Specific ionic interactions within a simple extension of the Gouy-Chapman theory including hard sphere effects

A field theoretical approach is used to describe the properties of ionic solutions near a charged wall. The properties of ions are accounted for by the hard sphere effect and interaction coefficients a(++), a(--) and a(+-) determining short range correlation in the system other than the hard sphere repulsion. The effect of these coefficients on the differential capacity curves depends on the polarization. For symmetric interaction coefficients, a(++) a(--) a(+-), we observe the Gouy-Chapman minimum in dilute solutions at the potential of zero charge (pzc) which becomes a maximum in concentrated solutions. This minimum is surrounded by two maxima. For a given hard sphere diameter the capacity at these maxima depends on a(++) or a(--) for the negative and positive charge density, respectively, and decreases with more positive coefficient values meaning more repulsive interactions between counterions. At extreme charge densities the capacity becomes independent of bulk ionic concentration, but we do not observe a limiting constant value as we do without the hard sphere effect. If the interaction coefficient a(+-) is different from a(++) and a(--), the capacity close to the pzc differs from the Gouy-Chapman value. The theoretical double layer capacity curves are compared with those obtained experimentally in various electrolytes on various metals as well as calculated using a different theoretical approach. (C) 2004 Elsevier B.V. All rights reserved.