Comparison of the Solvation Structure of Polarizable and Nonpolarizable Ions in Bulk Water and Near the Aqueous Liquid−Vapor Interface

Abstract

All-atom molecular dynamics simulations are performed on bulk and aqueous liquid−vapor interfacial systems for neat water and 1 M solutions of NaCl, CsCl, and NaI to more quantitatively characterize ion solvation in these environments and to more fully elucidate trends accompanying the introduction of ion polarizability in the context of a polarizable water potential. In the bulk systems, computed densities, hydration free energies, compressibilities, coordination numbers, and a variety of two- and three-atom distribution functions show that the polarizable ions exhibit smaller effective radii, a slightly increased propensity for bulk hydration (particularly for the anions), and no biased preference for contact ion or solvent-separated ion pairing relative to the corresponding nonpolarizable cases. For the interfacial systems employing long (>40 ns) sampling times, we obtain good convergence for a variety of interfacial characteristics such as slab thicknesses and interfacial widths, depth-dependent water and ion density profiles, surface excesses, and depth-dependent transverse distribution functions. We compare interface characteristics derived from fits to both a hyperbolic tangent and error function form and find negligible differences in the resulting parameters. For all three salt solutions, we observe that ion polarizability leads to a slight contraction of the liquid slab and simultaneous expansion of the interfacial thickness arising from the strong surface enhancement of polarizable anions. Computed values of the surface excess show that for the present Drude ion models and polarizable fluctuating charge water model the introduction of ion polarizability consistently leads to a slightly less-negative surface excess, implying a significant but incomplete cancellation of the surface enhancement by the corresponding subsurface anion depletion. Depth-dependent, transverse correlation functions for the oxygen−oxygen separation indicate slight water relaxation at the interface of both neat TIP4P-FQ water and the corresponding salt solutions.