Dependence of the vapor–liquid equilibrium on the attractive intermolecular forces

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Abstract
By means of fitting computer simulation data of 3D Lennard-Jones fluids, we present very simple analytical equations for the pressure and chemical potential as functions of the temperature and density. The standard thermodynamic requirement of liquid–vapor equilibrium, i.e., at a given temperature, the pressure and chemical potential in the gas phase are equal to the corresponding values in the liquid phase, leads to the determination of the vapor–liquid coexistence curve. By then using the Weeks–Chandler–Andersen separation of the intermolecular potential, we were able to determine the differing location of the coexistence curve in the phase plane as the intensity of the attractive forces is changed. This curve varies linearly with respect to the perturbative parameter of the Weeks–Chandler–Andersen theory. These results could be very useful in the study of vapour pressure curves of fluids composed of noninert molecules—ionic fluids, ferrofluids, polar fluids, etc.—which have a different intensity of the attractive intermolecular forces relative to the repulsive forces than the relation between the two forces given by the Lennard-Jones model. Finally, the variation in the location of the critical point with respect to differing intensities of the attractive forces is also dealt with.