Effective pair potentials in charged molecular systems:: Water mediated interactions

We study the effective interactions among ions of various valences immersed in a polar solvent by determining the effective pair potentials (EPPs) obtained after "contracting" (integrating out) the degrees of freedom of the solvent molecules. This is accomplished by extending the framework of the dressed ion theory to describe the formal solution of the reference interaction site model integral equations. It is shown here that these EPPs have in the asymptotic limit the expected Coulomb form, even for finite concentrations of the solute. At shorter distances, the molecular structure of the solvent induces noticeable deviations of the EPPs from the reference potentials corresponding to an analogous model system with a structureless dielectric background. These deviations remain almost invariant over the whole range of concentrations considered here. These EPPs are therefore suitable to be used as the input for computer simulations at the McMillan-Mayer level. We also analyze with some detail the consequences of the short range behavior of the EPPs involving multivalent ions. (C) 2002 American Institute of Physics.