Structure of the liquid–vapor interface of molecular fluids: Homonuclear diatomic molecules

The molecular dynamics method is used to simulate the liquid–vapor interface of fluids modeled on nitrogen and chlorine. Site–site Lennard-Jones potentials are used, with electrostatic interactions included in some simulations by means of point charges. The simulations yield the surface tension, surface thickness, density profile ρ(z), and density-orientation profile ρ(z, ϑ); the latter quantity gives the probability density for finding molecules whose axes are at an angle ϑ to the z axis at a height z in the interface, z being perpendicular to the interface. The calculated surface tensions are in good agreement with experiment. For the N2 models no detectable orientational correlations were observed, but for the Cl2 models such correlations were significant at low temperatures. In the absence of electrostatic forces the model Cl2 molecules on the liquid side of the Gibbs dividing surface showed a preferred orientation perpendicular to the surface, in qualitative agreement with first order perturbation theory. The addition of electrostatic forces decreased the tendency of the molecules to align perpendicular to the surface, and for very strong quadrupoles there appears to be a preference for a molecular alignment parallel to the surface. Simulations were made for systems of 216 and 1000 molecules to test the effect of system size. This increase in size caused the surface thickness to increase by about 23%, but had little effect on the other properties.