them, that is, by dipping strips of filter paper into them. If this is done every 24 hours to ferric chloride or ferric nitrate undergoing dialysis, pic- tures are obtained like those shown in Figs. 53 and 54. At the be- ginning of dialysis the molecularly dispersed solution ascends the paper without decomposition and concentrates high up, as shown by the dark bands at the tops of the colored columns. After i or 2 days, the upper concentrated salt zone begins to disappear while a second less-colored one appears below. The latter consists of colloid iron hydroxide which when first formed is highly dispersed; 1 N. Sahlbom, Kolloidchem. Beihefte, 2, 79 (1910). MECHANICAL i'Knri';R JM i';s oi' coij.dii) s\'s-n';Ms 255 With proLjressuij,^ dialysis, the niolrcularly dispersed salt dis- appears entirely at the expense "!' the iron h\ droxide, whieli ,L;radu- ally aequirt's the pmperties of a t_\pieal. posili\-el_\' ehar^red C(jl- Fig. 53. — Dialysis of a ferric clilori'le solution. (According to -V. Sahnu>in.) Fig. 54. — Dialysis of a feme nitrate soJution. ( .Vreortlui); to A'. .Siihllxnn .) loid and therefore tiseends filler paper lillle, il al all, as des( idlied on J). 15. Finally, a third jjlienomenon often ()l)ser\ed dining dialysis 236 SPECIAL COLLOID-CHEMISTRY deserves mention. When the separation of the molecularly dis- persed or electrolytic components of a system from the colloid is far advanced, a radical change in the state of the system often occurs. It may coagulate. This fact, which was already observed by Graham, shows that the presence of a certain amount of elec- trolyte is necessary to insure colloid stability. An example of this behavior is offered in Table 45, when the ferric hydroxide has been dialyzed 120 days. §30. Osmosis of Colloid Systems I. General Remarks and Literature.- — During dialysis, an increase in the volume of the dialj'zing liquid in the interior of the cell is often observed. This is the phenomenon of osmosis, known for a century and a half.^ Osmotic phenomena take place when- ever a dispersoid is brought in contact with a less-concentrated one or its pure dispersion medium under conditions which do not allow the "free" diffusion described in §28. This may be ac- complished by placing between them a so-called semipermeable or, better expressed, a selectively permeable membrane, in other words, a device which gives passage to the dispersion medium, but not to the dispersed phase. These devices are plainly nothing more than such as were used, for example, in the dialysis of colloid systems, as described in the previous paragraphs. In fact, os- motic phenomena may always be expected to appear during dialy- sis. Consideration of these osmotic phenomena discloses their close connection with the processes of diffusion and dialysis. Like the latter, osmosis represents an impeded diffusion. Osmosis, like free diffusion, tends toward the establishment of a uniform spatial distrihition of dispersed phase and dispersion medium. Since, in the presence of a dialyzing membrane, the dispersed phase cannot wander into the pure (or less concentrated) outer dispersion medium, the reverse occurs and the pure dispersion medium wanders into the dispersed phase. The result of this which represents the re- ciprocal of free diffusion, is an equalization, as far as possible, of the concentration of the dissolved substances in the different parts of the system. ' For a history of the development of our knowledge of osmosis see Wilh. Ostwald, Lehrb. d. allg. Chem., 2 Aufl., 652, Leipzig, 1903. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 237 The intensity of the tendency to bring about a uniform dis- tribution of dispersed phase and dispersion medium may be meas- ured by opposing this osmotic leveUng process by the hydrostatic pressure of a water column. The pressure thus made evident is called the osmotic pressure of the dispersoid.^ To make osmosis possible it is immaterial whether the selective permeability of the membrane is brought about by its sieve-like action, which holds back mechanically the dispersed phase, or by its selective proper- ties as a solvent in the sense that only the dispersion medium is soluble in it.- Osmotic pressure and osmotic phenomena like Brownian move- ment and diffusion velocity are markedly dependent on the spe- cific surface of the dispersed phase. Colloid solutions, therefore, show but slight osmotic pressures, provided they are not contami- nated with molecular or ionic dispersoids. Most colloids can only with difficulty be rid of such impurities which enter these systems in the process of their preparation or are necessary for their sta- bility. Such traces of impurities introduce great errors into pressure measurements which at the best yield but small values.^ It cannot, however, be denied that many typical colloids, espe- cially when of high dispersion, possess some osmotic pressure of their own. This follows as a necessary conclusion from the existence in them of Brownian movement and diffusibility. Measurements of the osmotic pressure of colloids have been made and discussed at special length by W. Pfeffer,* H. Picton and S. E. Linder,^ C. E. Linebarger,* E. H. Starling,'' C. J. Martin,* * In many textbooks, foUowing the lead of W. Nernst, we find it stated that osmotic pressure is the "cause" or "force" producing diffusion. This way of putting it is incorrect as the above remarks on the relation of diffusion to osmosis show and as J. J. van Laar (Vortrage uber d. thermodynam. Potential usw. Braunsch- weig, 1906) has long emphasized. The concept of osmotic pressure stands and falls with the presence and absence of a selectively permeable membrane. It contradicts every correct view of osmotic pressure to assume its existence in the absence of such a membrane, as in the processes of free diffusion. It is, however, correct to hold that the phenomena of diffusion, of osmosis and of Brownian movement all spring from the same source of energy as clearly evidenced by the close relations and analogies between them. ^ For details regarding such and other properties of membranes see the compre- hensive monograph of H. Zangger, Ergebnisse der Physiologic, 7, 99 (1908). ' With reference to the view that admixed electrolytes may constitute integral parts of the colloids see p. 143. ^ W. Pfeffer, Osmotische Untersuchungen, Leipzig, 1877. ' H. Picton and S. E. Linder, Journ. Chem. Soc. 63, 148 (1892). » C. E. Linebarger, Silliman's Am. Journ. Sci. (3), 43, 218, 416 (1892). ' E. H. Starling, Journ. Physiol., 19, 312 (1895-6); 24, 317 (rSgg). 8 C. J. Martin, Journ. Physiol., 20, 364 (1896). 238 SPECIAL COLLOID-CHEMISTRY A. Lottermoser/ B. Moore, W. H. Parker, H. E. Roaf, L. Adam- son, D. Bigland,- E. W. Reid,^ J. Duclaux,* G. Malfitano.^'R. S. Lillie,* G. Hufner, Gansser,' W. M. Bayliss,^ W. Biltz and A. von Vegesack' and others. Only the more important of their findings can be touched upon here. 2. Methods of Measuring the Osmotic Pressure of Colloids.— From what has been said it is clear that any dialyzing apparatus may be used to measure osmotic pressure. As dialyzing mem- branes, the earlier investigators generally used parchment paper. More recently collodion thimbles have been employed. C. J. Martin (/.c.) used clay cups impregnated with silicic acid gels; E. H. Starling (i.e.), the same impregnated with gelatine. For details the recent papers of W. Biltz and A. von Vegesack should be consulted. Fig. 55, which represents a cell used for osmotic pressure measurements, is taken fron their publications. Below is shown the collodion thimble. Of the two vertical tubes, one is used to fill the "osmometer," the other to record the pressure. The greatest source of error in the determination of the osmotic pressure of colloids lies in the disturbing effects of the presence of molecularly dispersed phases, especially electrolytes. Several schemes have been proposed to obviate the difficulty. Different investigators, especially B. Moore (with his collaborators) and J. Duclaux, have maintained that the accompanying electrolytes constitute integral parts of the colloid and are bound to it either chemically (see Duclaux) or at least through "adsorption." In other words, they hold the electrolytes to be essential to the maintenance of the colloid state. When they are removed the colloid is "denatured" and, as has been observed, "polymerized" >• A. Lottermoser, Anorg. Kolloide, Stuttgart, 1901; Z._ f. physik. Chem., 60,451 (1907). ^ B. Moore and W. H. Parker, Amer. Journ. Physiol., 7, 261 (1902); B. Moore and H. E. Roaf, Bioch. Journ., 2, 34 (r9o6); 3, 55 (1907); B. Moore and D. Bigland, ibid., 5, 32 (1909); H. E. Roaf and L. Adamson, Bioch. Journ., 3, 422 (1908); Journ. Physiol, 39 (1909); Quart. Journ. Physiol., 3, 75, 171 (1910); in part available only in abstract. 2 E. W. Reid, Journ. Physiol., 31, 439 (1904); 33. 12 (1905). * J. Duclaux, Compt. rend. 140, 1468, 1544 (1905); Journ. Chim. physique, 5, 40 (1907); I, 407 (1909); see also the review in Koll.-Zeitschr., 3, 126 (r9o8). * G. Malfitano, Compt. rend., 142, 1418 (rgod). ' R. S. Lillie, Amer. Journ. Physiol., 20, r27 (1907). ' G. Hufner and Gansser, Engelmann's Arch. f. Physiol., 209 (1907). * W. M. BayUss, Proc. Roy. Soc, 81, 269 (1909); KoU.-Zeitschr., 6, 23 (191). ' W. Biltz and A. von Vegesack, Z. f. physik. Chem., 68, 357 (1909); 73, 481 (1910). MECIIANICAf, l>ROI'i;k ril';S Dh' Col. I, (III) SVSriCMS 2]C) ^BlB^ into coarsely (lisperscd parliolcs, e\'en In Lhc point of (oa^ula- tion. That all lliis iiia\- orcur, as in the case of the all)Uinins, imisl be admitted, hut it caiiiiot he staled as a uni\'ersal truth. iVs R. S. Lillie [I.e.] has empluisi/ed, the jiresenee of eleitroUles is not essential to the e.xistenee of all metallic lu'ilrosols, and no reason can he assigned at present wh\" one phase t'annot he dis- persed into another to the point of colloid disper- sion in the eiitiri' ahsence of an\' elei. trol\ le. These remarks are not inti'iided to dem- the e.xisteirce of colloid-electrolyti' comj)le.xes. The\' are onh' made to emphasize that such discussion does not answer the ([uestion of wluit is the A'alue of the osmotic pressure of pure colloids thein- sel\'es and how it ma\' Ije measured, for theoretic- ally the colloids nrust ha\'e some, for the\' show Brownian mo\ement and dilTuse. The lollowiir.L,' measures ha\'e l.)eeR jiroposed to attain this end. At lirst si,!j:ht it would seem most satisfactor_\- to use niemhranes which permit a sharp dialytic separation of colloids and molecular dispersoids. In the course of the dialy- sis the molecular ilispersoids would then pass tlir(.)Ugh the membrane while the colloids would remain behind. The end pressure would then be that of the pure colloid. It is well to emphasize, at once, that these final osmotic pressures ha\-e almost in\"arialjl\' been found to be very lev. mometer of ]v. A second method consists in takinp; a limited ^''^' ^"^ ^- "'" W'Si-snck. volume of outer li(|uid and waiting until an equililiriunr has Ijeen establisheil between the concentration of the electrol_\'tes in this and the concentration ol those con- tained in the inner li(|uid. In t'onnection with this method it must be borne in mind that the equilibrium need not in any sense be s\-non\'mous with e(|uality of concentration in the tW(j lirjuids. .V whole series ol lads, one of which is the diflicultv of "washing out" the last traces ol electrolytes from precipitates, compels the conclusion that (ollo'uls /c;/i/ lo comciilnilr clrrlro/ylrs ii/mii lluinsclvcs and tliereb}' to increase the jjossibility of developing and exhibiting a greater osnrotic Fig. 240 SPECIAL COLLOID-CHEMISTRY pressure than is really due to the colloids themselves. Since