THE HISTORY AND SCOPE OF COLLOID CHEMISTRY

Part One CHAPTER ι

T H E HISTORY A N D SCOPE O F COLLOID

CHEMISTRY

Definitions Colloids are substances consisting of a homogeneous medium and of particles dispersed therein. Indian ink, the milky dispersions of sulphur, clays and humus, a soapy shaving cream, glue, a n d blood serum are all examples of colloids. The dispersed entities m a y eventually be large molecules. Colloidal particles are smaller than coarse, filterable particles but larger than atoms and small molecules. Any colloid contains particles whose diameter is about 0-000001-0-0005 m m , i.e. lm/x-500m/z or 10-5000 Â. Table 1 shows the dimensions of the dispersed entities in some examples. TABLE 1

Diameter of quartz grains in sand Diameter of human red blood cell Length of Bacillus coli Diameter of the particles in colloidal sulphur Dimensions of grippe virus Diameter of colloidal gold particles Length of the haemoglobin molecule Diameter of oxygen molecule

50,000-200,000 π\μ 7500 1500 50-500) 120 [Colloidal 1-100 I 2-8 ; 016

Colloidal particles are invisible in ordinary microscopes, a n d they pass the pores of ordinary filters. A term more general than that of colloid or colloidal solution is disperse system. The latter denotes any homogeneous medium containing dispersed entities of any size a n d state. Emulsions, for example, are composed of liquid particles dispersed in another liquid. Foams contain bubbles of gas. The degree of dispersion is a quantity varying reciprocally with the particle size ; emulsions are usually of a low degree of dispersion, a n d this means that the droplets are relatively coarse. The properties a n d the behaviour of colloids depend chiefly on the size and shape of the dispersed particles, as will be shown below. Historical data SELMI ( 1 8 4 3 ) was the first to investigate colloids systematically.

He

prepared colloidal solutions of sulphur, prussian blue a n d casein, perA l C.C

2

HISTORY AND SCOPE

OF COLLOID

CHEMISTRY

forming numerous experiments. H e came to the conclusion that these were not true solutions but suspensions of small particles in water. T h e English scientist GRAHAM (1861) is usually regarded as the founder of classical experimental colloid chemistry. H e investigated the diffusion of different substances, finding that some had a high rate of diffusion but that others moved very slowly. F o r instance, the ions or molecules of dissolved potassium hydroxide, magnesium sulphate and sugar are very mobile, whereas those of dissolved albumin or gelatin move relatively slowly. According to their diffusion rates, GRAHAM classified all substances into two groups: the crystalloids and colloids. He pointed out that the former can be easily crystallised, but not the latter. The difference between the two classes was still more pronounced if the substance in question were dissolved in water and separated from the pure solvent by a semipermeable m e m b r a n e : the crystalloids passed through the membrane easily, but the colloids did not (Fig. 1). FIG. 1.

Dialyser according to GRAHAM. C—colloid, M—membrane, W—water.

M

By means of this procedure, called dialysis, it was possible to purify a colloidal solution from admixed crystalloids. The name ' colloid ' was proposed by G R A H A M ; kolla in Greek means glue, and GRAHAM wished to stress that he considered all colloids to be more or less like glue. F o r colloidal solutions the name ' Sol ' was used. Some sols 4 under suitable conditions can be transformed into solid jellies or gels '. Although the work of GRAHAM was of fundamental importance, it was found later that his classification of all substances into crystalloids and colloids is not always convenient. Many colloids, e.g. some proteins, can be crystallised. On the other hand, almost all so-called crystalloids can be prepared in the colloidal state. One of the oldest examples is sulphur: there are sulphur crystals, sulphur sols, and amorphous sulphur. FARADAY (1857) was another British scientist who made interesting discoveries about colloids. H e prepared stable solutions of colloidal gold, and investigated some optical properties of these. A sharp beam of light passing through a gold sol, if observed from the side, appeared as a white path. This phenomenon, as FARADAY correctly pointed out, is caused by the particles of gold scattering light. In solutions of simple salts and other true solutions this phenomenon does not occur. TYNDALL found later (1869) that the light scattered by the colloidal particles is polarised. The stability of colloidal solutions or sols was investigated by

