Orientational structure at a vapor–liquid interface: Effect of electrostatic forces

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Abstract
The vapor–liquid interfacial density-orientation profile ρ(z1ω1) gives the probability of finding a molecule at height z1 in the interface with orientation ω1. We present a perturbation theory for this function, using the Mayer f function as the expansion functional. This ’’f expansion’’ has the advantage over the previously used ’’u expansion’’ that at first order it predicts orientational ordering in the surface due to electrostatic forces. The theory is used to study orientational structure in the surface for a model fluid in which the intermolecular potential consists of site–site Lennard-Jones terms plus a quadrupole–quadrupole term. When the quadrupole moment is set equal to zero, the theory predicts that the molecules on the liquid side of the dividing surface prefer an orientation perpendicular to the interface. As the strength of the quadrupole moment is increased, this preference for a perpendicular orientation is reduced, and for moderately strong quadrupoles it is replaced by a preference for an orientation parallel to the surface. These predictions are in qualitative agreement with recent molecular dynamics results.