Lyophilic and Lyophobic Colloids

colloidal systems differing in the intensity of molecular interaction between the substances in the dispersed phase and those of the liquid dispersion medium.

In lyophilic colloids, the particles in the dispersed phase undergo intensive interaction with the molecules of the the surrounding liquid. The particle surfaces are strongly solvated, and the specific free surface energy (surface tension) at the separation boundary is extremely low. The conditions necessary for the formation of lyophilic colloids are achieved at room temperature if the interphase (surface) tension does not exceed several hundredths min.m^–1 (dyne.cm^–1).

Lyophilic colloids are formed by the spontaneous dispersion of large clumps of a solid or drops of a liquid into minute colloidal particles (micelles). Such colloids are thermodynamically stable and therefore do not disintegrate when kept under the conditions necessary for colloid formation. The lyophilic colloids include critical emulsions (that is, emulsions formed near the critical displacement temperature for two interacting liquids), the colloidal dispersions of micellar surfactants (soaps, certain organic pigments, and dyes), and the aqueous dispersions of bentonite clay.

In reference to solutions of macromolecular compounds that form homogenous (single-phase) systems, the term “lyophilic colloid” is obsolete. Current Soviet physicochemical literature applies the term only to microheterogenous (multiphase), or colloidal-dispersion, systems.

In lyophobic colloids, there is little interaction between the particles in the dispersed phase and the surrounding medium. The interphase tension in these systems reaches a relatively high level: not less than several tenths min.m^-1 (dyne.cm^-1) at room temperature. Because of an excess of free surface energy, lyophobic colloids are thermodynamically unstable; that is, they show a constant tendency to decomposition. The decomposition of a lyophobic colloid involves the coagulation or coalescence of the colloidal particles, accompanied by a decrease in the free energy of the system. The aggregation stability (the ability to resist particle consolidation) of any lyophobic colloid is temporary, since it is determined by the presence of a stabilizer—a substance, adsorbed at the surface of the particles (drops), that prevents agglutination (fusion). Typical examples of lyophobic colloids include the hydrosols and organosols of metals, oxides, and sulfides; maximally dispersed (except critical) emulsions; and latices.