The effectiveness of the various electrolytes in bringing about this change is proportional to their valency; bivalent ions are from 70 to 80 times, and trivalent ions about 600 times as effective as monovalent ions.
Further, all sols in which the dispersed particles carry a charge of the opposite sign likewise precipitate both suspensoids and emulsoids.
A demonstration of the presence of an electric charge on the particles of a sol and a determination of its sign can be made by placing the solution in a U tube, with a layer of distilled water above the sol in each arm of the tube, and then passing an electric current through the contents of the tube, keeping the electrodes in the distilled water, so that the migration of the particles toward one pole or the other can be observed by their appearance in the clear water at that end of the tube; or by passing an electric current through the observation chamber of an ultramicroscope, in which the solution under examination has been placed, and observing the migration of the particles across the field toward either one or the other (positive or negative) electrode.
Emulsoids and suspensoids differ in their properties in the following respects. Suspensoids are always very dilute, containing less than 1 per cent of the dispersed solid; while emulsoids may be prepared with widely varying proportions of the two component liquids. Suspensoids have a viscosity which is only slightly greater than that of the liquid phase when it exists alone, and their viscosity varies with the proportion of dispersed solid which is present in the sol; while emulsoids have a very high viscosity in all cases. Emulsoids usually form stiff gels when treated with electrolytes; while suspensoids more commonly yield gelatinous precipitates under the same conditions.
Suspensoids and emulsoids which carry electric charges of opposite sign mutually precipitate each other. But emulsoids often protect suspensoids from precipitation by electrolytes, by forming a protective film around the particles of the suspensoids, which prevents the aggregation of the particles into the precipitate form.
ADSORPTION
If a sol be precipitated or coagulated by the action of an electrolyte, substances which may be present in solution in the liquid of the sol are carried out of solution and appear in the gel or precipitate. This phenomenon is known as "adsorption," which means the accumulation of one substance or body upon the surface of another body, as contrasted with "absorption," which means the accumulation of one substance within the interior of another. Since substances which are in the colloidal form have very large relative surface areas, it follows that the opportunity for surface adsorption on colloidal materials is very great.
Surface adsorption is a common phenomenon. It was extensively studied by the physicist, Willard Gibbs, who showed that adsorption will take place whenever the surface tension of the adsorbing body will be lowered by the concentration in its surface layer of the material which is available in the solution or other surrounding medium.
As applied to colloidal phenomena, adsorption may be exhibited in either one of four different ways, as follows: (1) A crystalloidal substance which is in solution may be adsorbed on the colloidal particles of a hydrosol, so that if the mixture be dialyzed, or filtered through a so-called "ultrafilter" (i.e., a filter with pores so small that it will retain colloidal particles) the dissolved crystalloid will remain with the separated colloidal particles, or the dissolved crystalloid will not react chemically as it would in a free solution. For example, if to a solution of methylene blue, which dyes wool readily, there be added a small quantity of albumin (a colloidal substance), the dye is adsorbed by the albumin and will no longer color wool with anything like the same readiness. (2) During gel-formation, electrolytes and other soluble substances which may be present in solution in the liquid may adsorbed out of the solution and appear in the gel. For example, a precipitate of aluminium hydroxide, or of silicic acid, is nearly always contaminated with the soluble salts which are present in the solution, and can be prepared in pure form only by repeated filtering, redissolving, and reprecipitating. (3) Colloidal substances may be removed from sols by being adsorbed upon porous materials like charcoal, fuller's earth, hydrated silicates, etc. For example, animal charcoal (or bone black) is used commercially for the clarification of sugar solutions, because it adsorbs out of these solutions the colloidal proteins, coloring matters, etc., with which they are contaminated. (4) Finally, colloids mutually adsorb each other, as in the case of the "protective colloids" previously referred to.
Certain characteristics of adsorption phenomena are of interest and importance from both the physiological and the industrial point of view. The following may be mentioned: (a) Amount of adsorption. Relatively more material is adsorbed out of dilute solutions than out of more concentrated ones. An increase of ten times in the concentration of the dissolved material results in only four times as much adsorption by the colloidal substance which may be introduced into the two solutions. In this, adsorption differs from chemical action, as the latter is proportional to the concentration of the reacting material which is present in the solution. (b) Adsorption out of different liquids, by the same adsorbing body, is different in amount. It is usually greatest out of water. Hence, many dyes may be adsorbed out of water by charcoal, porous clay, etc., and if the latter be then introduced into alcohol, or ether, the dye goes back into solution in these latter liquids. This process is often used industrially and in the laboratory for the purification of such substances when they are present in impure form in aqueous solutions. (c) Selective adsorption. Different substances are not adsorbed out of the same solvent to the same extent by the same adsorbing agent. Advantage is taken of this fact when filter paper is used in the so-called "capillary analysis" to separate different dyes, or other colloidal materials which have been stained different colors, into alternate layer by reason of the different rate at which the paper adsorbs the different materials out of the solution in which they are present together. (d) Similar relative adsorption by different adsorbing agents. Although different adsorbing agents may possess varying active surfaces and hence, variable adsorbing power, or rates of adsorption, they adsorb the same relative amounts of different materials; i.e., if substance A adsorbs more of X than it does of Z out of any given solution, substance B will likewise adsorb more of X than of Z out of the same solution; although the actual amounts adsorbed by A may be quite different from those adsorbed by B.