HISTORICAL

DATA

3

SCHULZE. Working mainly with inorganic colloids he found (in 1 8 8 3 ) that they can be precipitated quite easily. F o r instance, a red gold sol initially changes colour to blue on the addition of quite small amounts of sodium chloride ; in a short time the blue colour is changed into greyish brown. Then the sol becomes turbid, a n d finally precipitates slowly. SCHULZE investigated thoroughly this phenomenon of flocculation or coagulation, especially as regards the flocculating power of different reagents. Considerable progress in the investigation of colloids occurred at the beginning of the century. FREUNDLICH investigated adsorption phenomena, a n d discovered his law of adsorption ( 1 9 0 3 ) . SIEDENTOPF and ZSIGMONDY in 1 9 0 3 invented the ultramicroscope. This was based on the old observation of FARADAY a n d TYNDALL that colloidal particles scatter light strongly. If an intense beam of light is passed through a colloid a n d the path of the beam is observed with a microscope perpendicularly t o its incident direction a n d against a dark background, the separate particles can be detected. If the sol is sufficiently diluted the particles appear as rapidly moving coloured discs, which may be counted. F r o m this known number, the quantity of the substance and its density, the particle size can be determined. Many colloids were investigated in the following decades, a n d it was confirmed that colloidal particles are considerably larger than atoms a n d small molecules, although smaller than coarse, microscopic particles. Important contributions toward the solution of the problem of particle size, as well as of sedimentation, movement, and coagulation of particles

were made

by

SMOLUCHOWSKI

(1906),

SVEDBERG

(1906),

PERRIN ( 1 9 0 8 ) and EINSTEIN ( 1 9 0 8 ) .

The important conclusions of VON WEIMARN and W o . OSTWALD

were published at about the same time, VON WEIMARN showed from numerous examples that the so-called crystalloids of GRAHAM could be prepared in colloidal state. F o r instance, barium sulphate may be easily precipitated either as a microcrystalline precipitate or as a colloid. H e pointed out that many of the peculiar ' colloidal ' properties depended mainly on particle size. This was confirmed by OSTWALD. Even colloidal sodium chloride can be prepared by precipitation in suitable organic solvents in which it is practically insoluble. Moreover, VON WEIMARN pointed out that the many colloidal particles have a crystalline structure. This was later confirmed by means of X-ray analysis. Wo. OSTWALD and VON WEIMARN also proposed the first rational classification of colloids. The notion of disperse system was introduced, and particle size was taken as the chief factor in the classification and characterisation of colloids. ' Disperse system ' is, however, a very general term : not all disperse systems are colloids. There are three large classes of disperse systems.

4

HISTORY

AND SCOPE Disperse

Coarse dispersions >0-l/i

OF COLLOID

CHEMISTRY

Systems

Colloid-disperse systems 0-1 μ-\ τημ

Atoms, small molecules < l m/x

Decreasing particle size

There are many examples of the first, including soil, muds, coarse suspensions of silver chloride, and other turbid systems. Most emulsions are also coarse systems since the droplets can usually be observed in a microscope. On the other hand, the dimensions of atoms and of small molecules are about 0 T - 1 m/x, or 1-10 Â. Colloidal properties, however, are not strictly confined t o the above-mentioned limits of particle size. The figures 1 m/x-100 m/x were arbitrarily chosen by OSTWALD, although systems containing much larger particles, 200500 m/x or 0Ό002-0Ό005 mm, also show colloidal properties. As pointed out in the first decade of this century, colloidal particles are larger than molecules of water, benzene or sugar, but smaller than visible quartz grains, bacteria or blood cells. Some comparative examples of the magnitudes of these small objects are given in Table 1 on p. 1. Another classification of colloid systems was proposed some forty years ago by Wo. OSTWALD and is still valid. Any disperse system consists of a homogeneous medium and of particles, and both the medium and the particles may be either solid, liquid or even gaseous. In Table 2 are presented the most important practical examples of colloidal disperse systems. TABLE 2 Medium

