VAPOR-LIQUID-EQUILIBRIA OF STOCKMAYER FLUIDS COMPUTER-SIMULATIONS AND PERTURBATION-THEORY

Gibbs ensemble simulation data for Stockmayer fluids with mu*2 = 3.0 and mu*2 = 4-0 in the reduced temperature range of 0.77 (resp. 0-80)-0.98 and presented and compared with predictions based on the perturbation theories of Stell, G., Rasaiah, J. C., and Narang, H., 1972, Molec. Phys., 23, 393; 1974, 27, 1393. The description of the reference fluid is improved by applying the modified Benedict-Webb-Rubin equation of state instead of the Verlet-Weis implementation of the Weeks Chandler Andersen perturbation scheme. Second virial coefficients predicted by perturbation theory to order mu4 agree for Stockmayer fluids with mu*2 < 4 very well with exact values. Perturbation theory is capable of describing the low-density region of Stockmayer fluids with rather strong dipole moments. For these high values of the dipole moment, the Pade approximation of perturbation theory deviates significantly from the simulated coexistence curves in density and pressure. Compared with perturbation theory to order mu4, however, it is a far better approximation of the Stockmayer fluid coexistence curve. The behaviour of the Pade approximation in the critical region is not satisfactory.