colloid

A colloid sits in the middle of a spectrum. On one side is a true solution, where individual molecules dissolve and you see nothing but clear liquid; on the other are coarse particles you can watch settle. A colloid is in between: its particles are too small to see or to settle quickly, yet far larger than single molecules — they are simply suspended throughout, like the fine particles in fog or in a glass of milk.

By convention, colloidal particles range roughly from one nanometre to one micrometre. At this size they scatter light, which is why a colloidal dispersion often looks faintly hazy or opalescent (the Tyndall effect) even when no settling is visible. They are also small enough that random bombardment by solvent molecules keeps them in constant jiggling motion, called Brownian motion, which helps resist settling.

Pharmaceutical colloids are widespread. Some are lyophilic (solvent-loving), such as gelatin, acacia, or methylcellulose, which dissolve to form viscous, intrinsically stable dispersions used as thickeners and emulsion stabilisers. Others are lyophobic (solvent-fearing), such as colloidal silver or fine metal oxides, which are inherently unstable and must be protected by charge or by added stabilisers.