Dispersed particles

Common name of the system

Gaseous Gaseous Liquid Liquid Liquid Solid Solid

Liquid Solid Gaseous Liquid Solid Liquid Solid

Fog, mist, aerosol Dust, fume, aerosol Gas dispersions, foams Emulsions Sols, colloidal solutions Solid emulsions, some gels Alloys, glasses

The most important a n d common are the sols and emulsions. According to the above scheme, emulsions contain liquid particles, so that there is no justification for classifying as emulsions the silver halide-gelatin systems used in photography. Water remains the most important dispersion medium, especially in the fields of medicine, biology and analytical chemistry. In industry, however, many other solvents have gained considerable importance, colloidal solutions such as those of nitrocellulose in acetone and of rubber in benzene being widely used.

HISTORICAL

DATA

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During the last decades many new techniques have been introduced. Under the leadership of SVEDBERG the ultracentrifuge was developed in Uppsala (Sweden). By means of this instrument it is now possible to determine particle sizes a n d molecular weights of various substances in 3 7 the molecular weight region 1 0 - 1 0 . The ultracentrifuge is also able to yield information about the distribution of particle sizes. Very important information was also obtained from X-ray analysis, streaming birefringence, diffusion, light scattering, viscosity, electrophoresis ( 1) and from many other physical chemical techniques. Finally, great services have been rendered to colloid chemistry by the electron microscope, developed by German, British a n d American scientists between 1 9 3 2 - 1 9 4 0 . With it many colloidal particles a n d large molecules can now be seen and photographed. These include particles of colloidal gold, molecules of glycogen, tobacco mosaic virus, etc. Other important developments include the work of Sir ERIC RIDEAL, A D A M , SCHULMAN (in G r e a t Britain), and of LANGMUIR and H A R K I N S

(in the U.S.A.) on the interfacial phenomena. The study on monomolecular layers of substances spread on surfaces of liquids rendered invaluable information that aided considerably in the understanding of such phenomena as micelle formation, detergency, and the stability of emulsions and foams. While this physico-chemical approach added greatly to our knowledge of colloids, purely chemical investigations have also proved important. Some thirty o r forty years ago little attention was paid t o the purity of the colloid investigated. Often the substances were neither sufficiently pure nor well defined. PAULI in Vienna ( 1 9 2 0 - 1 9 3 8 ) was one of the first colloid chemists to direct attention to the chemical purification of inorganic colloids. Moreover, he founded the modern colloid chemistry of proteins. Valuable results were obtained by purely chemical methods by STAUDINGER a n d his associates in Freiburg (Germany) between 1 9 2 2 - 1 9 3 9 , and later. They investigated cellulose, rubber, starch, and many synthetic macromolecular substances. Mainly by chemical, purely preparative means, it was proved that the entities existing in the solutions of the mentioned substances are single large molecules, a n d n o t particles composed of many small molecules. STAUDINGER, as well as MEYER, M A R K , a n d others pointed out the im-

portance of particle shape. N e w problems of classification arose. STAUDINGER showed that cellulose molecules are so long that in bundles they may be seen in a microscope, b u t a t the same time they are each thinner than most colloidal particles. In the late thirties a n d in the forties the development of colloid chemistry proceeded rapidly. Many natural colloids, including some ( l)

More data about the history of colloid chemistry may be found in A . E . ALEXANDER and P. JOHNSON; Colloid Science (Clarendon Press, Oxford 1949), pp. 1-33.