such in- creases in concentration depend, as a rule, only on the cowcew^m^iow and not on the absolute amounts of the electrolyte present, they undergo progressive variation as osmosis takes place because of the movement of the liquid, and thus further complicate the problem. The following procedure has also been used. After the electro- lyte content of a colloid has been determined by analytical means, an amount is added to the outer liquid to bring its concentration up to that assumed to exist within the colloid. The excess of osmotic pressure exhibited by the colloid mixture is then regarded as the osmotic pressure of the colloid itself. To get a proper outer solution the dialysate or outer liquid, rich in electrolytes, is used against the original mixture, or a proper outer fluid is obtained by a filtration (see the following paragraphs) which separates the electrolyte solution from the colloid (J. Duclaux, I.e.). Finally, the maximum pressures observed in the osmosis of a colloid solution containing electrolytes has been taken as a con- venient method of arriving at the osmotic pressure of the colloid itself. As W. Biltz and A. von Vegesack {I.e.) have pointed out, this is the resultant of two processes; of the osmosis directed to- ward the inner liquid (endosmosis) and of that directed toward the outer (exosmosis), which latter parallels dialysis. These remarks make it clear that the methods for the quanti- tative determination of the osmotic pressure of colloid systems are not as yet worked out entirely. If we do not wish to determine the osmotic pressures of highly purified colloids or their final values to the point of utilizing a microscope to make readings and a micro-osmometer, then employment of a eonstant volume of outer liquid, with attainment of an equilibrium between the elec- trolytes present in both liquids, seems most expedient. It would, of course, be well to determine also the distribution of the electro- lytes between colloid and pure dispersion medium, in order to work out from the obtained values a proper equilibrium curve^ from which might then be extrapolated the osmotic pressure of the colloid when the concentration of the electrolyte equals zero. 3. Instability of Osmotic Pressure of Colloids. — One of the first things to be noticed when the osmotic pressures of colloids are measured, even though every effort is made to kee$) all external ' This would undoubtedly take the form of the adsorption isotherms. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 241 conditions the same, is their inconstancy. Not only do prepa- rations of one and the same substance, prepared by different methods, show different osmotic pressures, but shaking, stirring, standing, etc., all cause considerable change in them. The follow- ing examples illustrate this behavior. Table 46. — Intluence op Previous Treatment on Osmotic Pressure OF Albumin (According to E. W. Reid) Previous treatment Ash, percent Osmotic pressure of a r percent solu- tion in mm. Hg, Ovalbumin, twice crystallized and once washed Ovalhiimin, wn<;>iprl rpppptpHly 0.120 0.267 0.312 0.220 0-633 0.461 3-38 Ovalbumin, precipitated and once washed 4.82 15-71 u.OO 4.20 The same Precipitated bovine serum-albumin, repeatedly washed. The same, once washed . . . These experiments of E. W. Reid (I.e.) show that the osmotic pressure of one and the same substance (egg-albumin) varies at the same concentration between the values zero and 15.71 mm. of mercury. They also betray the important fact that the ash content of a colloid is not fundamentally responsible for the value of its osmotic pressure. The osmotic pressure of a preparation hav- ing the greatest ash content is zero, for example. The following example, taken from R. S. Lillie (I.e.) is intro- duced to illustrate the influence of shaking. Table 47. — Influence of Shaking on Osmotic Pressure of Gelatine AND of Egg-albumin (According to R. S. Lillie) 1. 25 percent gelatine Pressure in. mm., Hg. 1 .6 percent egg-albumin Pressure in mm., Hg. Pure gelatine Pure gelatine shaken Gelatine -|- -^ NaC! 4» fn Gelatine -|- -5 NaCl, shaken 48 m Gelatine -|- - „ NajSOj 40 m Gelatine -J — r NaaSOi shaken 16 4-2 5-3 2.6 2.9 2.4 2.6 Pure albumin Pure albumin shaken . . . ni Albumin -h „ NaCL... 40 Albumin + -g NaCl, shaken m Albumin + -^ NaT 4» m Albumni -|- - ^ Nal, shaken 3-' -I 31-3 9.0 8.9 6.6 242 SPECIAL COLLOID-CHEMISTRY It is a remarkable fact that while the osmotic pressure of gela- tine is increased by shaking, that of egg-albumin is decreased. Table 48 illustrates the influence of stirring on the osmotic pressure of colloid solutions. Table 48. — iNFunENCE of Stirring on Osmotic Pressure of Benzopur- PDRiN Solutions (According to W. Biltz and A. von Vegesack) A. Benzopurpurin low in electrolytes B. Benzopurpurin high in electrolytes Hours Height of fluid column Remarks Hours Height o£ fluid column Remarks 1 .0 9.41 Not Stirred s 1 . 22 Stirred. 2.5 9.62 Not stirred IS 1.25 Not stirred. 3-S 9-5° Not stirred 18 1-34 Stirred. 4.5 9.68 Stirred 5, min ■378 1.30 Stirred i hr. daily. 50 9.86 Stirred s min 426 1.24 Stirred 7 hrs. pre- 5-S 10.07 Stirred s min viously. 6.0 10.18 Stirred 5 min 450 1 . 26 Stirred 7 hrs. pre- 7.0 10.40 Stirred 5. min viously. 8.0 10.60 Stirred 5 min g.o 10.64 Stirred s min 10. 10.66 8.16 Stirred s min Not stirred 20.0 . 20.5 8.37 Stirred s min 8.92 9.08 Stirred s min Stirred 5 min 100 1. 14 21-5 Stirred during day. 22 .0 9. i6' Stirred 5 min 121 1. 19 Stirred 6 hrs. -■2^.0 9.29 Stirred .5 min I4S. 1 .09 Stirred 6 hrs. 24.0 8.98 Stirred 5 min 167 1 .10 Stirred 6 hrs. 25.0 8.99 Stirred 5 min 28. D 7.16 Not stirred 28.5 7.4s Stirred 5 min ' This table shows an increase in osmotic pressure with every stirring, even though the effect is but transitory. The increase, occurred three times in the data given. It is also apparent that solutions containing small amounts of electrolytes are more sensi- tive to this influence than those richer in these, which are scarcely affected. Gelatine behaves similarly, as shown in Table 47. In discussing the influence of time upon the osmotic pressure of colloids we need to distinguish between its variations when a colloid is simply left to itself in an osmometer and its variations if the same colloid is measured at different periods. The first MECHANICAL PROPERTIES OF COLLOID SYSTEMS 243 relation is evidenced in the left-hand column of Table 48. This benzopurpurin, showed a rise to 1.21 cm. after 310 hours; while the capillary rise in a similar tube amounted to 1.12 cm. The osmotic pressure was therefore o.qq cm. In illustration of the influence of age upon the solutions, these authors found a dialyzed solution of 0.00103 normal night-blue to yield a maximum osmotic pressure of 15.52 cm. of water after 2 days; after 6 days, 4.24 cm.; and after 11 days, 4.08 cm. When we survey these facts we are struck by the great in- constancy of the osmotic pressure of colloids as compared with that of molecularly dispersed solutions. The osmotic pressure of colloids is variable, being greatly modified by mechanical treat- ment, age, etc. Such sensitiveness is unknown in molecular dis- persoids. It is true, of course, that the experiments of W. Spring^ have shown that even ordinary salt solutions, for example, are not absolutely stable in their conductivity, their optical properties, etc., but these variations are very small when compared with those exhibited by colloids. The reasons for this great variability are to be sought in the changes of state of colloids, such as varia- tions in their degrees of dispersion, states of aggregation, etc., for which many different causes may be responsible, as will be dis- cussed later. The osmotic pressure of colloids, more especially of emulsoids, varies therefore as does their viscosity. 4. Influence of Concentration on Osmotic Pressure of Colloids. — The osmotic pressure of molecular dispersoids, as is well known, is governed by the important law of Pfeffer-van't Hoff : the osmotic pressure is directly proportional to the concentration. The relations in colloid systems are not so simple. Examples are known, in which the law holds approximately, but there are also those in which the osmotic pressure increases faster than the con- centration, or more slowly than this. Perhaps nothing better demon- strates the inappropriateness of applying without due considera- tion, the "solution laws" vahd for molecularly dispersed systems to colloid systems, than this variability of the concentration function of the osmotic pressure of colloids. The following results of W. Biltz and A. von Vegesack (I.e.) on purified congo red may serve to illustrate the first of the three ' W. Spring, Koll.-Zeitschr., 7, 22 (1910), where references to earlier papers on this subject may be found. 244 SPECIAL COLLOID-CHEMISTRY possibilities, namely, that wherein concentration and osmotic pressure are approximately proportional. W. M. Bayliss (I.e.) also noted this proportionality in concentrations ranging from 0.07 to I percent by weight. Table 49. — Relation of Osmotic Pkessure to Concentration in Dialyzed Congo Red Solutions (According to W. Biltz and A. von Vegesack) Concentration C Osmotic pressure in cm. t p g = const. 0.539 norm. 4. IS cm. U.770 1.08 8.15 cm. 0-75S 1.44 10.24 cm. 0.69s 1.80 14.00 cm. 0.778 2.1SS 14.62 cm. 0.678 ..87 18. 70 cm. 0.652 3-23 21.5s cm. 0.667 3-59 25.04 cm. 0.698 4-31 25.30 cm. 0.587 The constants are all of about the same order of magnitude. Gum arable behaves similarly according to W. Pfeffer {I.e.). Table 50. — Osmotic Pressure of Gum Arabic in Different Concentrations (According to W. Pfeffer) Concentration C Osmotic pressure in cm. Hg. C I percent 6-59 6.9 6 25-9 4-3 14 70.0 S-o 8 119. 6.6 The observations of J. Ducla,ux {I.e.) on the same substance and given in Table 51 should be compared with these. Some illustrations of how the osmotic pressure may increase more rapidly than the concentration are given in Table 51. P ... As readily apparent, the ratio p; increases greatly with nsmg concentration. This is altogether different from the behavior of molecular dispersoids, in which, so far as known, the opposite occurs as the concentration rises. That the experimental methods MECHANICAL PEOPERTIES OF COLLOID SYSTEMS 245 H a w o z; o U 1-1 ,_^ IH- H ^5 nl s h Q Ci o Si O H < w I a «,](J 00 f^ According to W. Biltz and A. von Vegesack {I.e.) the osmotic pressure of a 0.165 percent iron hydroxide sol amounts to.3.74 cm. after 4 hours; that of a 0.173 percent one to only 1.42 cm. after 5 hours. ■0, a " ° c3 ^ 2^ U Percent 1.6 2.S H d «.|0 vo CO in H In M « N CO 'S. g o\ in -J- " •- S u Percent 0.88 2.96 7-1 e «.o MD CO CO ty-) t^ C* Tj- VO ■0. . 10 'O r-* lo fO g M ii-> C?i i^ 4>- ^ d d 6 M «N 4 g \o i>. to fO 5 Tf irj o* r^ r-- j« d d d i-I M Tt 1 1 !;; a a. 8 ;d '^ «.,!J 6 6 6 6 ■tt . LO vO CO CO g t^ . . . ^ ^ M « lO g ro 10 t^ t- rO »j-) t^ t-i •a S •d c S about 1 . 4 about 2 . 7 about 5.0 about 8 . 7 about 12.0 b Ti- 00 m in M M C4 cs . in g w in U d « t^ t'l 00 d 00 \o m ^o w in t^ ci 1 M M u Percent about 0.15 about 0. 2 about . 4 about 0.8 about 1 . 84 00 3 -t in in rn fO about 2 about 3 about 3 about 8 246 SPECIAL COLLOID-CHEMISTRY used by Duclaux are not responsible for this behavior is shown by the experiments of W. Biltz and A. von Vegesack {I.e.) included in the table as controls. A relative decrease in osmotic pressure with increasing con- centration, a relation typical of concentrated molecular disper- soids, has been observed by B. Moore and H. Parker {I.e.) in sodium oleate solutions. Thus a 0.5 percent solution showed a maximum osmotic pressure of 14.4 mm. (at 55°), while a 3 per- cent solution showed one of 37.2 mm. (at 40°). The two quo- P tients, ^, are 288 and 124 respectively. In other words, a six- fold increase in concentration caused only a two and one-half fold increase in osmotic pressure. Table 52, containing the exceed- ingly careful experimental results of E. W. Reid {I.e.) on the osmotic pressure of repeatedly crystallized hemoglobin, illustrates strikingly what has been said. Table 52. — Relation of Osmotic Pressure of Hemoglobin Solutions TO THEIR Concentration (Actording to E. W. Reid) Concentration Temperature Osmotic pressure in mm., Hg. Osmotic pressure per I percent Hemoglobin _ i> C 2 . 