Colloids were originallydefined by Thomas Grahamin 1861as substances, such as starch orgelatin, which will not diffusethrough a membrane. He distinguishedthem from crystalloids (e.g.inorganic salts), which would passthrough membranes. Later it was recognizedthat colloids were distinguishedfrom true solutions by thepresence of particles that were toosmall to be observed with a normalmicroscope yet were much largerthan normal molecules. Colloids arenow regarded as systems in whichthere are two or more phases, withone (the dispersed phase) distributedin the other (the continuous phase).Moreover, at least one of the phaseshas small dimensions (in the range10-9-10-6 m). Colloids are classified invarious ways. Sols are dispersions of small solidparticles in a liquid. The particlesmay be macromolecules or may beclusters of small molecules. Lyophobicsols are those in which there isno affinity between the dispersedphase and the liquid. An example issilver chloride dispersed in water. Insuch colloids the solid particles havea surface charge, which tends to stopthem coming together. Lyophobicsols are inherently unstable and intime the particles aggregate andform a precipitate. Lyophilic sols, onthe other hand, are more like true socollective lutions in which the solute moleculesare large and have an affinity for thesolvent. Starch in water is an exampleof such a system. Association colloidsare systems in which thedispersed phase consists of clustersof molecules that have lyophobic andlyophilic parts. Soap in water is an associationcolloid. Emulsions are colloidal systems inwhich the dispersed and continuousphases are both liquids, e.g. oil-inwateror water-in-oil. Such systemsrequire an emulsifying agent to stabilizethe dispersed particles. Gels are colloids in which both dispersedand continuous phases have athree-dimensional network throughoutthe material, so that it forms ajelly-like mass. Gelatin is a commonexample. One component may sometimesbe removed (e.g. by heating) toleave a rigid gel (e.g. silica gel). Other types of colloid includeaerosols (dispersions of liquid orsolid particles in a gas, as in a mistor smoke) and foams (dispersions ofgases in liquids or solids).
