The effects of electrical charge on the adsorption of a weak cationic polyelectrolyte onto silica, silicon carbide and calcium fluoride

Adsorption of polyvinylimidazole, a cationic polyelectrolyte (pK ≈ 5.0), is measured on silica, silicon carbide, and fluorite as a function of pH, monovalent electrolyte (NaCl, KCl or KNO3) and sulfate ion concentration. Adsorption does not depend on the molecular weight in the range 10 000–450 000 Da. Except when the surface is highly positive (CaF2 substrate), the surface affinity for the polyelectrolyte is strong and the coverage is limited by the interaction forces between adsorbed polymer units. This qualitatively explains why adsorption depends on the composition of the solution. Electrophoretic measurements show that the strong increase in adsorption induced by adding sulfate ions is due to their binding with the polymer and also with the substrate in the particular case of CaF2 and SiC. The bell-shaped isotherms and adsorption maxima observed in this study agree qualitatively with recent models of polyelectrolyte adsorption.