76 percent 14-5° 12 mm. 4-3S 2 ,92 IS 12 4 II 4.S8 IS 17 3 71 4-95 IS 19 3 84 S-70 IS 17 3 SI 6.05 IS 22 '3 63 6.07 IS 23 3 79 Disregarding some slight irregularities, the decrease in the quotients is unmistakable. The differences are brought out most P plainly if the variation of the quotient, ^' is represented graph- ically. This has been done (in arbitrary units) in Fig. 53. The three different t3^es are easily recognized. It should be mentioned that J. Duclaux {I.e., 1910) has ob- served a minimum for the quotient in J the case of Berlin blue. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 247 though he has himself raised some doubts as to the reliability' of his measurements.^ The theoretical sigm'ficance of these different concentration, curves we shall discuss later (see p. 262). 5. Influence of Temperature on OsmoticPressure of Colloids. — C. J. ^lartin and W. M. Bayliss (I.e.) state that the osmotic Concentration — »■ KiG. 56. — Relation between concentration in colloid systems and the quotient of the osmotic pressure and concentration. pressure of albumin, hemoglobin and congo red varies rectilinearly with the temperature, in other words, directly with the absolute temperature. This statement would make Gay-Lussac's law valid for these solutions. The findings of B. Moore and Roaf (/.c), J. Duclaux (/.c), W. Biltz and A. von Vegesack (/.c), ' The measurements of W. Pfeffer on gum arable, given in Table 50, also show a minimum value for the quotient „• 248 SPECIAL COLLOID-CHEMISTRY contradict this. Moore and Roaf found the osmotic pressure of gelatine solutions to increase considerably faster than the absolute temperature. Technical night-blue solutions show an analogous behavior, according to the figures of W. Biltz and A. von Vegesack, contained in Table 53. Table 53. — Intlttence of Temperature on Osmotic Presstire of a 3.49 Normal Solution of Technical Night-blue (According to W. Biltz and A. von Vegesack) (° Temperature T° Osmotic pressure in cm. t T 273 6.21 0.022 25 298 10.81* 0.036 50 323 13-83 0.043 70 343 17.69 0.050 * Average of two experiments. J. Duclaux has observed the opposite to be true of iron hydrox- ide sol. In this, the osmotic pressure decreases not only relatively, but even absolutely, with rising temperature. There exists no analogue for this in the field of molecular dispersions. Duclaux found the following: Temperature Osmotic pressure (cm.) . t T '° (27s) 22 .9 0.083 25° (298 21.3 0.071 70° (343) 20. 9 0.061 Figuje 57 shows graphically how differently the osmotic pres- sure of different colloids varies with changes in the temperature. The dotted line represents the ideal case in which there exists simple proportionality between the two as is the case, at least approximately, in molecular dispersoids. It should now be pointed out that B. Moore and Roaf (I.e.) and R. S. Lillie (I.e.) observed interesting thermal after-effects or so-called hysteresis phenomena in gelatine solutions. Thus gelatine solution which has been heated continues to show a higher osmotic pressure for some time after cooling than when kept continuously at the lower temperature. The following Table 54 taken from R. S. Lillie illustrates this. It also shows that the differences first noted between the previously cooled and the pre- viously warmed gelatine become less with time. MECHANICAL PROPEETIES OF COLLOID SYSTEMS 249 Table 54.— Influence or Thermal History on Osmotic Pressure of I Percent Gelatine (According to R. S. Lillie) Osmotic Pressure at Room Temperature in Mm. Hg. Age of the solution Previously chilled on ice Previously warmed to 65-70° I day 5-0 6.4 5 days S-o 5-3 2 days 4.9 (chilled for long time previously) 6.0 I day S-7 6.2 I day S.6 6.0 Fig. 57.- fron hydroxide so/ Ni(jhl--blue Temperature 25 50 70° -Relation between the temperature of colloids and the quotient of osmotic pressure and absolute temperature. This behavior also is unknown among the molecular dis- persoids. 6. Influence of Added Substances on Osmotic Pressure of Colloids. — The influence of added substances upon the osmotic pressure of a given system, is, according to the classic theory of molecularly dispersed solutions, purely additive. In other words, the pressure exerted by the added substance is added to that of the original system. There exist exceptions to this rule, of course, and usually in the sense that the calculated osmotic pres- sures are found to be greater than those actually observed. The effect of added substances on the osmotic pressure of col- loid systems is more complicated. Under this heading also, 250 SPECIAL COLLOID-CHEMISTRY concentration and temperature functions are encountered which not onl}- do not correspond with any observed among molecular dispersoids, but which among themselves show great differences. The influence of added substances may be studied by adding them in equal concentration to both the inner and the outer hquid. The important experiments of R. S. Lillie {I.e.) were carried out in this way. Acids and alkalies may either increase or decrease the osmotic pressure of different colloids. Sometimes one and the same colloid may show both types of behavior. Often very small quantities of hydrogen or hydroxyl ions are sufficient to cause noticeable effects. W. M. Bayliss (I.e.) found the osmotic pres- sure of very pure (and highly dispersed) congo red to fall from 207 mm. to 120 mm. when the outer water (conductivity water) surrounding his osmometer was replaced by the same water satu- rated with carbon dioxide. The stronger acids produce, of course, still more marked effects. The addition of alkali in- ereases the osmotic pressure until a maximum is reached, beyond which it falls again. Table 55 gives a part of R. S. Lillie's results with gelatine.^ Table 55. — Influence of Aclds and Alkalies on Osmotic Pressure of 1.5 Percent Gelatine (According to R. S. Lillie) Influence of HCl Influence of KOH Concentration Osmotic pressure in mm. Hg. Concentration Osmotic pressure in mm. Hg. n/3100 HCl - n/2030 n/i5So n/1024 n/770 n/620 n/412 8 6 12 17 26 32 34 39 2 8 3 9 S 4 9 3 n/3100 KOH n/620 n/412 n/310 7-9 141 .23-7 25-1 29.0 As can be seen, low concentrations of acid lead to a slight but definite minimum of osmotic pressure. With higher concentrations 1 See also the analogous findings of H. E. Roaf {l.c) on hemoglobin. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 251 there occurs a sharp increase in osmotic pressure which rises steadily for a time with increasing concentration. R. S. LiUie thinks it probable that beyond a certain point a decrease in os- motic pressure would again occur. Within the concentration range studied, alkalies led only to an increase in osmotic pressure. Figure 58 shows graphically this variation of the osmotic 0.0010 .0020 Concentration .0030 norms/ Fig. 58. — EflFect of acid and alkali upon the osmotic pressure of a 1.5 percent gelatine solution. (According to experiments by R. S. Lillie.) pressure of gelatine solutions with the concentration of the added acids and bases. Contrary to the findings in the case of gelatine, the osmotic pressure of egg albumin is always lessened by the addition of hydrogen or hydroxyl ions. Table 56 shows this. In the case of the acids a definite minimum again appears. The type of curve, at least for albumin, is therefore not so funda- mentally different from that for acid gelatine. 252 SPECIAL COLLOID-CHEMISTRY Table 56- — Inflitence of Acms and Alkalies on Osmotic Pressure of 1.5 Percent Egg Albumin (According to R. S. Lillie) HCl KOH Concentration Osmotic pressure in mm. Hg. Concentration Osmotic pressure in mm. Hg. n/3100 HCl n/1240 n/620 n/412 n/310 25.6 20.7 14. 1 20.4 22.2 n/3100 KOH n/1240 n/620 n/412 n/310 25. 6 24.1 22.6 20. 2 18.0 17. q 0.009 ~m Concentratiorr '<6normal Pig. 59. — Effect of acid and alkali on the swelling of gelatine plates, experiments by Wo. Ostwald.) (According to To illustrate the varied influence of salts the following examples may be given. Technical night-blue contains a considerable ad- MECHANICAL PROPERTIES OF COLLOID SYSTEMS 253 mixture of electrolytes. If they are removed by dialysis its osmotic pressure increases, as shown in Table 57. W. M. BayHss {I.e.) obtained analogus results for congo red. The behavior of gelatine and albumin toward added salts has 0.001 .002 003 normal Concentration >■ Fig. 60. — Relation between internal friction (upper figure) and osmotic pressure (lower figure) in albumin solutions when acid or alkali is added in different concentra- tions. (From experiments by H'o. Paw// and his co-workers and ii. 5. Z.i7/!e.) Only the percentage increase in viscosity and not its absolute value could be given in the upper figure. been extensively studied (B. Moore and co-workers, R. S. Lillie, etc.). The following general truths are taken from the findings of R. S. Lillie: 254 SPECIAL COLLOID-CHEMISTRY Table 57. — Osmotic Pressure of Purified Night-blue and oe Night-blue Containing Electrolytes (According to W. Biltz and A. von Vegesack) Purified colloid Colloid containing electrolytes Concentration Osmotic pressure in cm. Concentration Osmotic pressure in cm. 1.30 S.8i I. 20 4.72 1-74 12.70 i-S8 S-io 2.17 16.64 1 .60 5-31 2.61 21.99 1 .96 6. 24 3-04 20.24 2.36 7.90 3-91 2S.32 2-73 9-42 4-34 32.18 3-49 II . 19 5-21 37-24 S-76 14.10 6.08 43-94 6.12 20.81 The addition of salts always causes a decrease in the osmotic pressure of these colloids. The degree of this decrease varies with the concentration and with the nature of the anion and cation. Generally speaking, the neutral salts of the alkali metals cause the smallest decrease. The salts of the alkaline earths are more effective and those of the heavy metals most effective of all, though they vary considerably among themselves. With salts having a common cation the order of the anions, when that most effective is given first, is about as follows: S04>Cl>N03>Br>I>CNSi The cations similarly arranged follow the order: heavy metals > alkaline earths > alkali metals. Table 58 details some of the actual experimental findings. If the validity of the above-mentioned conclusions is to be tested, the data of the original papers must be consulted for the experi- ments differ considerably among themselves. Figures 61 and 62 also show the complicated effects of the con- centration of the added salts upon the osmotic pressure. The original paper {I.e., p. in) must be consulted for the detailed data upon which these figures are based. ' For similar findings on hemoglobin see the work on H. E. Roaf (I.e.). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 255 Table 58. — Influence of Salts on Osmotic Pressure of Colloids (According to R. S, Lillie) Salts of the alkalies 1. 2 5 percent albumin P.bfl u 1. 25 percent gelatine Salts of the alkaline earths 1.25 percent albumin 1. 25 percent gelatine Pi bo m/24 NaCl m/24 NaBr ni/24 Nal m/24 NaNOa m/24 NaCN'S m/24 NaaSOi 21 m/24 m/24 m/24 m/24 m/24 m/24 m/24 m/24 m/24 m/24 KCl KBr KI KNO3 KCIO3 KBrOa KCNS K2SO4 KCOOCH; K2C204 m/96 MgCh m/96 CaCh m/96 SrCh m/95 BaCh !I.S 7.3 7.6 7.2 7.6 Is. 9 m/96 MgCh 3 .2 m/96 CaCh m/96 SrCh m/95 BaCh 2.7 3.r 2.7 Salts of the heavy metals Influence of different cations with com- mon anion 1.25 percent albumin 1. 