ESTIMATION OF THE SURFACE-TENSION OF POLAR FLUIDS - LONG-RANGE CONTRIBUTIONS

The theoretical prediction of the surface tension of polar fluids is c onsidered with the focus on the long-range contributions arising due t o orientational correlations among the dipoles. A set of three differe nt methods are applied to a simple model of a polar fluid: (i) A very simple analysis is presented based on an effective dipole-dipole poten tial and the use of the generalized van der Waals (GvdW) density funct ional theory which has proven to be quite accurate for Lennard-Jones f luids. The effective dipole-dipole potential retains, in a low-order a pproximation, the Lennard-Jones form of the full potential but adds te mperature dependence to the potential parameters. (ii) Molecular dynam ics (MD) simulations provide a far more accurate account of the short- range interaction in the polar fluid but suffer from numerical difficu lties associated with the need for a cutoff on the range of the intera ction to which the surface tension is particularly sensitive. (iii) Fo r this reason we introduce a hybrid MD/GvdW theory wherein the long-ra nge contribution to the surface tension is estimated by the correspond ing term in the GvdW result. Applications to the single-component flui ds composed of HCl or HBr are presented. The interaction potential is taken to be of either Stockmayer form or a closely related form where the dipole moment is created by two point charges. The results show th at the GvdW estimate agrees reasonably with experiment, and MD simulat ion and MD/GvdW calculation improves the xonvergence of the MD estimat e by inclusion of both direct long-range interaction and indirect effe cts due to changes in the bulk phases.