Classical molecular dynamics computer simulations have been used to investigate the thermodynamics and kinetics of sodium chloride association in polarizable water. The simulations make use of the three-site polarizable water model of Dang [J. Chem. Phys. 97, 2659 (1992)], which accurately reproduces many bulk water properties. The model’s static dielectric constant and relaxation behavior have been calculated and found to be in reasonable agreement with experimental results. The ion–water interaction potentials have been constructed through fitting to both experimental gas-phase binding enthalpies for small ion–water clusters and to the measured structures and solvation enthalpies of ionic solutions. Structural properties and the potential of mean force for sodium chloride in water have been calculated. In addition, Grote–Hynes theory has been used to predict dynamical features of contact ion-pair dissociation. All of the calculated ionic solution properties have been compared with results from simulations using the extended simple point charge (SPC/E), nonpolarizable water model [J. Phys. Chem. 91, 6296 (1987)]. The dependence on polarizability is found to be small, yet measurable, with the largest effects seen in the solvation structure around the highly polarizable chlorine anion. This work validates the use of some nonpolarizable water models in simulations of many condensed-phase systems of chemical and biochemical interest.

The molecular structure of the interfacial regions of aqueous electrolytes is poorly understood, despite its crucial importance in many biological, technological, and atmospheric processes. A long-term controversy pertains between the standard picture of an ion-free surface layer and the strongly ion specific behavior indicating in many cases significant propensities of simple inorganic ions for the interface. Here, we present a unified and consistent view of the structure of the air/solution interface of aqueous electrolytes containing monovalent inorganic ions. Molecular dynamics calculations show that in salt solutions and bases the positively charged ions, such as alkali cations, are repelled from the interface, whereas the anions, such as halides or hydroxide, exhibit a varying surface propensity, correlated primarily with the ion polarizability and size. The behavior of acids is different due to a significant propensity of hydronium cations for the air/solution interface. Therefore, both cations and anions exhibit enhanced concentrations at the surface and, consequently, these acids (unlike bases and salts) reduce the surface tension of water. The results of the simulations are supported by surface selective nonlinear vibrational spectroscopy, which reveals among other things that the hydronium cations are present at the air/solution interface. The ion specific propensities for the air/solution interface have important implications for a whole range of heterogeneous physical and chemical processes, including atmospheric chemistry of aerosols, corrosion processes, and bubble coalescence.

The molecular dynamics computer simulation technique is used to develop a rigid, four-site polarizable model for water. The suggested model reasonably describes the important properties of water clusters, the thermodynamic and structural properties of the liquid and the liquid/vapor interface of water. The minimum energy configurations and the binding energies for these clusters are in reasonable agreement with accurate electronic structure calculations. The model predicts that the water trimer, tetramer, and pentamer have cyclic planar minimum energy structures. A prismlike structure is predicted to be lowest in energy for the water hexamer, and a cagelike structure is the second lowest in energy, with an energy of about 0.2 kcal/mol higher than the prismlike structure. The results are consistent with recent quantum Monte Carlo simulations as well as electronic structure calculations. The computed thermodynamic properties for the model, at room temperature, including the liquid density, the enthalpy of vaporization, as well as the diffusion coefficient, are in excellent agreement with experimental values. Structural properties of liquid water, such as the radial distribution functions, neutron, and x-ray scattering intensities, were calculated and critically evaluated against the experimental measurements. In all cases, we found the agreement between the observed data and the computed properties to be quite reasonable. The computed density profile of the water's liquid/vapor interface shows that the interface is not sharp at a microscopic level and has a thickness of 3.2 Å at 298 K. These results are consistent with those reported in earlier work on the same systems. The calculated surface tension at room temperature is in reasonable agreement with the corresponding experimental data. As expected, the computed average dipole moments of water molecules near the interface are close to their gas phase values, while water molecules far from the interface have dipole moments corresponding to their bulk values.