Density-functional theory of the water liquid-vapour interface

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Abstract
An extended mean-field density-functional theory of the liquid-vapour interface of water is described. The theory generalizes standard mean-field theories of inhomogeneous molecular fluids by including quadratic orders of the anisotropic component of the intermolecular pair potential in the free energy functional. The pair interaction is modelled by an isotropic plus point dipolar and quadrupolar potential. Analysis shows that dipole-quadrupole coupling terms in the potential are responsible for inducing spontaneous polarization at the liquid-vapour interface. The direction of the surface polarization is determined by the sign of the axial component of the molecular quadrupole tensor, as in the earlier phenomenological theory of Stillinger and Ben-Naim. Explicit calculations are performed using molecular interaction parameters given by the TIP4P potential model for water, employed in recent computer simulations of the water interface by Wilson, Pohorille and Pratt. The preferred molecular orientations at the interface predicted by the theory are analysed in detail and compared with previous simulation results.