Classification of Colloidal Solutions

Colloidal systems can be classified along several independent lines, each capturing a different physical feature of the system.

By the physical state of the dispersion medium. The most commonly encountered colloidal systems have a SOLID dispersed phase suspended in a LIQUID dispersion medium. When that liquid medium is specifically water, the resulting colloid is called a hydrosol or aquasol; when the medium is an alcohol, it is called an alcosol; and when the medium is benzene, it is called a benzosol.

By the strength of interaction between phases. In lyophilic ('solvent-loving') colloids, a genuine attractive force or affinity exists between the dispersed phase and the dispersion medium — sols of protein and starch are classic examples. Because of this real attraction, lyophilic colloids are comparatively stable, resist precipitation, and — if they DO precipitate — can simply be brought back into the colloidal state by re-adding the dispersion medium, without needing any special chemical treatment. In lyophobic ('solvent-hating') colloids, by contrast, no attractive force exists between dispersed phase and dispersion medium — sols of gold, silver, platinum and copper are classic examples. Lyophobic colloids are consequently much less stable, precipitate readily even with a small disturbance, and — critically — CANNOT be regenerated simply by adding back the dispersion medium once they have precipitated; because of this they are called irreversible sols, and left alone they will even undergo SPONTANEOUS coagulation after a characteristic lifetime, without any added electrolyte at all. …