Structure of a diatomic fluid near a wall

Abstract

Molecular dynamics has been used to simulate a high density fluid of homonuclear diatomic molecules in the presence of a rigid planar wall. The molecular interactions were modelled by Lennard-Jones (12, 6) site-site potentials and (17, 3) wall-site potentials. The relative influence of attractive and repulsive forces on the interfacial orientational structure was studied by comparing with simulations in which the attractive components of either or both the site-site and wall-site potentials were absent. The results, presented as contour plots of the density-orientation profile ρ(z, θ), show that parallel orientation of the molecules is favoured when the centre-of-mass distance z is very close to the wall, but that perpendicular orientation is favoured at slightly larger distances. The presence of weakly attractive wall-site forces slightly increases the relative degree of parallel alignment compared with that in the absence of such forces. The presence of attractive site-site pair interactions diminishes considerably the amplitude of oscillations in ρ(z, θ) compared with their amplitude in the absence of such interactions, but does not modify the orientational preferences. This is attributed to increased cohesion of the liquid phase when the pair attractions are present, leading to a non-wetting state of the liquid-solid interface and a corresponding reduction in the density and wall-induced orientational anisotropy of the fluid layers near the wall. Site-wall density profiles were also obtained from the simulations and compared with the results of an interaction site approximation, integral equation theory: the agreement is only fair at best.