Birth, Life and Death of Colloids

Summary

Colloids are dispersions with particles corresponding to a molecular weight of at least 1000, but small enough to pass through filter paper and to show little sedimentation. There are two categories. Lyophilic colloids form spontaneously from the ingredients and they are thermodynamically stable. Lyophobic colloids have to be prepared in a roundabout way. They have to be protected against irreversible aggregation by a protective layer around each particle.

Lyophilic particles are formed by chemical reaction (e.g. by polymerization) or reversible association (as with soap micelles). Some are biocatalysts, some show rubber elasticity and many can serve as protective agents for lyophobic colloids. They can be destroyed by chemical degradation, but from a physical point of view they may live eternally. Lyophobic particles are formed by irreversible condensation, or by comminution. Monosized particles can be prepared. Most of their applications are based on the possibility to mix incompatibles, such as oil in water or insoluble substances in a solvent. They find many technical applications in paints, inks, magnetic tapes, photographic films and in the kitchen. They are destroyed by irreversible agglomeration if the protective layers are taken awayf but in some cases restoration of the dispersed state is possible. The solubility of lyophilics and the stability of lyophobic dispersions are both sensitive to small changes in the composition of the dispersion medium.