Non-monotonic Effects of Intrinsic Stiffness and Concentration of Polyelectrolytes on the Electro-Sorption

Abstract

The pure electro-sorption behaviors of semiflexible polyelectrolytes onto an oppositely charged planar surface are studied by means of mean-field theory based on the discretization of the Kratky-Porod wormlike chain model. The various polyelectrolyte intrinsic rigidities, surface charge densities, and monomer and salt concentrations are systematically considered. We focus on the effects of the intrinsic rigidities and monomer concentrations and investigate the adsorption/desorption limit and adsorption amount under specific conditions. By analyzing the density profiles of the species, we find that in the salt-free systems, the critical adsorption surface charge density increases (the adsorption amount decreases) monotonically with monomer concentrations and intrinsic rigidities, while in the saline solution, the intrinsic rigidity- and monomer concentration-dependent adsorption behaviors are more complicated and interesting. The adsorption behaviors of polyelectrolytes in the low-salt concentration ( 0.2 M for a given monomer concentration 0.01 M in our work), the critical adsorption surface charge density and adsorption amount show non-monotonic dependence on the polyelectrolyte intrinsic rigidities. Moreover, at some intermediate salt concentrations, the adsorption amount first increases and then decreases with monomer concentrations and the monomer density ρₘ* at which the maximum adsorption amount occurs increases with salt concentrations but decreases with intrinsic chain rigidities. Additionally, we find that the rigidity-dependent adsorption behaviors of polyelectrolytes at high salt concentrations can be understood by those of neutral semiflexible polymers near a non-electrostatic attraction surface, while the monomer concentration-dependent adsorption behaviors of strongly charged polyelectrolytes considered in this work are qualitatively different from the ones of neutral polymers in the entire range of salt concentrations.