Dynamics of Adsorbed Counterions of Surface-Anchored Polyelectrolytes

Dynamics of counterions adsorbed on the main chain of polyelectrolytes are investigated by broadband dielectric spectroscopy. Sodium polystyrenesulfonate (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 are effectively detected and measured as a function of the molecular weight, salt concentration, grafting density, and temperature, as well as the type of counterions. The key findings are the decrease of activation energy for the counterion to desorb from the main chain with the increase of the molecular weight of the polyelectrolyte, salt concentration, and atomic number of the alkali counterion. The results demonstrate that the effective charge density of the main chain is reduced for higher molecular weights and higher salt concentrations, indicating the occurrence of enhanced counterion adsorption. The dependence of the dynamics of counterions on the 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 the polyelectrolyte chain and its counterions is a delocalized and correlated process.