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 interface along the orthobaric curve between 277 K and 548 K. Molecular dynamics simulations of 500 and 1000 water molecules are performed using a slab geometry with the long-range interactions calculated using the Ewald sum. The coexisting densities and the surface tension are predicted accurately over this temperature range. The model predicts different preferential orientation of the molecular dipoles on either side of the Gibbs dividing surface. The components of the second-order nonlinear susceptibility in the interface are calculated using the five independent elements of the molecular hyperpolarizability (the Kleinman relationships are not assumed). The susceptibility profiles are presented and the integrated susceptibility is positive in agreement with experiment. The temperature dependence of the components of susceptibility tensor is in reasonable agreement with experiment and in good agreement with density functional theory. The calculated electrostatic surface potential and the corresponding temperature coefficient are both negative at room temperature. The negative sign of the surface potential is due to the large quadrupole contribution whereas the dipole contribution is responsible for the negative sign of the temperature coefficient. The model has been extended by adsorbing phenol at the interface and the orientational distributions for the water in the presence of adsorbate are presented.