PNNL

Abstract

We describe our molecular dynamics simulation studies of the inter-ionic potentials of mean force (PMF) for the Ca2+-Cl- ion pair in water. Ion-water and ion-ion polarizable potential models were constructed to reproduce experimental ion-hydration enthalpies as well as the electronic-structure calculations of the minimum energy for the Ca2+-Cl- interactions. Our study predicted a PMF with no contact ion-pair state and a shallow solvent-separated ion-pair state. In addition, our computed radial distribution function, gCa-Cl, for a 1-M CaCl2 electrolyte solution indicated that there is no contact-ion pair but, instead, showed a maximum near 5 Å. This observation is consistent with our PMF calculation. These new results are discussed and compared to the recent classical and ab initio PMF calculations on the same system reported by Timko, De Castro, and Kuyucak (J. Chem. Phys. 134, 204510 (2011)). This work was funded by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences (BES), of the U.S. Department of Energy (DOE). Battelle operates PNNL for DOE. The calculations were carried out using computer resources provided by BES.