The form of the density profile at a liquid-gas interface

Abstract

The form of the density profile at the liquid-gas interface of some model fluids is investigated using a simple free-energy functional. For fluids with attractive Yukawa potentials we find that the profile decays exponentially into the bulk regions with a decay length greater than the appropriate bulk correlation length; the two lengths become closer and diverge in the same fashion at the critical temperature. We contrast this decay with that found for fluids with attractive van der Waals potentials where the profile decays asymptotically as z ^-3. The results of numerical calculations suggest that the effects of van der Waals' potentials would be difficult to observe in experiments or in computer simulations of liquid-gas interfaces. We compare the results of our calculations for fluids with truncated Lennard-Jones potentials with those of recent simulations. The theory provides a rather good account of the interfacial width and surface tension as a function of temperature. Our results show that truncation of the potential has a significant effect on the surface properties. We comment briefly on attempts to infer capillary-wave contributions to interfacial quantities from comparison of theory and simulation.