Deviation from corresponding-states behaviour for polar fluids

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Abstract
The influence of the strength of the reduced dipole moment μ* 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 μ*2=5 and μ*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 Padé 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.