Anisotropic van der Waals model of the liquid–vapor interface

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Abstract
We study the properties of the liquid–vapor interface in a model system with anisotropic long-ranged attractive interactions, so chosen that exact calculations (using the density functional formalism) can be carried out and for which most results of the classical van der Waals theory of the liquid–vapor interface hold true without approximation. In particular there is an ‘‘intrinsic’’ interface profile of finite width, which near the critical point takes on the hyperbolic tangent form. However, contrary to approximate calculations of Evans, all pair correlations (except for trivial k=0 correlations) are short ranged. Indeed they are identical to those in a nonuniform (one phase) hard sphere system in an external field chosen to produce the desired density profile. The long-ranged anisotropic interactions suppress long wavelength (capillary wave) density fluctuations, which in a realistic system produce both long-ranged pair correlations parallel to the interface and a nonuniversal density profile.