Adsorption of zinc on colloidal silica, triple layer modelization and aggregation data

In view of the control of the formation of silica conversion coatings on galvanized substrates, the interaction between silica nanoparticles and dissolved zinc species was studied as a function of pH. The influence of the concentration of zinc species, mostly Zn2+ and ZnOH+, on the ionization of the silica surface was measured and modelized using the triple layer model and a single set of interfacial parameters (inner and outer dielectric capacitances, ionization and adsorption equilibrium constants, surface density of active sites). The critical coagulation (gelation) concentration (CCC) of zinc salt in the colloidal silica dispersion was measured between pH 7 and 9.5. It was observed that the CCC’s correspond almost exactly to the zinc adsorption level where it balances the surface charge of the silica particles, showing that the DLVO theory applies in this case.