Classification of Colloids: Lyophilic, Lyophobic, Multi-molecular, Macromolecular and Associated Colloids

Colloids are classified along two entirely independent lines: how strongly the dispersed phase interacts with, and is attracted to, the dispersion medium, and how the individual colloidal particles are themselves actually built up at the molecular level.

Classification by interaction with the dispersion medium. A lyophilic ("solvent-loving") colloid is one in which the dispersed phase has a strong natural affinity for the dispersion medium — starch, gelatin, gum and proteins dispersed in water are classic lyophilic (specifically hydrophilic) colloids. Lyophilic sols form spontaneously, simply by bringing the two substances into contact, are thermodynamically quite stable, and — crucially — are reversible: if the dispersion medium is evaporated away, the dry residue can usually be re-dispersed again into a colloid by simply re-adding the solvent and stirring. A lyophobic ("solvent-hating") colloid, by contrast, has little or no natural affinity between phase and medium — metal sols such as gold sol or silver sol, and sols of metal sulphides such as $\text{As}_2\text{S}_3$, are classic lyophobic colloids. Lyophobic sols must usually be prepared by special methods (not by simple mixing), are considerably less stable, require a stabilizing agent to prevent the particles from aggregating, and are generally irreversible — once precipitated out, the solid cannot usually be brought back into colloidal form just by re-adding solvent. …