Integral equation theory for the electrode-electrolyte interface with the central force water model. Results for an aqueous solution of sodium chloride

Abstract

The structure of an aqueous solution of sodium chloride at a planar surface is investigated by integral equation techniques. With the central force water model the aqueous electrolyte is modelled as a mixture of sodium and chloride ions, and partially charged hydrogen and oxygen atoms interacting via effective spherically symmetric pair potentials. The correlation functions obtained from the Ornstein-Zernike equation with reference hypernetted chain closure give a good description of the bulk structure (e.g., hydrogen bonded water network, solvation shell). With the bulk information and the Wertheim-Lovett-Mou-Buff equation we have calculated the density profiles at the uncharged and charged surfaces. The rather rigid ice-like water structure found previously at the neutral surface strongly repels the ions. Steric interactions between the ions of different sizes and the ice-like water structure dominate the ionic distribution near the surface. This model electrolyte also responds differently to opposite charges on the surface. We found the asymmetry in the differential capacitance curve determined entirely by the response of the interfacial water structure.