Protonation of silica particles in the presence of a strong cationic polyelectrolyte

Potentiometric titrations and electrophoresis techniques were used to study the charging behavior of aqueous silica particle suspensions in the presence of poly(N,N-diallyldimethylammonium chloride) (DADMAC). The main finding is that the particles undergo charge reversal with increasing pH, which can be directly seen by electrophoresis and as a common crossing point in the titration curves at different salt levels. These features resemble the charging behavior of an amphoteric oxide surface. With increasing polymer dose, the charge reversal point shifts towards higher pH. These trends are well reproduced by a modified Stern model. The main effect of the adsorbed cationic polyelectrolyte is to further deprotonate the surface silanol groups, and thereby to induce additional negative charge on the surface.