Mean spherical approximation based perturbation theory equation of state for Stockmayer fluids

We propose a new mean spherical approximation (MSA) based perturbation theory (PT) equation of state for dipolar fluids, which can be modelled by the Stockmayer potential. Our equation of state contains a Lennard-Jones equation of state and an excess term, which takes into account the contribution of the dipole-dipole interaction. The prediction of the latter term is based on the analytical solution of the MSA for a dipolar hard sphere fluid. Pressure data calculated from our MSA based PT equation of state at different isotherms and reduced dipole moments [mu* = mu/root(sigma(3) epsilon)), where mu is the dipole moment and sigma, epsilon are the parameters of the Lennard-Jones potential] are compared with Monte Carlo (MC) simulation and Gubbins-Pople-Stell perturbation theory results. At low density both theories give good agreement with MC simulation results. At higher densities the MSA based PT gives better agreement with the simulation data. The vapour-liquid coexistence curves are predicted well for mu*(2) < 2 reduced dipole moments by both PTs, but for higher mu* values our MSA based PT results are in better agreement with the simulation data. The equilibrium pressure and enthalpy of vaporisation data predicted from the MSPI based PT are in better agreement with the appropriate simulation data than those of the Gubbins-Pople-Stell PT. A comparison between the mu* dependence of the theoretical and MC simulation critical parameters is also made,where, with the exception of the critical density, the MSA based PT data are in better agreement with the MC ones.