25 percent gelatine 1.25 percent alb umin I.2S percent albumin to a) 0) u d ^t ^, lf{ .2 t;i .2 s a v u C flfi : s fl r .aaj : en 6a 60 U 6s 8 oE 21. s S.4 20.8 5.4 m/96 MnCh 6.9 m/192 C0C12 2.0 m/48 Li CI 5-4 m/48 LiCl 2.9 m/96 Co CI 2* 5.6 m/192 CuCh 3.3 m/48 NaCl S.6 m/48 NaCl 2.6 m/96 CdCh* 4.1 m/48 KCL S.9 m/48 KCl 2.4 m/96Pb(N03)2* 2.8 m/48 NH4C1 4.5 m/48 NH4CI 2.6 m/96 CuCl2* 1.6 'A precipitate is formed. It is interesting to compare the behavior of the two colloids toward the same added substance. While the salts of the alkali metals produce about the same effect upon both (the sulpho- cyanate having the least effect, the sulphate the greatest) almost opposite effects are produced on albumin and gelatine when 256 SPECIAL COLLOID-CHEMISTKY other salts are used. Among the alkaline earths, SrCl2 and MgCl2 produce a greater effect on albumin than CaCl2 or BaCl2. When gelatine is used the reverse is the case. Of the salts of the heavy metals, CuCl2 affects albumin more than CoCl2. The opposite is true for gelatine. Such contrary effects are not so evident when the cations are compared. On the basis of the investigations of S. Posternak,^ Wo. Pauli^ and R. Hober,' we are, no doubt, correct in referring these dif- Na Acetate q Nal Q ^ \ NaCNS o-\_. -V-oAfe CNS c,NaBr f/aBr,Na^SO^c^ \ bAto Acehatey^ NaNO^a.^ ^^^'^~~~~— ~~ oAfe/ Naa,NaF 0*==^ 1 1 ^NaCI 1 '"/as '"Ua Concentration ■"/a* Fig. 61. — Effect of salts upon the osmotic pressure of gelatine. (According to R. S. Lillie.) ferences in behavior to the differences in the reaction of the two colloids. Fresh (native) albumin, such as R. S. LilHe used, has a slightly alkaline reaction, while commercial gelatine is always acid. The differences in the effects of an added salt upon an "acid" or an "alkaline" albumin so far as its internal friction was concerned were discussed in §25. It is of much interest that the osmotic pressure of colloid systems should also be so greatly dependent on the acid or alkaline reaction of the colloid. 1 S. Posternak, Ann. de I'Inst.-Pasteur, 15, 85 (1909) 2 Wo. Pauli, Hofmeister's Beitr., 5, 27 (1903). 'R. Hober, Hofmeister's Beitr., 11, 35 (1907). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 257 The influence of electrolytes on the osmotic pressure of col- loids may show hysteresis. The after-effects of temperature were discussed on p. 249. If, in the osmotic study of a gelatine -j- acid mixture, the outer liquid is replaced by distilled water, the pressure column gradually sinks. But to attain its original level Concenhnahon Fig 62. — Effect of salts upon the osmotic pressure of albumin. (According to R. S. Lillie.) requires days, and maybe weeks, before the osmotic pressure of the pure gelatine is again reached, even when the acid which dialyzes out very rapidly, is constantly removed by frequent changes of the water (R. S. Lillie). Such lagging before equilib- rium is finally attained is unknown in the osmosis of molecular dispersoids. 17 2s8 SPECIAL COLLOID-CHEMISTRY Our knowledge of the influence of non-electrolytes on the osmotic pressure of colloids is still limited. An investigation of this question would doubtless bring out many interesting facts. Table 59 reproduces some of R. S. Lillie's {I.e.) results, in which but small differences of both a positive and a negative nature appear. Obviously, higher concentrations of alcohol, acetone, etc., might cause a decided decrease in the osmotic pressure of these colloids. Table 59. — Influence of Non-electrolytes on Osmotic Pressure of Colloids (According to R. S. Lillie) Egg Albumin 1.25 percent 1.6 percent J Added substance Osmotic pressure in mm., Hg. Added substance Osmotic pressure in mm., Hg.j m/6 cane sugar jn/6 dextrose 22.4 21.5 21.8 m/6 glycerine m/6 urea 29.4 29.5 27.9 Gelatine 1.25 percent 6.2 S-S m/6 cane sugar 6.6 m/6 dextrose 5-7 m/6 dextrose 5-8 m/6 glycerine 5-6 m/6 glycerine 5-9 m/6 urea 6.6 m/6 urea 7-3 7. On the Theory of Osmotic Pressure of Colloids. — In the classic theory of osmosis in molecularly dispersed systems, as formulated by J. H. van't Hoff, on the basis of W. Pfeffer's {I.e.) experiments, the absolute concentration, in other words, the number of moleeules in the unit volume alone determines the amount of the osmotic pressure (at constant temperature). The osmotic pressure is directly proportional to the number of molecules and to the absolute temperature. Sv. Arrhenius assumed a dissocia- tion of the molecules into ions, in the case of the electrolyte's in which a gram molecule in the unit volume shows a higher osmotic pressure than that calculated. On the other hand, when une^- MFjCHANICAL PROPERTIES OF COLLOID SYSTEMS 259 pectedly low osmotic pressures were observed, as in high con- centrations of different substances, it was held that there occurred association, polymerization, etc., of single molecules to larger aggregates, or that the dissolved substances combined with the dispersion medium to form solvates, etc. But whatever the ir- regularities observed, they were uniformly reduced to either an ■increase or a decrease in the number of particles actually present to the unit volume as compared with their calculated number. The number of particles has, in other words, in this classic theory of solution, been regarded as the most important if not the sole variable. ^^'hen the osmotic phenomena of dispersed systems are viewed in a more general way, especially in connection with other forms of movement, as Brownian movement and diffusion, it becomes evident that several other variables, not considered in the classic theory of osmosis, play an important part. They are the degree of dispersion and the type of the dispersed phase, together with such associated properties, as degree of hydration, etc. It makes no difference in the classic theory of osmosis what is the size of the dispersed particles, or whether we deal with molecularly and ionically dispersed phases or with coarse dispersions. Nor does the type of the dispersed phase matter, or its degree of hydration, except in so far as through hydration a portion of the solvent may be withdrawn, thereby causing an increase in the molar concen- tration. With any given substance in a given dispersion medium, each particle, no matter what its type or size, behaves like a molecule, and if A'^ particles (Avogadro's number) are present in the unit volume, the system will exert unit osmotic pressure. It is evident that we may not thus assume the independence of osmotic pressure, say of the degree of dispersion, when we come to deal with systems which have not a maximum degree of it, as in colloid solutions. To do so would be to deny the importance of the relations between osmosis, diffusion and Brownian movement. We cannot ascribe the small pressures exhibited by colloids to a low "molar" concentration of the colloid phase. Just as certainly as highly dispersed phases possess a greater Brownian movement and a higher diffusion coefficient, even independently of their concentration, equally certainly must they show a greater osmotic pressure than less dispersed ones, other conditions being equal. 26o SPECIAL COLLOID-CHEMISTRY Only a theory of osmotic phenomena that considers the degree of dispersion of the system, in addition to concentration and tempera- ture, can prove universally valid for all dispersed systems. It is not difficult to bring experimental proof for such theo- retical deductions. In fact, no one who tries to account for the great sensitiveness of the osmotic pressure of colloids to different influences, can escape considering changes in those characteristic variables of colloids, namely, their degree of dispersion and their type, as responsible for it. The variations in the osmotic pressure must be explained by the same kind of changes by which we explain, for example, the variations in their internal friction, namely, "changes in state." The influence of degree of dispersion upon osmotic pressure is very evident in congo red. W. M. Bayliss {I.e.) prepared a pure and highly dispersed congo red by allowing NaOH to diffuse into its free acid contained in an osmometer.^ While the free acid is pronouncedly colloid, as betrayed by the fact that it is readily resolved ultramicroscopically, congo red prepared in the manner described, cannot be thus resolved. But it can be as soon as traces of electrolytes are added. Even the carbon dioxide of the air suffices to do this. At the same tinie, the osmotic pressure of the system decreases. All factors which cause a decrease in degree of dispersion, as the addition of electrolytes, ageing, shak- ing, etc., reduce the osmotic pressure. Other factors which in- crease the osmotic pressure, as the addition of alkalies, also make the ultramicroscopically heterogeneous structure give way to an optically homogeneous one. The fact observed by J. Duclaux^ that the osmotic pressure of a red gold hydrosol is considerably greater than that of a blue one also belongs here. We have every reason for believing that blue gold sols are not as highly dispersed as red ones. The view advanced here that changes in the state of a colloid, more especially variations in its degree of dispersion and its type, are of particular significance in determining its osmotic pressure, is perhaps most clearly demonstrated by the close analogies be- tween the osmotic phenomena exhibited by colloids and their in- ' The similar behavior of freshly prepared silicic acid is discussed on p. 232. ^ J. Duclaux, Compt. rend., 148, 295 (1909); for a description of the special method used by this author in determining the osmotic pressure see this paper and KoU.-Zeitschr., 3, 134 (1908). MECHANICAL PEOPERTIES OF COLLOID SYSTEMS 261 ternal friction and swelling. The close relationship between these processes is brought out not only by emphasizing that age, pre- vious thermal history and mechanical treatment affect all of them in the same general way, but by the fact that they do this often down to the minutest details. This is clearly apparent when we compare the influence of acids and alkalies on the osmotic pressure (R. S. LilHe) with their effect upon the internal friction (Wo. Pauli, etc.). Still more striking, perhaps, is a comparison of the effects of acids and alkalies on the osmotic pressure of 1.25 percent gelatine solutions (R. S. Lillie) with those of these same substances on the swelling of gelatine discs (Wo. Ostwald).^ Here the agreement is perfect even to details (see Figs. 58, 59, pp. 251, 252).- In connection with these facts the influence of added substances on the viscosity of gelatine solutions, as given on p. 173, should also be studied. As a matter of fact, the relation between osmosis and swelling is close even when the question is viewed from a theoretical stand- point. In the place of a selectively permeable membrane, we have the structure of the material undergoing swelling which hinders the movement of the dispersed phase into the dispersion or swelhng medium. The process leading to the highest attainable homogeneous (spatial) distribution of swelling substance and swelling producing medium, is possible only if the structure and the specific surface of the swelling body change simultaneously, while the spatial continuity of the two phases to each other re- mains. If this continuity is destroyed, as by increase of tem- perature, above a critical value, then instead of swelling, .solution occurs. Besides these analogies between the osmosis and the swelling of colloids (as well as between osmotic and swelling pressures), characteristic differences also exist between them. In the process of swelling, a radical change in state, namely, an increase in degree of dispersion takes place. In the osmotic processes of molecularly dispersed systems, the specific surfaces, etc., of the dispersed particles remain constant and only the re- ' Wo. Ostwald, Pfluger's Arch., 108, 563 (1905). ^ According to R. S. Lillie the acid minimum is about one-tenth that found by Wo. Ostwald in his experiments on swelling. But since the latter minimum is practically identical with that of the viscosity ma.ximum of dilute gelatine solutions as found by P. von Schroeder (p. 173) and agrees fully with the acid maximum for albumin solutions (see H. Handovsky, Koll.