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HISTORY

AND SCOPE

OF COLLOID

CHEMISTRY

enzymes and other proteins have been prepared in a highly purified state and investigated by both chemical and physical methods (NORTHROP, STANLEY). Many new synthetic high polymers have been intro( 2) duced in science a n d industry. New physico-chemical views a n d methods have gained importance in the understanding of colloidal phenomena. The importance of colloid chemistry Research in colloid chemistry is becoming increasingly important in various branches of pure chemistry, industry, medicine and many other fields. Adsorption, dialysis and coagulation are important in preparative chemistry, in analytical chemistry (co-precipitation, washing of precipitates, filtration problems, chromatographic adsorption analyses), in soil chemistry, in clinical work and in the preparation of pharmaceutical mixtures such as emulsions. Colloid chemical approaches are very important in dealing with numerous technical and industrial problems. Solutions of such practically important materials as cellulose, rayon, rubber and starch are colloidal. Soaps and the many new synthetic detergents dissolve in water to form colloidal solutions the deterging property of which is investigated as a colloid chemical problem. Surface activity and wetting are important in the textile industry, in dyeing, and in the separation of pulverised ores (flotation). Quite new in science and in everyday life are the many synthetic polymers—' nylon ', ' Orion the silicones, the polymethacrylates for safety glass, synthetic rubber, PVP, the synthetic blood plasma substitute, and so on. These new substances have been prepared by organic chemists by polymerisation and polycondensation reactions. The final products are colloids. Colloid chemical views and methods are essential in the final evaluation and characterisation of the polymers, e.g. the viscosity of the solutions yields information about the length of the dissolved macromolecules. The tensile strength of fibres depends on this length of the macromolecules. The possibilities to develop new plastics and high polymers are unlimited, and colloid chemical considerations will always be of great value in this field. Furthermore, colloid chemical views help to solve problems of heterogeneous catalysis—e.g. the catalytic action of fine palladium depends on the degree of subdivision or dispersion of the metal. The adhesion of paints and glue, and problems of lubrication involve colloid chemical considerations. The lubricating action of graphite depends on its laminar structure : the substance is split easily into small leaflets of colloidal dimensions. In many industries there arises the problem 2

( ) See, for instance, A. SCHMIDT and C . MARLIES; Principles of High-Polymer Theory and Practice (McGraw-Hill, New York 1948). W. H. CAROTHERS; Chem. Revs. 8, 353 (1931). P. J. FLORY; Chem. Revs. 39, 137 (1946).

IMPORTANCE

OF COLLOID

CHEMISTRY

7

of filtering very fine particles from liquids, the breaking of oil emulsions, the stability of emulsions for emulsion polymerisation and the preparation of drilling muds of definite viscosity and specific weight. Colloid chemical problems are encountered in photography, printing, tanning and the ceramic industries. Also of importance is the adsorption of gases on porous materials, the precipitation of dust, smoke ( 3) and fogs. The colloid chemical points of view are important in agricultural chemistry. The fertility of soil depends on the relative amount of colloids in the soil. The clay and humus substances are the most important colloidal ingredients of soil. The higher the amount of these ingredients in soil the better the soil holds water and plant nutrients ; a coarse soil does not hold water and from it the inorganic plant nutrients are lost easily by leaching. Especially wide are the applications of colloid chemistry to biology and medicine. Blood and protoplasm are complicated colloidal solutions. Skin, muscle and the many different tissues are gels possessing quite peculiar structures. The most important substances in the human and animal body are the proteins which are colloids. Simple colloid chemical considerations about the shape of protein particles have proved to be of considerable importance. The proteins which serve as building materials possess long molecules (collagen of skin and bones, myosin of muscle), whereas the proteins of blood and milk have globular particles. Rods and threads, of course, are suitable for building, but useless in circulation since they will easily clog the capillaries. In blood clotting the particles of the fibrinogen are transformed ( 4) into long, fibrous structures leading to gelation. The phenomena of swelling and hydration are connected with many biological problems such as that of ageing : the proteins and other colloids in a young body are more hydrated than in an older body. Further, there are the problems connected with the action of narcotics (alcohol, ether) on tissue proteins. As examples may be cited the change of degree of aggregation of the colloids concerned, alteration in the permeability of body membranes, the different colloidal characteristics of normal and pathological blood proteins. We are far from the over-optimistic belief that all the problems of life will be solved by colloid chemical means—the purely chemical approach is doubtless of equal importance —but there will always be wide, unexplored fields of investigation open (5) for the colloid chemist in biology and medicine . 3)

< J ALEXANDER; Colloid Chemistry, Principles and Applications, 4th ed. (Van Nostrand, New York 1937), 5th ed. 1944; 6th ed. 1946. 4) <5 J. D . FERRY; Advances in Protein Chemistry, 4, 1-78 (1948). ( ) B. JIRGENSONS, Organic Colloids (Elsevier, Amsterdam; Van Nostrand, New York; Cleaver-Hume Press, London 1958).