Simulation and density functional study of a simple membrane. II. Solvent effects using the solvent primitive model

The simple membrane, supporting charge densities sigma(1) and sigma(2)=-sigma(1) on its inner and outer surfaces that we studied previously is extended to include solvent molecules, modeled as hard spheres. In addition to the electrostatic potential, the membrane can interact with the surrounding electrolyte by a short-range van der Waals-type potential that can be attractive or repulsive. The fluid beyond the outer surface is four component electrolyte consisting of the hard sphere solvent, two species of cations, and one species of anions. The membrane is impermeable to one of the cation species so that the fluid in the membrane and beyond the inner surface is a three component electrolyte. Monte Carlo simulations and density functional theory are used to study the density profiles of the electrolyte and the charge-electrostatic potential relationship for the membrane surfaces. The presence of the solvent molecules leads to oscillatory profiles. The potential versus charge relationship is strongly influenced by the solvent density. The density functional results are in good agreement with the simulations. (C) 2000 American Institute of Physics. [S0021-9606(00)50626-6].