| Abstract: |
Electrical charge acquired by colloids or solid surfaces in aqueous systems plays a significant role in determining their properties. Homo- and heterocoagulation and flocculation, nonspecific ion adsorption and ion exchange, electrode process rates, and the configuration of organic macromolecules are all affected to some extent by surface charge or potential. Over the past decade, several lines of research and model development involving solids or surfaces as different as metallic mercury, virtually insoluble oxides, silver halides, proteins, polystyrene latexes, ionizable monolayers, and clays have converged. For colloids and hydrosols that contain acidic, basic, or amphoteric functional groups accessible at their surfaces, it is now possible to present a single, conceptually simple and easily visualized model capable of predictively modeling the development of surface charge and potential, the zeta potential of the solid, the conductivity of the sol, electrolyte adsorption densities, and related properties. It is our purpose to review the origins of this model, the methods of characterizing solids and colloids for its use, and the results obtained when it is applied to inorganic oxides, polystyrene latexes, and clays. We are convinced that the fundamental concepts embodied in the model provide a more realistic understanding and representation of ionic processes at surfaces than any single previous model, though it is a direct outgrowth of several earlier approaches. We hope that this review will stimulate a more unified view of organic and inorganic colloids and that it will trigger systematic study of the ionization and complexation reactions at surfaces and of the appropriate intrinsic equilibrium constants for surface functional groups. |
