Adsorption of Cationic Polyacrylamide onto Monodisperse Colloidal Silica from Aqueous Electrolyte Solutions

Adsorption of a low linear charge density cationic polyacrylamide (CPAM) onto monodisperse silica particles has been studied over a wide range of KCl concentration (10(-3)-1 M) and pH (4-9). In parallel the surface charge density of silica particles with and without adsorbed polyelectrolyte has been determined. The adsorbed amount (Gamma) as a function of electrolyte concentration (cs) exhibits a maximum at cs approximately 0.1 M. As pH increases from 4 to 6, Gamma increases and then either reaches a plateau (cs 0.1 M). These features can be rationalized in terms of the interplay between the different factors that govern polyelectrolyte adsorption. The effect of polyelectrolyte adsorption on the surface charge density of silica is largest at low cs. It decreases with increase of the salt content and vanishes at cs > 0.1 M. This suggests that at electrolyte concentrations lower than 0.1 M, the polyelectrolyte plays a dominant in surface charge regulation. The ratio of the adsorbed polyelectrolyte charge to total surface charge is close to unity at high pH. It increases drastically with decreasing pH and increasing salt. A large overcompensation of the substrate charge by polyelectrolyte at these conditions is established in the present work.