Peter J. Dyer and Peter T. Cummings. Department of Chemical Engineering, Vanderbilt University, Nashville, TN 37235-1604
We developed force fields for several monovalent ions utilizing Gaussians charges that are polarizable, designed to work with Gaussian charge polarizable model (GCPM) for water. We have compared the effect of using polarization within the ions, the effect of the ion on the dipole of the neighboring water molecules, the structure of ion-water clusters, surface verses bulk solvation, water and ion orientation, residence time of the 1st solvation shell, diffusivity, structure and free energy of solvation at infinite dilution. These properties have been compared to published ab initio and experimental results in order to evaluate the ion-water force fields. The addition of the Gaussian charges softens the electrostatic interactions of the ion to its neighboring water molecules. We also found that kosmotropes and chaotropes affect the magnitude of the induced dipole depending on the number of water molecules in the different sized clusters, where kosmotropes result in a large induced dipole of the ion-single water complex, which decays with the number of water molecules to the bulk value, while chaotropes have a lower induced dipole for the ion-single water complex, increasing with the number of water molecules until the bulk value. We observe an increase in the induced dipole moment of water molecules located in the first hydration shell around an ion. This increase is , most significant for smaller cations, and we found evidences for the induced dipole of water molecule to be related to the structure of water molecules around the ion, not just radial distance from the ion center. We also calculated free energy of solvation for the anions and cations, finding good agreement with experimental data.