-Zeitschr., 7, 192, igio) Lillie's figure evidently represents an error either in measurement or calculation. 262 SPECIAL COLLOID-CHEMISTRY lation of number of particles to unit volume changes. But when colloid systems are under discussion the processes of swelling and osmosis again agree; in fact, the osmosis of colloid solutions might well be termed a "swelling of liquids" in contrast to the usual swelling of solids. In considering the enormous effect of acids and alkalies on the osmotic pressure of colloids one might try to save the classic con- ception of osmosis by assuming an increase in the molecular con- centration of the albumin particles, caused, say, by hydrolytic cleavage. But examination of this idea leads to an exactly opposite conclusion, for, as Wo. Pauli^ has emphasized, and as St. Bur- garsky and L. Liebermann first showed, the observed freezing points of mixtures of acid and alkali with albumin are not as low as those obtained by adding together the effects which albumin and the added substance produce alone. A decrease in the molar concentration therefore occurs, either by chemical or adsorptive union of albumin with electrolytes. The addition of acids and alkalies as emphasized in the discussion of viscosity on p. 174 leads to the formation of a larger number of albumin ions which are capable of holding more water than the neutral albumin particles. The emulsoid properties of the system, originally relatively low, are, therefore, greatly increased, as betrayed, for example, by the rise in its internal friction, indifference toward salt, etc. The remarkable effects of concentration and of temperature on the osmotic pressure of a colloid will some day, no doubt, be similarly explained through the changes in the state of the colloid produced by them. It need but be recalled that the degree of dispersion and the type of the dispersed phase are, at times, a function of the concentration and the temperature as discussed on p. 35. When the degree of dispersion decreases with rise in concentration, as in soap solutions, then the (relative) osmotic pressure must decrease. Actually this is found to be true not only for soap solutions but also for hemoglobin (see Table 52 on p. 246). Analogous considerations hold for the effects of temperature on the osmotic pressure of different colloid systems. The many and complicated possibilities for great variations in ' Wo. Pauli, Pfliiger's Arch. (1910). Festschr. f . E. Hering. Prof. Pauli was kind enough to place the proof sheets of this article at my disposal. MECHANICAL PROPERTIES • OF COLLOtD SYSTEMS 263 behavior, especially among the emulsoids belonging to the number of the complex dispersoids, may be foreseen, especially when the additional variations which may be introduced through changes in the electrical properties are kept in mind. The suspensoids, which assume but one form, will show a simpler behavior. That this is so is borne out by the observations on dyes of the suspen- soid type, as congo red, benzopurpurin, etc., as studied by W. M. Bayliss (I.e.), W. Biltz, A. von Vegesack (I.e.) and others. The problem of the future is more the problem of analyzing the type of these various colloid changes than that of settling whether or not the observed peculiarities can be explained on the basis of the classic theory of osmosis. In a word, then, the osmotic pressure of most colloids is by no means only a function of the number of particles in the unit volume, but varies with the changes in the state of these systems, more espe- cially with the changes in the degree of dispersion and the type of the dispersed phase. The value of the osmotic pressure is therefore a more complex function in the case of colloids than in molecularly dispersed systems, and may not offhand be made identical with the latter. In fact it seems impossible, for these reasons, to assign absolute values to the osmotic pressure of coUoidally dispersed systems. This is true of all emulsoid and complex dispersoids, while simpler relations, resembling those valid for the molecularly dispersed systems, seem to exist in the case of suspensoid systems (see the succeeding paragraphs). Perhaps future investigators will find it best to reserve the concept of osmosis for molecular dispersoids and to use another term like hydration (solvation) for the phenomena observed in colloid and coarsely dispersed systems. Such a term would constantly bring to mind the impor- tant difference between the two kinds of phenomena. 8. Determination of the "Molecule Weight" of Colloid Systems by Osmotic Means. — As is well known, the molecular weight of a dissolved substance may be determined from the os- motic pressure of a molecularly dispersed solution, by the follow- ing formula: c T M = (22.4 X 76o)-Thir' P-J- in which M represents the molecular weight sought, 22.4 the "normal" osmotic pressure of a gram-molecule of the molecularly 264 SPECIAL COLLOID-CHEMISTRY dispersed substance at 0°, c the concentration (in percent), p the observed osmotic pressure in mm. of Hg., T\, the observed absolute temperature, and Tq, 273°. Since J. H. van't Hoff first formulated this law the different investigators who have measured the osmotic pressure of colloids have, also, in many cases tried todeduce therefrom their "molecular weight." Indeed, there exist but few publications on the osmotic pressure of colloids in which there is not a column devoted to their "molecular weight" as calculated from the osmotic pressures. Many examples could be given of this. The striking thing about these "molecular weights" of colloid systems is their great absolute value and their great variaiility under different conditions. The former seems obvious enough in view of the low values found for the osmotic pressure of colloids of even simple chemical composition. The second, however, accord- ing to which the molecular weight varies under different cir- cumstances is, strictly speaking, a contradiction in terms, for by definition, the molecular weight is a constant. But 'disparities between the molecular weights of substances, as deduced from osmotic measurements and from analysis, have been observed in molecularly dispersed systems also. In other words, the simple proportion between osmotic pressure and concentration, as demanded by theory, has not always been observed to hold even here. Thus W. M. Bayliss {I.e., 1910) cites the fact that the molecular weight of alcohol dissolved in benzene rises from 50 to 208, is quadrupled, in other words, in passing from a con- centration of 0.494 percent to one of 14.63 percent. It must be left to the students of the molecularly dispersed solutions to inter- pret these contradictions between their fundamental equation and its applications. But so far as the colloids are concerned, such a calculation of molecular weight from osmotic measurements can never be attempted with safety because it is wrong in principle. Not even the sense of the variations in the osmotic pressure of colloid solutions and their concentration need he the same in all cases. On p. 245 it was pointed out that, according to J. Duclaux, the osmotic pressure of iron hydroxide sol increases more rapidly than its concentration. Thus, while we generally observe an increase in the molecular weight with increasing concentration, due to the relatively smaller increase in the osmotic pressure, in the MECHANICAL PROPERTIES OF COLLOID SYSTEMS 265 example just quoted we observe a decrease in molecular weight, even to one-tenth the original. The "molecular weights" of acid and alkali albumin would even be found to yield complicated curves with maxima and minima related to the concentration of the added electrolytes. In fact, two or three entirely different concentrations of acid or alkali would be found in which the molecu- lar weights of the albumin, or its combination with an electrolyte, would be the same, thereby contrasting with the molecular weights observed in all other concentrations. Similarly, salts would affect the "molecular weight," making it either rise or fall, depending solely upon the concentration of the added electrolyte. Depending upon the acid or alkaline reaction of the colloid, the "molecular weight" of a colloid might be either raised or lowered on adding a salt. With rising temperature, the molecular weight of some colloids would be increased, of others decreased. The "molecular weight" of a colloid would be changed by shaking or stirring, by ageing and by being warmed either slowly or rapidly. These illustrations will suffice to demonstrate the impropriety of applying the ordinary concept of "molecular weight" to colloid solutions.^ It is hard to see how a "constant" which varies between several hundred and infinity with concentration alone, as in soap solutions, can be of any value in the physico-chemical char- acterization of a system. In this condemnation of the value of "molecular weight" determinations of colloidally dissolved substances by osmotic methods, ^ it is not maintained that there may not exist transition systems between colloidally and molecularly dispersed systems in which there is at least an approximate proportionality between osmotic pressure and concentration, and therefore a proper basis for the calculation of the molecular weight. In fact, W. M. Bayliss {I.e.) discovered such a system in congo red, freshly pre- pared by the method described above (see also W. Biltz and A. von Vegesack, I.e.). This dye when fresh and free from electrolytes, is highly dispersed, as evidenced by its ultramicro- scopic properties, its considerable osmotic pressure, etc. In this pure condition a 0.465 percent solution yields an osmotic pressure of 60 mm. of water. If now, by the formula given above, 1 See in this connection J. Duclaux, Compt. rend., 148, 714 (1909). 2 For the determination of the "molecular weight " of colloids by indirect methods, as by measuring the vapor tension, the boiling or freezing points, etc., see p. 142. 266 SPECIAL COLLOID-CHEMISTRY the molecular weight of the pure congo red is calculated, the answer is a value of 90 to 95 percent of that obtained by analytical methods (696.47). W. Biltz and F. Pfenning obtained similar results. This shows that pure congo red behaves like a typical molecular dispersoid, at least in its osmotic relations. The applicability of the above formula to the determination of the molecular weight of this dye is also evidenced by the direct proportionality existing between concentration and pressure, in other words, the constancy of the quotient ^, as evidenced m Table 49, on p. 244. In cases of this type, and only in such, are molecular weight determinations by this method justified. Moreover, the considerable electrical conductivity of pure congo red solutions as studied by W. Biltz arid A. von Vegesack further shows that we deal in this case with a molecular dispersoid rather than with a colloid, for high conductivity is not characteristic of typical colloids. 