Solutes

Solutes, or sols (from the Latin solutio—solution), are colloidal solutions. Classification. In the series of dispersed systems, solutes occupy an intermediate place between suspensions and emulsions on one side and true solutions (molecularly or ionically dispersed systems) on the other. Their optical heterogeneity is not noticeable under a microscope, but can be detected by means of an ultramicroscope or by the Tyndall phenomenon. The size of their particles fluctuates on average between 100 and 1 nm. Dispersed systems differ not only by the size of their particles but also by the aggregate state of the phases constituting them. If solid or liquid microscopic particles are suspended in a liquid medium, there are suspensions in the first case and emulsions in the second. Such a division was continued on more finely dispersed colloidal systems. Correspondingly, according to the proposal of Ostwald (W. Ostwald), solutes are divided into suspensionoids and emulsionoids, or, according to Weimarn (Weimarn), briefly into suspensionoids and emulsionoids. Since the concept of aggregate state refers not to individual molecules but only to their accumulations or aggregates, the indicated classification loses its meaning as one proceeds to more finely dispersed systems and is completely inapplicable to true solutions. An even more important criterion for the classification of solutes apparently represents the relationship of the colloid to the solvent constituting its dispersion medium. Freundlich proposed to divide colloids into lyophilic and lyophobic depending on whether they possess an affinity for the solvent or not. In the case of hydrosols, whose dispersion medium is water, there is a corresponding division into hydrophilic and hydrophobic (Perrin). Strong hydration of a colloidal particle brings it closer to the liquid state. In this connection, hydrophilic (or lyophilic) colloids are for the most part at the same time emulsionoids, while hydrophobic (or lyophobic) ones are suspensionoids. The properties of colloidal solutions belonging to these two main groups differ from each other in many respects. Viscosity. For suspensionoids, as well as for most coarse suspensions, the suspended particles do not noticeably increase the viscosity of the hydrosol, which remains practically equal to the viscosity of water. On the contrary, emulsionoid, lyophilic solutes are characterized by very high viscosity compared not only with pure solvent but also with solutions of crystalloids. Thus, a one-percent solution of NaCl has 1.6%, and a solution of cane sugar 2.5% greater viscosity than distilled water at the same temperature; the addition of one percent of gelatin raises the viscosity of water by almost 30%. Such high viscosity of emulsionoids is apparently due to their strong hydration. Swollen to a significant size, colloidal particles collide with every movement of the liquid, and their elastic resistance is summed with the true viscosity of the solution. This viscosity differs from the ordinary viscosity of true solutions by some characteristic features. If a sol of gelatin is subjected to heating, then immediately after returning to the original temperature its viscosity will be significantly lower than before heating. Only after a certain time does it rise to its original value. This change indicates the structural character of the viscosity, which obviously depends to a considerable extent on the gluing of individual colloidal micelles, gradually fusing into a complex network. The latter probably also determines some elastic properties observed in solutes as opposed to typical liquids. Surface tension. The surface tension of water is also strongly changed by emulsionoid colloids—in contrast to suspensionoids, which in this respect behave as indifferent suspensions not affecting their dispersion medium. The reduced surface tension of blood is to a considerable extent due to the colloids contained in it. As is known, substances that lower surface tension are adsorbed on the boundary surfaces. Therefore, adsorption of agar, gelatin, gum arabic and other emulsionoids is observed both on the surfaces of contact of the solution and the liquid and solid bodies contained in it, and on the boundary with air. The adsorption of colloids gives rise, in some cases, to a phenomenon highly characteristic of emulsionoids—the formation of surface films. The content of colloid in the surface layer (and with it its viscosity) can increase strongly, giving it the strength of a solid film. Solutes are surrounded by a thin layer of gel—a phenomenon which represents the colloidal model of the formation of a shell by cytoplasm. Relation to electrolytes. The greatest difference between the two types of solutes manifests itself in their relation to electrolytes. Suspensionoids are characterized by high sensitivity to the content of electrolytes and often precipitate from the solution in the presence of a very small amount of them, whereas non-electrolytes do not exert such a precipitating action. Electrolytes, in particular neutral salts, precipitate similarly and more coarse suspensions. The formation of river deltas, muddy deposits in places where muddy fresh water mixes with sea water, is probably due to a large-scale "salting out" of the suspended mud occurring here. The stability of lyophobic solutes depends mainly on the electric charge of the colloidal particles. The repulsion caused by these charges counteracts the tendency of colloidal particles to glue together under the influence of surface forces into larger aggregates and precipitate from the solution. Therefore, any agent that destroys the electric charge causes the precipitation of a suspensionoid, its coagulation. Such an agent is in particular salts, the action of which on solutes is the more significant the greater the charge and the more strongly the ion of opposite sign to the charge of the colloidal particle is adsorbed. Emulsionoid, lyophilic solutes are characterized by significantly greater stability, significantly lower sensitivity to the addition of electrolytes. Their colloidal particles are held in a suspended state mainly due to the forces of chemical affinity binding them to the solvent. Thus, their stability has the same nature as the stability of true solutions, to which they are much closer than suspensionoids. Therefore, instead of the sign and magnitude of the charge determining the action of electrolytes on suspensionoids, a greater role here belongs to influences that change the relation of suspended particles to the solvent, influences which Freundlich called lyotropic (see Gomberg's series).