Colloid, Dispersed Phase and Dispersion Medium

The systematic study of colloids traces back to Thomas Graham, who observed that solutions of sugar, urea or sodium chloride diffused readily through a parchment membrane, while glue, gelatine and gum did not diffuse through in the same way. He named the freely-diffusing substances crystalloids and the slowly-diffusing ones colloids, from the Greek 'kola' (glue) and 'eidos' (like) — reflecting the fact that his original examples of colloids were, quite literally, glue-like substances.

It was later realised that this behaviour is not an intrinsic, fixed property of particular chemical substances at all — essentially ANY substance can be converted into the colloidal state simply by reducing its particle size into the colloidal range, roughly 1 nm to 200 nm. This size-based understanding is the modern definition used throughout the chapter: a colloid is a homogeneous-LOOKING mixture of two substances in which one substance, present in the smaller proportion, is finely dispersed throughout the other substance, present in the larger proportion. Within a colloidal system, the substance present in the larger amount — the one do …