THE FREE-SURFACE OF WATER - MOLECULAR-ORIENTATION, SURFACE-POTENTIAL AND NONLINEAR SUSCEPTIBILITY

The SPC/E model of water has been used to simulate the vapour-liquid i nterface along the orthobaric curve between 277 K and 548 K. Molecular dynamics simulations of 500 and 1000 water molecules are performed us ing a slab geometry with the long-range interactions calculated using the Ewald sum. The coexisting densities and the surface tension are pr edicted accurately over this temperature range. The model predicts dif ferent preferential orientation of the molecular dipoles on either sid e of the Gibbs dividing surface. The components of the second-order no nlinear susceptibility in the interface are calculated using the five independent elements of the molecular hyperpolarizability (the Kleinma n relationships are not assumed). The susceptibility profiles are pres ented and the integrated susceptibility is positive in agreement with experiment. The temperature dependence of the components of susceptibi lity tensor is in reasonable agreement with experiment and in good agr eement with density functional theory. The calculated electrostatic su rface potential and the corresponding temperature coefficient are both negative at room temperature. The negative sign of the surface potent ial is due to the large quadrupole contribution whereas the dipole con tribution is responsible for the negative sign of the temperature coef ficient. The model has been extended by adsorbing phenol at the interf ace and the orientational distributions for the water in the presence of adsorbate are presented.