.SOME RECENT ADVANCES in COLLOIDS' JAMES W. McBAIN Stanford University, Stanford University, California
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VERY student, every'iudustrialist, every biologist has had occasion to reflect upon the fact that classical physics, classical chemistry, and even the most modem physics of nucleus and quantum lie somewhat remote from everyday observations and experience. The cause lies in the fact that practically all the matter with which we commonly deal is in the colloidal condition and here the unit is not the separate atom, or even chemical molecule, but the much larger colloidal particle. The colloidal particle is the brick from which larger structures are made and if we cannot see the architecture of a house in the pile of bricks dumped on the site, still less can we infer it from the clay of which the bricks were originally made. The colloidal particle is often highly organized within itself and yet its external behavior is often as dependent upon the material that coats its surface as in the appearance of a bouse upon its coat of paint or stucco. This stabilizing material on the colloid particle may determine the behavior of the particle even when its actual amount is as small a fraction of the particle as the weight of paint is to that of the house. Now it used to be taken for granted that the arrangement and behavior of these common materials of industry and life would be so complicated as to be largely a matter of empirical experience, greatly depending upon every vicissitude of their prev~oustreatment and history. To me the most important development of recent years has been the ever-increasing demonstration that colloids are subject to law and order and that the behavior of a colloidal particle is predictable once we know its composition and organization and most especially thenatnre of the stabilizing agent on its surface. 'Presented before the Pacific Division of the American Association for the Advancement of Science, Stanford University. Stanford University, Calif.. June 27. 1939.
Some of the most interesting colloids are those which provide their own stabilizing agents by virtue of the chemical groupings which they expose upon their surface. For example, if you wish to create a soap-like detergent all you need is to provide a polar structure capable of organizing itself into colloidal particles by putting, let us say, all the insoluble hydrocarbon portions side by side and end to end, leaving exposed a water-soluble polar group such as the carboxylate of the old fashioned soap or the sulfate or sulfonate of the newer synthetic detergents of which thousands have now been patented. Or still better, we may create a long chain by polymerization of some simple substance like ethylene oxide into long parallel chains, with just enough oxide groups on the surface to keep the particle soluble, and with almost any kind.of a strong polar group on the end to make it into a superlative detergent, even though it is no longer a salt or an electrolyte and is no longer affected by hard water or acids, and so forth, like its soapy ancestors. The soap-like substances introduce us to the enormous class of materials that form colloidal electrolytes. These include all the dyes, the protein salts, the gums, and most of the biocolloids. They have two outstanding properties expressed by the name colloidal electrolytes. They possess dissociable or electrolytic groups which may be paired off with any other ions as desired, and they have so pronounced a tendency to ksociate into stable colloidal particles that the usual theories of electrolytic behavior are completely submerged. One surprising property of many of these detergents has lain buried as far as many of us are concerned in the pharmaceutical and technical literature of the last century and has only within the last few years been brought to light and carefully established by English and American investigators. The property is that dilute solutions of detergents in water can actually dissolve very
appreciable amounts of dyes and hydrocarbons and other substances that are otherwise notoriously insoluble in water. The product is a true stable colloidal solution maintaining reversible equilibria with its constituents and subject to the ordinary methods of pbysical chemical study. Here again we find the same problem being approached in a very different manner by scientific workers in other fields. The organic and biochemists have labeled this "the choleic acid principle" because bile acids share this property of detergents. Desoxycbolic acid carried the fat-soluble and otherwise water-insoluble vitamins as has been shown by several investigators a t Berkeley. This explains bow such materials are carried in the blood stream and can take part in physiological processes. It gives a possible clue to the mechanism of digestion in animals or of the behavior of chlorophyll in plants. It has been used by the Mayo Clinic to render available Vitamin K which prevents certain persons from bleeding to death after operations. It is evident that there is a clear and simple rationale behind the behavior of these almost innumerable and easily varied colloidal electrolytes. They are subject to such laws as the phase rule of Willard Gibbs. In fact, the phase relations of ordinary soap solutions are as definite as those of brass or steel, but far more complicated and much more interesting. From the way in which I have developed the opening theme of this subject, i t is clear that great stress is being placed upon a knowledge of actual structure. For this the most important tool is the examination by X-rays. Remarkable advances have been made in the X-ray study of proteins, wool, hair, feathers, silk, muscle, rubber, synthetic rubbers, the natural gums, and other polysaccbarides. The study of these natural polymers, which include also all the textiles, bas inspired all large chemical organizations thrpughout the world to discover and design new polymers of desirable characteristics due equally to the interior structure of the particle and the exterior groups which together govern its mechanical and chemical properties, some of which are unexpectedly favorable. These newly created substances are commonly referred to as the resins and plastics, and they promise another minor industrial revolution. They are making all nations more selfsufficient, because they can replace expensive imported materials by new ones of superior properties. Thus a metal-like tin can be advantageously supplanted by a plastic, e. g., in steel rolling, with only water a sa lubricant. Some of the new properties are rather unexpected, such as the base exchange with the exposed groups of certain resins which make them available as water softeners. We need no elaboration of the remarkable possibilities opened up by these plastics when we see what a prominent and attractive place they hold at both the Golden Gate and New York Expositions, and when we read in the latest issue of our Scientific American that a complete airplane fuselage can be molded in two hours, in one-half of the time required ordinarily to finish one square foot of an ordinary plane.
