Vapour pressure of non-polar fluids and their repulsive and attractive contributions

Using a simple equation of state, based on the Weeks-Chandler-Andersen sepa ration of the intermolecular potential, we have obtained the contributions of repulsive and attractive intermolecular forces to the thermodynamic prop erties of coexisting vapour and liquid phases of a Lennard-Jones (LJ) fluid . In order to obtain the vapour pressure of real non-polar fluids, we take th e LJ fluid as a reference model, and propose a new perturbative contributio n, which is dependant on the temperature and on the acentric factor of the substance. Using the complete perturbed equation, we determine the correspo nding repulsive and attractive contributions to the vapour pressure of non- polar fluids. The results show that the attractive vapour pressure of non-p olar fluids increases with increasing acentric factor, i.e., larger deviati on of the molecular shape from spherical symmetry. This procedure could be extended to separate the repulsive and attractive c ontributions of the intermolecular forces to other thermodynamic properties of non-polar fluids as well as of polar fluids and fluid mixtures.