9. On the Moleculo-kinetic Theory of Osmosis in Colloid Systems. — In view of the successful applications that have been made of moleculo-kinetic conceptions to the quantitative study of the phenomena of movement exhibited in colloid systems, it may be asked if they may not also be of service in the theory of the osmotic pressure of these systems. A. Einstein and M. von Smoluchowski^ have considered this question. They conclude that the osmotic pressures of two equally concentrated hut differently dispersed phases are inversely proportional to the cubes of the radii of their particles."^ In other words, P2 IrO' This highly interesting conclusion has not yet been tested experimentally. It is of interest that the above conclusion was reached on the basis of considerations in which it was assumed that the Boyle- Gay-Lussac law (direct proportion between pressure and con- centration as well as absolute temperature) was valid. This assumption holds, of course, only at great dilutions. The Sved- ' M. von Smoluchowski, Boltzmann-Festschrift, 626, Leipzig, 1904. ' See The Svedberg, van Bemmelen-Gedenkboek, 131, 1910. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 267 berg^ tried to determine indirectly the validity of the Boyle-Gay- Lussac law for colloids. An equation governing local changes in the motion of particles showing Br ownian movement, so far as extent and frequency are concerned, may be derived from the equation of von Smoluchowski {I.e.). A detailed exposition of this second equation and the considerations leading to it cannot be given here. The Svedberg, however, found highly diluted gold and mercury sols to obey it. He concluded, therefore, that the Boyle-Gay- Lussac law used in deriving the formula would also have to be valid for greatly diluted colloid systems. It is perhaps too early to concur entirely in this conclusion, since the number of mathe- matical assumptions in the formula is exceedingly great. Be- sides, Svedberg's figures (see especially their graphic represen- tation on p. 555 of his paper. I.e., 1910) themselves show that the law holds strictly only at a transition point, for only in very dilute concentrations is there strict agreement between observa- tion and theory. Deviations from the rule, and therefore from the Boyle-Gay-Lussac law, begin to appear in the case of a mercury sol as soon as its concentration amounts to }y^.io~^° normal, or about 0.000,000,000,3 percent by weight. In view of the slight practical significance of the concentration range over which it is valid, the law appears to be an ingenious theoretical deduc- tion rather than a means of studying quantitatively the depend- ence of osmotic pressure in colloid systems on concentration and temperature. Addendum: Other Types of Movement in Dispersoids The phenomena of movement observed in colloid systems under the influence of an electric current will be discussed at another time. At this point we merely wish to mention the phe- nomena of movement which occur under the directive influence of heat and light. The botanists F. StahP and W. Sachs^ ob- served such directed movements in small solid and liquid particles while attempting in 1876 to determine to what extent the thermal and heliotropic movements of unicellular organisms (such as zoospores) depended upon biological properties and in how far they were merely passive. The directive influence of light on the 1 The Svedberg (I.e.), as well as Zeitschr. f. physik. Chem., 73, 547 (1910). 2 F. Stahl, Bot. Ztg., 715 (1876); Verh. d. phys.-med. Ges. Wurzburg, 14 (1879). ' W. Sachs, Flora, 241 (1876). See also E. Strassburger, Jenaisch. Z. f. Naturw., 12 (1878). 268 SPECIAL COLLOID -CHEMISTRY movement of dispersed particles was later studied in detail by G. Quincke.^ W. R. Whitney and C. J. Blake^ have studied such light and heat effects on the movement of colloid particles in col- loid gold. The directive influence of light on crystallization and sublimation should also be mentioned here.^ §31- Filtration and Ultrafiltration of Colloid Systems I. Filtration of Colloid Systems. — A property which dis- tinguishes colloid solutions from coarse suspensions is the ability of the former to pass unchanged through filter paper. It is by this means that we recognize the formation of a colloid solution when we wash a precipitate with pure water. While typical colloids pass through all filter papers, somewhat coarser systems begin to be held back by hardened filter papers and by clay and porcelain filters as those ofBerkefeld, Reichel, Chamberland and Pukall. The filtrability of a dispersoid depends upon the size, shape and rigidity of its particles, upon the filtration pressure and the nature of the filter, more especially the size of its pores.* To determine the approximate size of the dispersed particles it is therefore well to know the average size of the pores of different filters. Such determinations, as made by H. Bechhold,^ are given in Table 60. Table 60. — Size or Pores in Filters (According to H. Bechhold) Filter Average size of pores (permeability to water) Size of largest pores (permeability to air) Ordinary thick filter paper Filter paper No. 556 (Schleicher and Schiill).. Filter paper No. 602 (extra hard, Schleicher and SchuU). Chamberland iilter 3 -Si" I-7M 0.89-1.3M i-i.Sm 0.23-0.4IM 0. i6-o.i75(U Reichel iilter . . ' G. Quincke, Report Brit. Assoc. Advanc. Science, Glasgow, 60 (igoij; Drude's Ann. d. Physik., 7, 701 (902). ^ W. R. Whitney and C. J. Blake, Journ. Amer. Chem. Soc, 26, 1347 (1904). ^ See the summary of J. M. Eder, Photochemie, 3 Aufl., 123, Halle, 1906. ■■ Details regarding this question may be found in the paper of E. Hatschek, J. Soc. Chem. Industry, 27, 538 (1908); also Koll.-Zeitschr., 6, 254 (1910); 7, 81 (1910). ' H. Bechhold, Zeitschr. f. physik. Chem., 64, 328 (igS). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 269 The original paper must be consulted for details of the methods used by Bechhold in arriving at the assigned values. As the table shows, typical colloids, with particles having a diameter of less than o.i//, must be just able to pass through the filter lowest in the list. But even with pores of this size by- effects, known as "adsorption" effects, often appear, due to the action of the filter itself upon the dispersed phase. These lead to retention of the dispersed phase and so to a clogging of the pores of the filter. At other times coagulation processes occur due to this surface action. Whenever any of these things take place, filtration cannot, of course, any longer tell us anything definite regarding the size of particles in a dispersoid. 2. Ultrafiltration of Colloid Systems. — After W. Schmidt^ and F. Hoppe-Seyler^ had found that solutions of albumin and of gum became more dilute by being filtered through animal membranes, C. J. Martin' discovered that colloidally dissolved materials could be completely separated from their dispersion medium by being filtered through organic or inorganic gels. To give these a proper support he used Chamberland filters and impregnated them with gelatine or silicic acid. He could then filter liquids under 30 to 100 atmospheres of air pressure without breaking the filter. By using this method, he was able to separate from the albumin a clear fluid containing salt but entirely free of protein. Table 61 contains the more important of his results. Such filtration through gels was next used by French investi- gators (Borrel and Manea, 1904; G. Malfitano, 1904; J. Duclaux, 1905)^ to separate the dispersed phase from the dispersion medium in different organic and inorganic colloids. They usually em- ployed collodion thimbles as filters. H. Bechhold" took an important step forward in this problem of filtration when in 1906 he discovered the permeability of gels to be a function of their concentration. He found dilute gels to be more permeable than more concentrated ones. For details re- 1 W. Schmidt, Poggendorf's Ann., 337 (1856). ^ F. Hoppe-Seyler, Virchow's' Arch., 9, 245 (1861). ' C. J. Martin, Journ. Physiol., 20, 364 (1896); see also E. \V, Reid, ibid., 27, 161 (1903); A. Craw, Zeitschr. t. physik. Chem., 52, 569 (1898); Proc. Roy. Soc, 77, 172, ill (1899). ^ For the history of filtration through gels see J. Duclaux, KoU. -Zeitschr., 3, 134 (1908); also H. Bechhold, ibid., 3, 226 (1908). ' H. Bechhold, Z. f. Elektroch., 12, 777 (1906); Koll. -Zeitschr., i, 107 (1906); 2, 3 (1907); Zeitschr. f. physik. Chem., 60, 237 (1907); 64, 328 (1908). 270 SPECIAL COLLOID-CHEMISTRY garding his methods his original publications must be consulted. As a rule, he used ordinary filter papers as a foundation, impreg- Table 61. — Filtration through Clay Cells Impregnated with Silicic Acid (According to C. J. Martin) Impermeable to Partially permeable to Readily permeable to Egg albumin. Alkali albumin. All albumoses. Serum albumin. Acid albumin. Urochrome (pigment of Egg globulin. Caramel. urine). Serum globulin. Biliverdin (bile pigment). All crystalloids. Fibrinogen. Dextrin. Caseinogen. Nucleoalbumin. Hemoglobin. Glycogen. Soluble starch. Soluble starch (amylodextrin) Table 62. — Ultrafiltration (According to H. Bechhold) The disperse phase is held Dispersoid back by a following gelatine gel of the concentration in Remarks percent Platinum sol (Bredig) 2 Average size of particles about 44MM (Zsigmondy). Colloid iron hydroxide 2 Casein (of milk) 2.S Colloid gold containing so- 3 About 40iifi. dium lysalbinate (Zsig- mondy). CoUargol (v. Heiden) 3-S About 20MM. I percent hemoglobin solu- 4 tion. I percent gelatine solution. . . 4 Serum albumin 4-4 -S Molecular weight — 15,000 down to 3000. Protalbumoses '. . . . 4-S Deutero-albumoses A 8 Molecular weight about 2400. Deutero-albumoses B and C. 10 Traces pass through. 10 Small amounts pass through ; molecular weight about 965. All rrvstalloids Pass through. MECHANICAL PROPERTIES OP COLLOID SYSTEMS 271 XSn \ T -I — I -I — I nating them with various gels, as acetic acid-collodion, gelatine- formaldehyde, etc. Table 62 gives a survey of his results. It is evident that the filter becomes less permeable as the concentration of the gel rises. A hardened 10 percent gelatine niter holds back even molecules of the size of those contained in dextrin. A proper series of filters makes it possible to distinguish, within the range of colloid dispersion, between systems of different degrees of dispersion, and these are then found to correspond with a differentiation between them made on optical grounds. For this reason H. Bechhold has named his method "UltrafiHration.'" Recently A. Schoep^ has described a simple method of ultrafiltration, in which is eliminated the disad- vantage of having to work with high pressures.^ He found that filters of different degrees of per- meability could easily be made by adding to collodion solutions different amounts of glycerine and castor oil. Dialyzing thimbles may be made from such mixtures by the methods described in the practical introduction on p. 10. The dry collodion thimbles become progressively more permeable (within certain limits) as the amount of glycerine or castor oil in them is increased. Fig. 63 illustrates Schoep's simple method. We cannot advantageously discuss the theory of this variable permeability of gels of different concentrations until we have treated their general structure. In conclusion, it must be mentioned that undesirable by- effects, such as adsorption of the disperse phase by the filter, occur in ultrafiltration, also. Ultrafiltration yields dependable results, therefore, only if checked up by other methods. 'A. Schoep, Bull. Soc. Chim. Belg., 24, 354 (1910); Koll.