Structure of a simple fluid near a wall. I. Structure near a hard wall

Abstract

The wall‐particle Ornstein–Zernike equation is used to obtain the density profile near a hard wall for a fluid with pair potential consisting of a hard‐sphere core and Lennard–Jones attractive tail. Several closure approximations are compared: these include the wall‐particle analogs of the PY, HNC, EXP, and RHNC approximations. We also examine a new class of approximations motivated by the special properties of surface structure, where attractive interparticle forces have much greater influence than in the case of bulk structure. These new approximations are of a mean‐field (MF) nature, treating the fluid as equivalent to a hard‐sphere fluid in a certain effective potential. The numerical results suggest that two new approximate closures, the MF–PY and MF–EXP approximations, bracket the true behavior of the fluid near a hard wall. There are striking differences between these approximations, however, as there are among all those that we have investigated (especially in the region close to the wall). Despite the relative sophistication of our approximation techniques, when viewed in terms of analogous bulk‐fluid techniques, more ’’exact’’ computer‐simulation results are needed before it will be clear which of our approximations is the most reliable.