An equally important colloid tool is the centrifuge, more specially in the form where it is freed from convection or stirring and called the ultracentrifuge. The original development of this invaluable means for determining particle size is due to the genius of Svedberg, but more recently equally powerful and incomparably less expensive forms have been developed by Beams, Wyckoff, and a t Stanford University. Now we have models which should be available in every university and industrial laboratory, both for research and for instruction. The least expensive and potentially the most powerful and versatile are the opaque or analytical ultracentrifuges, where the liquids are taken out and analyzed by any appropriate method. They are no longer confined to the study of large colloidal particles, but are successfully applied in sedimenting ordinary molecules. Another important function of centrifuges and ultracentrifuges is to concentrate and segregate interesting materials or important impurities that might otherwise be present only in inaccessible traces. In colloidal matter their presence or their complete elimination may make an all-important difference in behavior. A third tool which is producing unexpectedly definite results with protein systems is the observation of movement in the electric field particularly in electrophoretic equipment developed by Tiselius and MacInnes, where the effects are kept under prolonged observation and intensified by the expedient of mechanically compensating their electrical movement, while keeping them under critical optical observation with the well-known Schlieren method of Toepfler. This separates even very similar constituents. In fact;.it has been found that protein constituents that 'stay together in moderate electrical fields will in higher fields travel separately a t differentrates. Dr. Eloise Jameson has also found that it is possible by mere feeding to alter the proteins of the blood, and even to introduce antibodies which can be separated in the electrophoretic cell and afterward studied by the methods of bacteriology, or in the ultracentrifuge, or in the biochemical laboratory. The proteins occupy the forefront of attention and are the subject of a symposium somewhere every few months. The methods of chemistry, and study with enzymes, and surface phenomena, all show that proteins have a long chain poly-peptide structure. X-rays confirm this for many proteins such as silk, wool, and keratins, but X-rays and the ultracentrifuge have shown that some proteins like egg albumin, hemoglobin, and insulin have a nearly spherical or globular form, until they are denatured or spread on surfaces. The cyclol cage structure suggested to account for this bas not found favor, and some alternative, explaining proteins as built up from sub-units, is being sought. Another colloidal material recently come into prominence whose structure bas been determined by X-rays. and whose fine particles can be segregated by a highpower centrifuge and whose exposed reactive groups can be altered and replaced in an electric field is clay. The
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replacement of these active groups by others is familiar in the formation of deltas a t the mouths of rivers where the silt reacts with the heavy metal salts in sea water. Clay consists of sheets laminated like a club sandwich whose surfaces are all different from their exposed edges. All this is of primary interest in soil science, but the triple laminated or bentonite type is especially useful in the drilling muds of the petroleum industry. The mechanical properties of these muds can be changed in a spectacular way by the addition of very small amounts of tanning agents. This is no strained analogy but turns out to be a direct parallelism with the tanning of swollen skin to make leather by putting inactive groups upon the tom exposed ends and edges and by blocking off any other reactive groups on the surface. I t has long been known that a suspension of the finest clay particles when evaporated yields a highly tenacious film, but Hauser has produced commercially promising films-wrapping and insulating materials of this nature that are water- and fire-resistant. This even promises to be the beginning of an inorganic plastics industry, a distinct extension of the age-old ceramics. Much theoretical speculation has appeared in recent years to account for the curious phenomena of tactoids, Schiller layer thixotropy, and coacervation, to give each its technical title. Tactoids are the doubly refracting concentrated patches that spontaneously appear in dilute colloidal solutions of certain rod-shaped or flat particles, such as vanadium pentoxide or the tobacco mosaic virus protein. Thixotropy has long been familiar as that property of many colloid systems of setting to a jelly when undisturbed, and liquefying each time that they are shaken or stirred. Evidently some structure is temporarily destroyed, which subsequently rebuilds itself. Coacervation is the mutual precipitation out of solution of two colloids added together, familiar for example in one of the chief methods of disposal of sewage. Some of these coacervates may be readily separated again into their two constituents by suitable treatment, jls for example, in an electrical field.
In all these phenomena there are three current alternative explanations; one involving loose interlocking of the particles a t certain points of contact; another, less favored, that the particles are surrounded by liquid that is rendered practically rigid by the proximity of the particles; and the third, now much discussed, that the particles may be held apart a t almost microscopic distances by a balance of attractive and repulsive forces. The Dutch and Freundlich consider that the electrical forces are repulsive over great distances. They balance this repulsion by deducing a residual van der Waals' attractive force increasing with the size of the particle, and thus explain how the particles are held in more or less stable array a t a distance from each other. On the other hand Langmuir and Levine revert to the classical view that since the system as a whole is electrically neutral and the charged particles are interspersed with equal amounts of oppositely charged ions, the electrical forces are not repulsive, but attractive. Langmuir then calculates an extra osmotic effect to produce a repulsion to balance this electrical attraction. The author, however, is impressed by the fact that thixothropy is most prominent in non-conducting, non-aqueous liquids where there are no ions either to attract or repel. Coacervation, too, occurs in these completely' nonionizing liquids. Excellent although most annoying examples are found by industrialists who attempt to put asphaltic bitumen and coal tar pitch into the same oil, although the whole system consists almost entirely of hydrocarbons. In general, the reviewer is impressed by the f a d that most of the prominent phenomena of colloid systems have been demonstrated in non-aqueous, non-ionizing media, and therefore he questions these electrical effects as being responsible for the same phenomena in aqueous systems. In the foregoing review it will be noticed that the only subjects mentioned are those that are being studied simnltaneously in many countries: .chiefly in Russia, Holland, England, Japan, the United States, and elsewhere.