-Zeitschr., 8 (igii). See also A. von Lebedew, Zentralbl. f. Physiol., 23, 767; 24, 511 (1910). ^ Emulsoids may be separated from their dispersion medium, with Schoep's filter, only when pressure is used. Fig. 63. — A. Schoep's arrangement for ultra61tration. AUTHOR INDEX Adamson, L., 237, 238 Albanese, V., 156 Alexander, J., no Alexandrow, N., 131, 142 Allen, 74 Amagat, E. H., 117, 120 Amann, J., i, 60, 6g, 232 Ambronn, H., 65 Antonow, G. N., 189 Arrhenius, S., 94, 142, 179, 219, 223, 258 Avogadro, 213, 259 Axelrod, S., 156 B Bachmetjew, Z., 91, Baker, F., 157 Bancelin, M., 153 Barus, C, 118, 119, 120 Baumhauer, H., 97 Bayliss, W. M., 137, 238, 244, 247, 250, 253, 260, 263, 264, 265 Bechhold, H., 5, 268, 269, 270, 271 Beck, K., 181, 183, 184 Beer, ± Behrens, H., 62 Bemmelen, J. M. van, 23, 87, 88, 89, 106, 124, 130, 136 Berkefeld, 268 Berthelot, 106 Berzelius, J., 52 Beyerinck, M. W., 138, 183 Bigelow, L., II Bigland, D., 238 Biltz, W., II, 140, 156, 157, 158, 159, 161, 162, 167, 168, 169, 182, 230, 232, 238, 240, 241, 243, 244, 245, 246, 247, 248, 254, 263, 265, 266 Blake, J. C, 131, 13S. 225, 226, 268 Bodaszewski, L. J., 196 Bodenstein, M., 94 Bodlaender, 94 18 273 Borrel, 269 Bose, M., 183 Botazzi, F., 131, 146, 147, 148, 149, 151, 156. 173. 186, 189 Bousfield, W. R., 210 Boyle, 266, 267 Brauer, 160 Bredig, G. 24, 89, 92, 93, 135 Broglie, M. de, 196, 198 Brown, H.T., 131, 132 Brown, R., 191 Bruni, G., 130 Bruyn, Lobry de, 27, 139 Bugarsky, St., 131, 143, 262 Buglia, G., 181, 183, 186, 189 Burnett, Th. C, 131, 141, 142 Biitschli, 0., 62, 106 Buxton, B. H., 229, 230 Calcar, R. P. von, 228 Cavazzani, E., 156 Chamberland, 268 Chaudesaigues, P., 198, 199, 206 Chick, H., 156, 1 57, 178 Cholodny, P. J., 121, 135 Cohen, E., 94 Cotton, A., II, 58, 64 Coudres, Th. Des., 92 Craw, A., 269 Curie, P., 90 D Dabrowski, 206 Daguin, io5 D'Errico, G., 131, 147, 148, 149, 151 Denning, Du Pr6, 156 Doelter, C, 57 Doerinckel, Fr., 140 Donau, J., 76, 13s Donnan, F. G., 68, 83, 84, 183 Drucker, K., 54, 75, 112, 145 Drude, P., 77 2 74 AUTHOR INDEX Duclaux, J., II, 131, 137, 210, 238, 240, 244, 24s, 246, 247, 248, 260, 264, 26s, 26g DuUbergj P., 131 Du Pr6 Denning, 156 Ebbinghaus, K., 181, 183 Eder, J. M., 268 Eduardoff, F., 140 Ehrenhaft, F., 43, 196 Einstein, A., 133, 211, 213, 214, 221, 222, 223, 224, 266 Engelmann, W., 64 Erb, W., s6 Errico, G. d', 131, 147, 148, 149, 151 Exner, F. M., 197, 201, 203 Exner, S., 218, 222, 223 Fano, G., 156 Faraday, M., 52 Fichter, F., 15 Fick, A., 215 Fischer, M. H., 94, iii Flemming, W., 156, 162 Frankenheim, M. L., 57, 62, 183 Free, E. E., 14 Frei, W., 156, 186, 189 Freundlich, H., 50, 55, 67, 68, 94, 100, 112, 178, 188, 230 Frey, W., 177 Friedenthal, H., 131, 132 Friedlander, J., 7, 55, 56, 105, 146, 182, 184 Fuchs, C., 68, 195 Galdi, F., 156 Galeotti, G., 106, 178 Gansser, 238 Garrett, H., 140, 156, 161, 162 Gatin-Gruszewska, Z., 131, 132, 142, 143, 147 Gaunt, R., 157 Gay-Lussac, 247 Gefifcken, G., 136 Genthe, A., 146, 148 Giampalmo, G., 178 Gibbs, W., 77, 106, 189 Gilbaut, H., 116, 117, 120 Gladstone, J. H., 131, 142 Gokun, 156, 159, 161, 169 Goldsborough, 136 Gouy, G., 194, 19s, 196, 211 Graham, Thomas, 9, 24, 31, 39, 40, 75, 99, 14s, 216, 217, 219, 22s, 227, 228, 229, 232, 236 Groschuff, E., 232 Guinchant, 116 Guthrie, F., 130 H Haber, F., 68 Hamburger, H. J., 94 Hammarsten, O., 56 Handovsky, H., 156, 160, 174, 17s, 176, 177, 261 Hantzsch, A., i Hardy, W. B., 156, 173, 179 Hartl, F., 140 Hatschek, E., 5, 92, 138, 153, iS4i 162, 163, 178, 183, 268 . Heen, M. P. de, 127 Heidenhain, M., 68 Henri, V., 50, 156, 191, 192, 198, 199, 205, 206, 212 Herz, W., 94 Herzog, R. O., 219, 223 Heyer, R., 228 Hibbert, W., 131, 142 Hober, R., 54, 94, 230, 256 Hoff, J. H. van't, 68, 84, 128, 133, 142, 144, 243, 258, 264 Hoffmann, F., 105 Hofmeister, 172 Holde, D., 46, 103, 183 Hooker, M. O., iii Hoppe-Seyler, F., 269 Hiifner, G., 218, 238 Hulett, G., 74 Hulshof, 115 Humphrey, E., 154, 162 Huth, M. E., 112 Jahn, St., 192 Johanott, Phil., 77 Just, J., 103 AUTHOR INDEX 275 K Karsten, 125 Kasarnowski, H., 219 Kassel, R., 146 Kaufler, F., 95 Kelvin, Lord, 138 Kirchof, F., 157 Kohnstamm, 77, 113 Konowalow, D., 130, 141 KiJrner, T., 131 Kossonogow, J. R. von, 70 Krafft, F., 129, 131, 142, 143, 229, 230, 232 KruUa, R., 62 Kruyt, H. R., los, 106 Kuenen, J. P., 108 Laar, J. J. van, 237 Lallemand, 106 Lalou, is6 Laqueur, E., 156, 173, 177, 179 Lebedew, A. von, 271 Lecoq, 205, 206 Lehmann, O., 57, 62, 64, 65, 68, 69, 79, 191, 192, 194, 196 Lemoine, G., 104 Levites, S. J., is5, 157, 158, 164, 165, 166, 169, 177 Lewis, Wm. C. McC, 92, 189, 190 Liebermann, L., 131, 143, 262 Liesegang, R., 93 Lillie, R. S., 145, 238, 239, =41, 248, 249, 2SO, 251, 252, 253, 2SS, 256, 257, 258, 261 Linder, S. E., 34, 124, 125, 130, 131, 136, 141, 186, 187, 221,. 224, 22s, 229, 233. 234, 237 Linebarger, C. E., 131, 237 Link, 62 Ljubavin, N., 131 Lobry de Bruyn, C. A., 2,7, 139 Lodge, O., gi Loeb, J., 8s Loffler, B., 121 Lorenz, R., 103 Lottermoser, A., 131, 238 Ludeking, Chr., 122, 126, 130 Liippo-Cramer, 93, 97, 140 Luther, 145 M Madsen, Th., 219, 223 Malfitano, G., 131, 228, 238, 269 Maltezos, C, 196 Manea, 269 Martici, A., 181 Martin, C. J., 136, 237, 238^ 247, 269, 270 Maxwell, 68 Mayer, A., 156 Mayer, H., 28 Mcintosh, 95 Mecklenburg, W., 31, 194, 204, 211 Mellor, J. W., 94 Mensbrugghe, G. van der, 68, 196 Metz, G. de, 117, 118 Meyer, W., 131 Michaelis, L., 35, 68, 85, 86, 156 Michel, 131 Mittasch, A., 106 Molisch, H., 192, 194, 196 Moore, B., 237, 238, 246, 247, 248,, 253 Morris, G. H., 131, 132 Moruzzi, G., 131, 156 Mostynski, B., 156 Mouton, H., II, 58, 64 MuUer, A., 52, 156 Miiller-Thurgau, 91 Mylius, F., 232 N Nernst, W., 94 Neuberg, C, 98 Neumann, W., 50, 56, 100, 178, 188, 230 Noyes, A. A., 50, 51 O Odfo, Sven, 136, 149, 150, 154 Oettingen, H. von, 7 Oholm, L. L., 223 Oker-Blom, M., 226, 227 Oppenheimer, 94 Ostwald, Walther, no, 138 Ostwald, Wilhelm, 3, 28, 60, 62, 63, 66, 68, 73, 74, 91, 93, 94, 9S, io3, 105, 112, 121, 127, 139,. 142, 145, 181, 215, 216, 217, 224, 236 276 AUTHOR INDEX Ostwald, Wolfgang, 23, 24, 25, 39, 46, so, 52, 68, 71, 82, 102, 107, 109, no, III, 146, 148, 162, 181, 183, 1S4, 185, 221, 252, 261 Paal, C, 100 Pappad^, N., 130, 131 Parker, W. H., 237, 238, 246 Paterno, E., 131 Pauli, Wolfgang, 40, 156, 169, 173, 174, 17s, 176, 177, 179, 225, 226, 253, 256, 261, 262 Pawlow, P., 91, 106, 108 Pelet, L., 60 Perrin, J., 30, 50, 51, 68, 153, 192, 194, 19s, 197, 199, 204, 20s, 206, 207, 208, 209, 210, 211, 213, 214 Pfeffer, W., 237, 243, 244, 247, 258 Pfenning, F., 230, 266 Pickering, S. U., 46, 138, 183 Picton, H., 34, 124, 125, 131, 136, 141, 186, 187, 221, 224, 225, 226, 229, 233, 234, 237 Pieroni, A., 93 Pockels, A., 77, 186 Posnjak, G., 104, 144 Posternak, S., 256 Potts, H. E., 183 Prange, A. J., 134 Preuner, G., 229, 230 Procter, H., 177 Pukall, 268 Quincke, G., 52, 62, 63, 64, 107, 117, 124, 125, 126, 186, 187, 196, 268 Quincke, H., 122 R Raflfo, M., 93, 136, 149 Ramsden, W., 186 Rankin, 106 Raoult, F., 128 Rayleigh, Lord, 77, 85, 186 Reg&zy, E. von, 224, 226 Reichel, 268 Reid, E. W., 238, 241, 246 Reissig, J., 134 Reynold, 77 Richter, B. J., 52, 59 Ringer, W. E., 156 Roaf, H. E., 237, 238, 247, 248, 250, 254 Robertson, T. B., 95, 131, 141, 142, 143. 183 Rodewald, H., 122, 123, 127, 143 Rohland, P., 100 Rontgen, W., 116 Rose, G., 121 Rossi, G., 156 Rothe, R., 105 Rothmund, V., 182, 183, 184 Rotinjanz, L., 105 Rucker, 77 Sabanejew, A., 130, 131, 142 Sachs, W., 267 Sackur, O., 156, 173, 177, 179 Sahlbom, N., 15, 16, 156, 187, 234, 235 Samec, 156 Scala, A., 69, 72, 135, 137 Scarpa, O., 156 Schade, H., 36, 79, 94, 107, 108 Scheffer, G., 219 Schenck, R., 183, 189 Schidrowitz, P., 156 Schmidt, C, 62 Schmidt, W., 122, 126, 269 Schneider, 116 Schneider, J., 103 Schoep, A., 271 Schorr, K., 156, 160 Schroeder, 185 Schroeder, P. von, 156, 157, 158, 161, 164, 165, 169, 170, 171, 172, 173, 261 Schutt, 77 Seddig, M., 198, 203, 204 Siedentopf, H., 27, 29, 58, 87, 88, 106, 198, 199, 200 Simon, J., 156, 159, 160, 183 Smith, A., 105 Smits, A., 129, 130, 131 Smoluchowski, M. von, 153, 154, 179, 211, 213, 214, 221, 222, 223, 224, 266, 267 Spring, W., 243 Stahl, F., 267 AUTHOR INDEX 277 Starling, E. H., 237, 238 Stas, 97 Stefan, 219 Steiner, H., 159, 161, 162, 167, 168, 169, 170, 171, 172, 173 Steinwehr, H. von, 92 Stodel, 156 Stokes, G., 209, 210, 212, 214 Stoltzenberg, H., 112 Strassburger, E., 267 Strutz, A., 143 Suzuki, S., 232 Svedberg, The, 31, 76, 99, 133, 140, 194, 19s, 197, 198, 1997 200, 201, 202, 203, 204, 205, 206, 211, 212, 218, 219, 221, 222, 223, 267 Tammann, G., 107, 130, 131, 141 Teague, O., 229, 230 Teletow, J., 92, 93 Thomson, J. J., 95, 206, 210 Traube, Moritz, 164 Traube-Mengarini, M., 60, 69, 72, 13s, 137 V Vanino, L., 134, 140 Vegesack, A. von, 11, 140, 156, 158, 182, 238, 240, 242, 243, 244, 245, 246, 247, 248, 254, 263, 26s, 266 Victorow, C, 156, 173, 186, 189 Vignon, L., 219, 230 Vogelsang, H., 62 Voightlander, F., 217, 226 Vries, H. de, 218 W Waals, van der, 77, 115 Wagner, R., 156 Washburn, G. H., 31 Weber, C. O., 156 Wedekind, E., 97 Weimarn, P. P. von, 24, 33, 45, 54, 57, S8, 59, 60, 61, 62, 63, 6s, 89, 90, 91, 92, 97, 99, 100, loi, 102, IDS, 107, 108, no, 139 Weinmayr, J., 185 Wenzel, 93, 96, 97 Whitney, W. R., 131, 135, 225, 226,, 268 Wiener, Chr., 194, 19s, 196, 201, 211 Wigand, A., 106 Wild, A., 60 Wittich, J. von, 226 Wolff, L. H., 27, 139 Woudstra, H. W., 147, 148, 149, 151,, 152, 159, 160 Wullner, A., 128 Zangger, H., 161, 237 Zeiss, 200 Zirkel, F., 62 Zlobicki, L., 186, 187 Zoja, L., 174, 177 Zsigmondy, R., 27, 29, 30, 32, 42, 50, 52, 58, 76, loi, 13s, 140, 191, 193,. 194, 202, 215, 223, 228 SUBJECT INDEX Acid, arsenious, 103 Adds, and viscosity, 174, 175; and Brownian movement, 205; and osmotic pressure, 250 Adsorption, 95, 185, 239, 271 After-effects, 248 Agar-agar, viscosity of, 164 Age, and viscosity of suspensoids, 152; and viscosity of emulsoids, 157 Albumin, viscosity of, 175; diffusion of, 142, 224, 226, 250; osmotic pressure of, 252, 254, 255; fil- trability of, 270 Alcohol, and viscosity of gelatine, 159; and viscosity of emulsoids, 159 Alcohol-sol, 41 Alkalies, and emulsoids, 160; and viscosity, 174, 175; and Brownian movement, 205; and osmotic pres- sure, 250 Allocolloids, 103 Allotropism, 105 Analysis, colloid, 3, 12, 16; capillary, 15, 234 Arsenic trisulphide, 225 Arsenious acid, 103 Associated liquids, 3, 103 Avogadro's constant N, 213, 259 B /3-Gelatine, 164 Beer's law, i Behavior of electrified sulphur, 69 Benzopurpurin, 158; osmotic pressure of, 242 Boiling point, 128; of colloids, 130 Boundaries, 21 Brownian movement, 191; char- acteristics of, 191; independence of, 194; measurement of, 197; photography of, 198; rotary motion of, r99; uniformity of, 200; velocity of, 200; Svedberg's law of, 200; and specific surface, 201; and concen- tration of dispersoid, 202; and vis- cosity, 202, 203; and temperature, 203; and added substances, 205; of rubber, 205; and electrical charge, 205; and gravity, 206; and Stokes' law, 209; kinetic theory of, 210; and molecular weight, 214 Calcium, colloid, 202 • Capillary analysis, 15, 234 Capillary phenomena, 72 Capillary pressure, 91 Caramel, 245 Casein, 142, 178, 270 Castor oil, 184 Catalysis, 94 Cellulose, 181 Chalk-sacs, 192 Chamberland filter, 268 Chemical energy, and specific surface, 93 Chemical heterogeneity, 22 Cinematograph, 198 Classification, of Zsigmondy, 29, 34; of disperse systems, 29, 33 Closed phase, 25 Clotting, 40 Coagulation, 40 Coalescence, 63 Collodion, 271 Colloid analysis, 3; special, '12; outline of methods of, 16 CoUoid-chemical nomenclature, 40 Colloid ice, 106 Colloid metals, 32, 202, 212, 260 Colloid solution (see also colloids and colloid systems), 4; differentiation of, from true, 6, 9; optical properties of, 6; vapor tension of, 128; boUing 279 280 SXJBJECT INDEX point of, 130; freezing point of, 131; saturation in, 134 CoDoid state, 2, 14; and independence of chemical composition, 2; theory of, 3, 21; concept of, 21, 99; universality of, 99 Colloid systems, reversible and ir- reversible, 