Molecular orientation at a gas–liquid interface

Abstract

In a fluid of molecules with orientation-dependent intermolecular forces there will be preferred molecular orientation near a gas–liquid interface. The present knowledge of such effects is discussed for some simple model fluids, for the case of a plane surface. The property of direct theoretical interest is the density–orientation profile, rho;(z, ω), which gives the density of molecules at height z in the interface having an orientation ω relative to space-fixed axes. It has been studied both theoretically and via computer simulation for several model fluids, including those with site–site Lennard-Jones, site–site Lennard-Jones with distributed point charges, and Lennard-Jones plus multipole intermolecular potentials. Theoretical calculations can be based on perturbation theory or integral equation methods. In the latter approach there are several alternative integro-differential equations for rho;(z₁ω_–) which provide rigorous starting points for the theory. Results from perturbation theory are compared with those from molecular dynamics simulation for several model potentials. Preliminary results from integral equation theory are given.