Colloids

the traditional term for maximally disperse (microheterogeneous) systems. The particles in the dispersed phase in colloidal systems, colloid particles, usually have dimensions of 10^-7 to 10^-5 cm. In a gas or liquid, these particles freely and independently experience intense Brownian movement, uniformly occupying the entire volume of the disperse medium. Such freely disperse colloids as smoke or sols are resistant to sedimentation—that is, their particles do not precipitate. Colloid particles collide in the process of Brownian movement or upon mixing. If upon collision there is no particle enlargement resulting from conglomeration (coagulation) or fusion (coalescence), then the colloid is resistant to aggregation (for example, lyophilic and stabilized lyophobic colloids).

Colloids form upon condensation of substances in a homogeneous medium (such as supersaturated solutions, vapor, or supercooled fluids) if the nucleation centers of a new dispersed phase that have formed in the medium—that is, tiny drops or crystals—are not able to grow to dimensions exceeding 10^-5–10^-4 cm. Condensation is often accompanied by chemical reactions that result in the formation of poorly soluble compounds. Another means of producing colloids is dispersion, which is spontaneous in the case of lyophilic systems and forced in the case of lyophobic systems. The existence of aggregation-resistant liquid lyophobic colloids is always brought about by the presence of surface-active agents (stabilizers) in the disperse medium. These substances produce an adsorption-solvate protective layer on the particle surfaces and prevent the close approach and coagulation of particles under the action of short-range forces of molecular attraction. The close approach of particles may be impeded by the disjoining pressure of the liquid dispersion medium, which is solvated by molecules or ions of the stabilizer in the adsorption layer; by electrostatic repulsion of like-charged ions adsorbed on the particle surfaces; or by enhanced structural viscosity of the surface protective layer, the “structural-mechanical barrier.”

Nonstructured liquid colloids, or sols, have a number of properties that are intermediate between those of coarsely disperse systems and solutions. When the protective action of the stabilizer is reduced in lyophobic sols (astabilization, or destabilization, of the system), structured colloids called gels form. Solid microheterogeneous systems are also considered colloids; they consist of fine-grained bodies (such as some minerals, alloys, and crystallized glasses) in which the size of the structural elements or inclusions does not exceed the limits of colloidal dimensions.