extremely disperse colloidal systems with a liquid dispersion medium. Traditionally sols are sometimes called colloidal solutions. The particles of the dispersed phase, micelles, participate independently of one another in vigorous Brownian movement and therefore do not precipitate under the influence of gravity. Their dimensions usually do not exceed 10^-10−10^-7 cm. Sols with an aqueous dispersion medium are called hydrosols; those with an organic medium are called organosols.

A distinction is made between lyophilic and lyophobic sols. Lyophilic sols form spontaneously and do not break down with time. Lyophobic sols, on the other hand, break down gradually because of spontaneous coagulation or coalescence of the particles, although if the system contains a stabilizer, the process can be very slow. Hydrosols of soaps and soaplike surface-active substances, as well as hydrosols and organosols of certain organic pigments and dyes, are lyophilic sols; synthetic latices and hydrosols and organosols of metals are typical lyophobic sols. When the liquid dispersion medium solidifies without separation of a new phase— that is, in vitrification—so-called solid sols form, in which motion of particles with respect to one another is impossible; ruby glass is an example. Systems of minute solid or liquid particles uniformly distributed in a gaseous medium (air) are called aerosols. [9–666–l]