40; volume and density, relations in, 115, 120; concentra- tion-variable and complex, 136; supersaturation in, 138; viscosity of, 145; dialysis of, 227; osmosis of, 236 CoUoidality, 32 Colloids, recognition of, i; diffusion of, 9, 10, 142, 215, 219, 224, 226, 250; suspension and emulsion, 12; lyo- phiUc and lyophobic, 13, 51, 52; coagulation of, 13, 16; viscosity of; 13; electrical properties of, 14, 15; mutual precipitation of, 16; as disperse heterogeneous systems, 23; specific surface of, 28; character- istics of, 39; nomenclature of, 39; thermal coefficient of expansion in, 126; molecular weight of, 140, 263; effects of temperature on, 150; surface tension of, 185; movement in, i9i;diffusioncoefficientsof, 219; osmotic pressure of, 237; filtration of, 268, 270 Complex dispersoids, 36, 136 Concentration, and disperse systems, 35; effects of, 47, 48, 49, 136; and viscosity, 165, 183 Concentration-variable systems, 36, 136 Condensation, 87; theory of, 88 Congo red, 244, 260, 265 Copper ferrocyanide, 245 Cosmic dust, 43 Critical mixtures, 37, 82, 105 Crystal formation, 59, 62 Crystalline constitution of colloids, 56 Crystallinity, concept of, 56; of col- loids, 58; theory of, 58 Crystallization, 56, 62 Crystalloids, characteristics of, 39, loi Crystals, liquid, 62 Cube, increase in surface with division of, 27 D Degree of dispersion, 4, 26; and dif- fusion velocity, 220 Density and colloids, 1 24 Determination of osmotic pressure of colloids, 263 Dializability, 231 Dialysis, 10, 227, 228, 229 Dialyzers, 229 Diffusibility, 215, 216 Diffusion, 9; apparatus, 10, 216; co- efficients, 219; of colloids, 215, 216, 222; of serum albumin, 227 Diminution of surface, 84 Discontinuity of matter, 96 Disintegration tension, 82 Disperse phase (see also disperse systems), 25 Disperse systems, 24, 32; classification of, 29, 33, 42; ionic, 31; of gold, 32; concentration-variable, 35 ; tem- perature-variable, 36; complex, 36; solid -|- solid, 43; solid -f- liquid, 43; solid -|- gas, 43; effect of con- centration on, 45; energetics of, 66; effects of electrolytes on,. 174 Dispersion, 4, 26, 31, 33, 77; and viscosity, 181 Dispersion medium, 25 Dispersions, 31 Dispersoids (see disperse systems) Droplet formation, 86 Dust cosmic, 43 Dyes, surface tension of, 188; dialysis- of, 230 Dynamic surface tension, 67, 190 E^-alBumin (see also albumin), (14 '■ 250' \> EinHein-Smoluchowski formula, 211 Electrical charge (see also electrolytes), and viscosity, 179; and Brownian movement, 205 Electrical energy and specific surface,. 92 Electrical fountain, 68 Electrical heart, 68 ' SUBJECT INDEX 2«I Electrolytes, effects of , on viscosity, 151; and viscosity of suspensoids, 151; and gelatine, 158; and colloid diffusion, 224; and osmotic pressure of colloids, 250 Electrophoresis, 16 Emulsoids, 12; general properties of, 49, 34, 124; crystallinity of, 64; and suspensoids, 124; viscosity of, iSS, 157, 161, 164, 165, 168, 169, 173; and alcohol, 159; and alkali, 160; theory of viscosity of, 178; filtrability of, 271 Emulsion colloids (see emulsoids) Emulsions, 5, 183 Energetics of dispersoids, 66 Energy, surface, 74, 77; and specific surface, 92, 93, 97 Expansive surface tension, 68, 69, 70 Ferments, 94 Filter paper, 15; and capillary analysis, 23s, 268 Filters, 5; ultra, 12, 268 Filtrability, 268 Filtration, 5, 268 Fluid mixtures, critical, 105 Fluorescence, 8 Foams, in Fog, 43 Formation, of crystals, 59; of droplets, 87 Formula, of Hatschek, 153; of Gibbs, 189; of Einstein-Smoluchowski, 211; of Svedberg, 200 Freezing point, 128; of coUoids, 131 Gas + gas dispersions, 43 Gas + liquid dispersions, 43 Gas + solid dispersions, 43 Gelatine, no, 139, 159, 165, 270; viscosity of, 157, 158, 164, 165, 171, 173; surface tension of, 187, osmotic pressure of, 241; thermal history of, 247; and acids and alkalies, 173, 250; swelling of, 252, 261; as filter medium, 269 Gelation, 40 Gels, 24, 40; permeability of, 269 Gibbs' theorem, 189 Glycogen, 142, 178 Gold, 32, 222, 225, 260 Gravity and Brownian movement, 206 Gum arable, 142, 244, 245 Gutta percha, 182, 192, 19s, 205, 209 H Hatschek's formula, 153 Heat, 248, 267 Hemoglobin, 246, 262 Heterogeneity, 3, 21, 23; concept of, 21; chemical and physical, 22 Hofmeister series, 172 Homogeneous liquids, 3 Hydrates, 39 Hydrogels, 41 Hydrosols, 41 Hylotropic changes, 3 Hysteresis, 248 Ice colloids, 106 Ice cream, no Increase of surface, 78 Independence of Brownian movement, 194 Instability of mechanical suspensions, S; of osmotic pressure of colloids, 240 Internal friction (see viscosity) Ionic dispersoids, 31 Ionic series, 172; and osmotic pressure of colloids, 254 Iron hydroxide, 233, 234, 245, 270 Iron nitrate, capillary analysis of, 23s Isocolloids, 4, 8, 102; of water, 107 Isodispersoids, 4, 8, 102, 107 Isomeric compounds, 3 K Kinematograph, 198 Kinetic theory and Brownian move- ment, 207 2»2 SUBJECT INDEX Latex, 205 Law, Beer's, i; Wenzel's 96; mass, 142; Svedberg's, 200; Stokes', 209; van't Hoff's, 264 Light, and Tyndall effect, 7, 8; and colloid movement, 267 Light cone of Tyndall, 7, 8 Liquid crystals, 52 Liquid + gas dispersions, 49 Liquid + liquid dispersions, 48 Liquid + solid dispersions, 48 Liquids, heterogeneous and homo- geneous, 3 Lyophilic colloids, 13, 52 Lyophobic colloids, 13, 52 M Mass law, 142 Mastic, 209 Masticized rubber, 182 Matter, discontinuity of, 96 Measurement of osmotic pressure of colloids, 238 Mechanical suspensions, 4; instability of, 5 Membranes, liquid, 87, osmotic, 237 Metameric compounds, 3 Metastyrol, 3, 104 Methods of studying diffusion, 216; of dialysis, 228 Microns, 29 Milk, 181, 192 Minerals, 43 Mixtures, critical, 37, 82, 105 Molar surface energy, 3 Molecular dispersions, 4, 31, 54; viscosity of, 145 Molecular weight of colloids, 140, 142, 263; and Brownian move- ment, 214 Moleculo-kinetic theory of osmosis, 266 Movement in colloids, 191 N N. (Avogadro's number), 213, 259 Nicol prism, 8 Night-blue, 140; viscosity of, 158, 163; osmotic pressure of, 248, 252,254 Nomenclature, colloid-chemical, 39. Normal liquids, 4 Nucleus, 138 O Oil, S, 183 Optical behavior of colloids, 6, 58 Osmosis (see also osmotic pressure), 236; kinetic theory of, 266 Osmotic pressure, of benzopurpurin, 242; of night-blue, 248, 252, 254; of gelatine, 250, 255; of albumin, 252, 254; and molecular weight, 263 Osmotic pressure of colloids, 144, 237, 254; instability of, 240; and pre- vious treatment, 241; and shaking, 241; and stirring, 242; and time, 242; and concentration, 243; and temperature, 247; and added sub- stances, 249; and acids and alkalies, 250; theory of, 258 Particles, size of, 30 Peptization, 40 Permeability of filters, 268; of gels, 269 Phase rule, 105 Phases, 22; closed, 25; disperse, 25; physical state of, 42 Phosphorus, 104 Photography of Brownian movement, 198 Physical heterogeneity, 21 Platinum, 94, 202, 212 Polydisperse systems, 35, 54, 138 Polydispersoids, 35, 54 Polymeric compounds, 3 Polysuspensoids, 54 Pores in filters, 268 Practical introduction, i Precipitation, 40 Pressure, capillary, 91; osmotic, 237, 238 Prism, Nicol, 8 SUBJECT INDEX 283 Protective action, s Proteins, no, 139, 142, 270; viscosity of, 156, IS9, 162, 165, 171, 173; and acids and alkalies, 175; surface tension of, 187; diffusion of, 227; osmotic pressure of, 241; thermal history of, 247; and acids and alkalies, 171, 250; swelling of, 252, 261; as filter medium, 269 Pukall filter, 268 R Radiant energy, 97 Radio-activity, 98 Recognition of colloids, i Reichel filters, 268 Reversible systems, 40 Rosin, ss, 184 Rubber, 182, 192, 195; masticized, 182; Brownian movement in, 205 Rule of Gibbs, 189; of Svedberg, 200; of Einstein-Smoluchowski, 211 Sacs, diffusion, 10; chalk, 192 Salol, 60 Selenium, 104 Serum-albumin, viscosity of, 175; dif- fusion of, 227 Serum-protein, 178 Shear, 154, 162 Silicic acid, as filter medium, 269 Silver, 75, 19S; viscosity of, 152 Size of particles, 30 Smoke, 43 Smoluchowski-Einstein formula, 211 Soap, 143, 183, 262 Solid -|- gas dispersoids, 43 Solid + liquid dispersoids, 43 Solid -f- solid dispersoids, 43 Sols, 24, 60; alcohol, 41; water, 41; sulphuric acid, 41; sulphur, 149 Solubility, of salol, 60; of silver chloride, 75 Solutions, true, 4, S4I colloid, 4, 6, 9; molecular-disperse, 4, 6, 9; optical behavior of, 6; super- saturated, 33, 60; vapor tension of, 128; boiling point of, 130; freezing point of, 131; saturation in colloid, 134. I'SS; supersaturation in colloid, 138 Solvates, 38, S4 Special colloid-chemistry, 115 Specific surface, 26, 72, 90, 92, 93; of colloids, 29; electrical energy and, 92, 93; chemical energy and, 93; and Brownian movement, 201 Starch, 154 State, colloid (see also colloid state) 2, 14; theory of, 3, 21; concept of, 99; universality of, 99 Stokes' law, 209 Strong coUoidality, 32 Styrol, 104 Submicrons, 29 Sulphur, 60, 104, 105, 149; viscosity of, 150 Supersaturated solutions, 33 Surface, discontinuous diminutions in, 84 Surface energy, molar, 3; of first order, 66, 74; of second order, 67, 74;, and other energies, 71; reciprocal effects of two kinds of, 79, 80 Surface increase, 78 Surface tension, 61; dynamic, 67, 190; static, 67, 190; negative, 68; expansive, 68; properties of, 69, 70; and specific surface, 76; of colloids, 118; of gelatine, 187; of dyes, 188 Surfaces, 21, 27; in colloids, 29 Suspension colloids, 12; general properties of, 49, 54, 124; viscosity of, 146, 150, 151, 152; effects of temperature on, 150 Suspensions, mechanical, 4, 5; colloid, 49; viscosity of, 153 Suspensoids (see suspension colloids) Svedberg's law of Brownian movement, 200 Swelling of gelatine, 252, 261 Systems, 22; disperse, 24, 32; classi- fication of, 29, 33; submolecularly disperse, 32; supermolecularly dis- perse, 32; polydisperse, 35; concen- tration-variable, 35, 136; tempera- ture-variable, 36; colloid, 40, 115, 284 SUBJECT INDEX .120; reversible and irreversible, 40; dispersoid, 43 Temperature, effects of, on colloids, 150; and viscosity, 168; and Brownian movement, 203; and hysteresis, 247 Tension, surface, 61, 67, 79, 80; dis- integration, 82 Theorem of Gibbs, 189 Theory of condensation, 88; of osmotic pressure of colloids, 258 Thermal coefficient, and expansion in colloids, 136 Thermal history, 247; and viscosity, 159 Thorium hydroxide sol, 245 Time, and viscosity of emulsoids, 157; and osmotic pressure of colloids, 242 Tobacco smoke, 43 Transition phenomena, 39 Transition system, 12, 39 True dispersions, 30 True solutions, 4, 54 Tyndall phenomenon, 7, 8 U Ultrafiltration, 12, 268 Ultramicrons, 29 Universality of colloid state, 99 V Van't Hoff's laws, 264 Vapor pressure, 128 Vapor tension, 128 Vectorial constitution of colloids, 56, 58,64 Viscosimeter, 145 Viscosity (see also viscosity of emulsoids), 13; of colloid sys- tems, 14s; of molecular disper- soids, 14s; of suspensoids, 146, 150, 151; of sulphur, 150; and electrolytes, 151; mechanical theory of, in suspensoids, 152; of silver, 152; of suspensions, 153; of emulsoids, 155, 157, 161, 164, 165, 166, 167, 168, 169, 171, 173, 175, 177, 180, 184; of gelatine, iS7> 158, 164; of benzopurpurin, 158; and thermal history, 159; and inoculation, 161; of night- blue, 161, 162; of agar-agar, 164; and concentration, 165, 183; theory of, 178; and electrical charge, 179; and degree of dispersion, 181; and type of disperse phase, 184; and Brownian movement, 202, 203; and character of dispersion medium, 203 Viscosity of emulsoids (see also viscosity) and age, 157; and electrolytes, 157; and mechanical treatment, 161; and concentration, 165; and tem- perature, 168; and added sub- stances, 169, 173, 175; and non- electrolytes, 177; and electrical charge of dispersion phase, 179 W Water, isocoUoids of, 107 Weak colloidality, 32 Wenzel's law, 96 Zoospores, 267 Zsigmondy, classification of, 29