Recent developments in the understanding of colloid stability

The stability of suspensions and emulsions against coagulation is governed by the forces between the particles. The main forces are: van der Waals attraction, electrostatic repulsion, and the repulsion due to the interaction of adsorbed large molecules. It is stressed that coagulation and redispersion are rate phenomena. Smoluchowski's classical theory on coagulation as modified by Fuchs to include interaction forces between particles has been the basis of theories of colloid stability. Corrections to these theories are needed to account for the hampering of Brownian motion when the particles are close together. Furthermore it is pointed out that the duration of a Brownian collision is long enough to allow redistribution of the double layer and of the conformation of adsorbed macromolecules, but too short for adaptation of the surface charge or of the adsorption of macromolecules. Repeptization requires the existence of a distance of closest approach between particles which is of the order of one or two solvent molecules.