Molecular solvent model for an electrical double layer: Reference hypernetted-chain (RHNC) results for solvent structure at a charged surface

Abstract

We report results of solving the full reference hypernetted-chain (RHNC) theory for a large, multiply charged macroion at infinite dilution in a solvent of hard spheres with point dipole and quadrupole moments chosen to represent liquid water. We obtain results for the restructuring of this model solvent next to the macroion surface for a range of surface charges and macroion sizes up to 30 solvent diameters. Although we are unable to solve the theory for larger particles, when this largest macroion is neutral we find a solvent orientational structure in the surface layer that is in good qualitative agreement with computer simulation results for water-like models at a planar surface. Our RHNC calculations show that this surface structure proves surprisingly resistant to the effect of surface charges as high as 17.5 μC/cm2, apparently because even such strong fields cannot compete with the still stronger intermolecular forces of water-like models. There is, nevertheless, strong oscillatory behavior in both the polarization density and the mean electrostatic potential and we discuss the implications of this for theories and models of the electrical double layer.