Structure of a diatomic fluid near a wall. I

Abstract

In recent years there has been considerable interest in the density profiles of dense monatomic fluids near solid surfaces. A natural extension of this work is to consider the density and orientational structure of a polyatomic fluid near a solid wall. As a first step in this direction we have calculated the wall-site correlation functions of symmetric hard diatomic molecules near a hard planar wall. These have been obtained from a generalization of the bulk RISM integral equation theory to the situation of a polyatomic fluid in the presence of a spherical particle whose radius is allowed to become infinite. We compare our results with Monte Carlo simulations and find qualitative agreement, although somewhat worse quantitatively than in bulk fluids. The results indicate that with increasing density and elongation, the molecules in the layer nearest to the wall increasingly prefer orientations parallel to the wall. Difficulties associated with solving the wall-RISM theory in the case of polyatomic molecules whose sites are non-equivalent are discussed. Some alternative integral equation theories for the wall-site correlation functions, suggested by variants of an approach used to derive the bulk RISM approximation, are examined and shown to be less accurate than the wall-RISM theory.