Molecular dynamics study of gas-liquid aqueous sodium halide interfaces. I. Flexible and polarizable molecular modeling and interfacial properties

Gas - liquid interfacial structures of NaCl and NaI aqueous solutions are investigated via molecular dynamics simulations using a flexible and polarizable water model we have developed. The new five-site model of water aims at suitably describing interfacial properties, including vibrational sum frequency spectroscopy where both flexibility and polarization are crucial. The performance of the water model is systematically examined and demonstrated by a number of properties of bulk and interface, including density, vaporization energy, dipole moment, diffusion coefficient, radial distribution function, infrared and Raman spectra of the O - H stretching region, surface potential, and surface excess of ions. The orientational structure of surface water is investigated in detail in connection with the issue of surface solvation of anions. These investigations will be utilized to analyze the sum frequency generation spectra in relation to the orientational structure at the molecular level.