Ions at membranes: a density functional approach

Abstract

We present application of the density functional method to describe the structure of an ionic fluid in contact with a semipermeable membrane. The membrane is built of two differently charged plates, separated by a distance Δ. In addition to the electrostatic potential, created by the walls delimiting the membrane, our model assumes the existence of a short range van der Waals potential between the membrane and the surrounding liquid. The electrolyte beyond one of the outer membrane surfaces is a three component restricted primitive electrolyte consisting of two cations and one anion. The membrane is impermeable to one of the cations so that the fluid in the membrane and beyond its second delimiting surface is a two component restricted primitive electrolyte. The theory also allows for the presence of a primitive solvent, the molecules of which can penetrate the membrane. We also study a membrane consisting of a single charged plane. A comparison of the Monte Carlo results with theoretical predictions shows that the density functional theory is capable to reproduce the simulated results quite well. Additionally, we perform some model calculations, demonstrating the usefulness of the proposed approach.