Molecular Dynamics Simulation Studies of the Limiting Conductances of CaCl2 using Extended Simple Point Charge and Revised Polarizable Models

Abstract

We have carried out molecular dynamics (MD) simulations of the limiting conductances of CaCl2 in ambient and supercritical states as a function of water density using extended simple point charge (SPC/E) and revised polarizable (RPOL) models for ions and water molecules. Both models predict the limiting conductances of CaCl2 in supercritical water that are a linear dependence on water density. The effect of the electronic polarization on the limiting conductances is too small to cause a deduction in the lower water density of 0.6 ∼ 0.7 g/cc in this study. The most significant effect of the electronic polarization is appeared in a decrease in the ion–water potential energy and, as a result, an increase in the limiting conductances for both ions. Different charge distributions of water molecules in the first hydration shell around the ions lead the opposite behavior of the induced dipole moment with water density for a positive and a negative ion in supercritical water; the induced dipole moment of Ca2+ decreases with increasing water density but for Cl-, the opposite is observed. The same kind of opposite behavior due to the structure of water molecules around the ions is also found in hydrogen-bond correlations of water around the ions and of bulk water; hydrogen bonding around Ca2+ persists longer than in bulk water whereas the opposite is observed for Cl-.