DEVIATION FROM CORRESPONDING-STATES BEHAVIOR FOR POLAR FLUIDS

The influence of the strength of the reduced dipole moment mu* of a Stockmayer fluid on thermodynamic phase behaviour is systematically investigated by means of computer simulations. Gibbs ensemble simulation data are presented for Stockmayer fluids with mu*2 = 5 and mu*2 = 6 in the reduced temperature range of 0.78-0.98 (respectively 0.99). Critical properties are estimated from Gibbs ensemble Monte Carlo data. The critical temperature increases strongly with increasing dipole moment, and the critical density shows a decrease for higher dipole moment values. The critical pressure increases only slightly, with the resulting reduced compressibility factor showing a clear decreasing trend. These trends are correlated with the reduced dipole moment. The Pade approximation of perturbation theory, compared with four other theories, follows the 'exact' critical-property correlations closest. The simulation results show that dipolar forces alone are sufficient to induce a deviation from corresponding-states behaviour: with increasing reduced dipole moment, the saturated-liquid densities and enthalpies of vaporization increase.