Fluid phase equilibria using molecular dynamics: the surface tension of chlorine and hexane

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Abstract
In this paper we demonstrate that the direct molecular dynamics method can be used to predict accurate fluid phase equilibria for molecular fluids. The method is applied to chlorine and n-hexane to calculate the coexisting densities, vapour pressure, and surface tension as a function of temperature. Chlorine is modelled as a rigid diatomic molecule, and n-hexane as an isotropic united-atom model. For hexane we use two sets of parameters for the intermolecular potential. The main difference in the parameters is the strength of the repulsion-dispersion interaction of the terminal methyl group εCH3 /k = 90·44 K (model I) and = 114 K (model II); systematic differences in the calculated properties are found for the models. For chlorine, the liquid-vapour densities and vapour pressures are in excellent agreement with experimental results, and with those previously calculated using the Gibbs ensemble Monte Carlo method (GEMC). Good agreement with the experimental surface tensions is obtained. For hexane, the calculated properties are in better agreement with experiment for model I. The coexisting densities calculated in this work are in very good agreement with those calculated using the GEMC method.