This work utilizes a combination of theory and experiments to explore adsorption on (ellipsometry) and forces (AFM) betweeen silica surfaces, immersed in solutions containing highly charged cationic polyelectrolytes. We investigate salt responses, as well as how adsorption and surface interactions depend upon the average degree of polymerization, and polymer type. For all kinds of polyelectrolytes we studied, at low concentrations of simple salt, the adsorption increases with ionic strength, reaching a maximum at intermediate levels (about 200 mM). The adsorption then drops but retains a finite level even at very high salt concentrations, indicating the presence of nonelectrostatic contributions to the adsorption. The adsorption of polymer with shorter chain length increases more slowly in lower salt region but drops stronger in higher salt region comparing with longer chain polymer. We shall demonstrate that our recently developed correlation corrected classical density functional theory prediction captures the overall adsorption behaviours, even under the assumption that the adsorption is of purely electrostatic origin, since a silica surface usually carries a relatively high charge at pH 9, which means we have specifically chosen to investigate adsorption under conditions where electrostatics dominate the interactions. However, the short-ranged non-electrostatic surface affinity was also estimated by matching predicted and experimental slopes of adsorption curves at high ionic strength. Additionally, since the surface charge density of silica varies with different salt concentration, the titration surface effects were also considered in the theoretical treatment. This leads to a quantitative improvement, as compared with experimental data, especially at low salt. Given these estimates for the nonelectrostatic part and titration response, experimental adsorption data are essentially captured with quantitative accuracy by the classical density functional theory.
fei.xie@teokem.lu.se
