Quasichemical and structural analysis of polarizable anion hydration

摘要

freeenergies using the polarizable AMOEBA force field. The total hydration free energies display astronger dependence on ion size than on polarizability. The quasichemical approach exactlypartitions the solvation free energy into inner-shell, outer-shell packing, and outer-shell long-rangedcontributions by means of a hard-sphere condition. The inner-shell contribution becomes slightlymore favorable with increasing ion polarizability, indicating electrostriction of the nearby waters.Small conditioning radii, even well inside the first maximum of the ion-water(oxygen) radialdistribution function, result in Gaussian behavior for the long-ranged contribution that dominates theion hydration free energy. This in turn allows for a mean-field treatment of the long-rangedcontribution, leading to a natural division into first-order electrostatic, induction, and van der Waalsterms. The induction piece exhibits the strongest ion polarizability dependence, while thelarger-magnitude first-order electrostatic piece yields an opposing but weaker polarizabilitydependence. The van der Waals piece is small and positive, and it displays a small ion specificity.The sum of the inner-shell, packing, and long-ranged van der Waals contributions exhibits littlevariation along the anion series for the chosen conditioning radii, targeting electrostatic effects(influenced by ion size) as the largest determinant of specificity. In addition, a structural analysis isperformed to examine the solvation anisotropy around the anions. As opposed to the hydration freeenergies, the solvation anisotropy depends more on ion polarizability than on ion size: increasedpolarizability leads to increased anisotropy. The water dipole moments near the ion are similar inmagnitude to bulk water, while the ion dipole moments are found to be significantly larger thanthose observed in quantum mechanical studies. Possible impacts of the observed over-polarizationof the ions on simulated anion surface segregation are discussed.