Ions at membranes: a density functional approach

We present application of the density functional method to describe the str ucture of an ionic fluid in contact with a semipermeable membrane. The memb rane is built of two differently charged plates, separated by a distance De lta. In addition to the electrostatic potential, created by the walls delim iting the membrane, our model assumes the existence of a short range van de r Waals potential between the membrane and the surrounding liquid. The elec trolyte beyond one of the outer membrane surfaces is a three component rest ricted primitive electrolyte consisting of two cations and one anion. The m embrane is impermeable to one of the cations so that the fluid in the membr ane and beyond its second delimiting surface is a two component restricted primitive electrolyte. The theory also allows for the presence of a primiti ve solvent, the molecules of which can penetrate the membrane. We also stud y a membrane consisting of a single charged plane. A comparison of the Mont e Carlo results with theoretical predictions shows that the density functio nal theory is capable to reproduce the simulated results quite well. Additi onally, we perform some model calculations, demonstrating the usefulness of the proposed approach.