Polyelectrolyte brushes with added salt

Using molecular dynamics simulations, we study completely charged polyelectrolyte brushes with added salt at moderate electrostatic coupling strength. The full Coulomb interaction of monomers, counterions, and salt ions is treated explicitly. In the (nonlinear) osmotic brush regime, the brush thickness h is found to decrease with the salt concentration c(s), obeying a weak power law, h similar to c(s) (-1/3). This result is in agreement with the scaling prediction by Pincus. The transverse ion distribution obtained in the simulations can be understood by means of a modified Donnan approach which takes into account the self-volume of polyelectrolyte chains.