Dynamics of Adsorbed Counterions of Surface-Anchored Polyelectrolytes

Dynamics of counterions adsorbed on the main chain of polyelectrolytes is investigated by broadband dielectric spectroscopy. Sodium polystyrene sulfonate (NaPSS) grafted on planar surfaces is adopted as the model system. Taking advantage of the giant dipole at the surface layer created by charge separation, the dynamics of adsorbed counterions on polyelectrolyte chains is effectively detected and measured as a function of molecular weight, salt concentration, grafting density and temperature. The key findings are the decrease of activation energy for counterion to desorb from the main chain with the increase of molecular weight of the polyelectrolyte and salt concentration. The results demonstrate that the effective charge density of the main chain is reduced for higher molecular weight samples and higher salt concentration, indicating the occurrence of enhanced counterion adsorption. The dependence of dynamics of counterions on grafting density reveals the effect of overlapping of electric fields of individual chains, with chain conformation change as a secondary process. The results demonstrate that the interaction between polyelectrolyte chain and its counterions is a de-localized and correlated process.