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Methods for the estimation of the differential molar heat capacity, the difference between the heat capacity of the solid and the liquid form of organic compounds at their melting point ∆c p -(T m ), are presented. Three schemes are considered: the first involves use of group contribution methods for the prediction of solid heat capacity (c p S ) and liquid heat capacity (c p L ); the other two, empirical correlations through the entropy of fusion at the melting point ∆S f (T m ). Recommendations for the different categories of organic compounds are made that provide substantial improvement over the commonly used assumption of ∆c p ) 0, in the prediction of ideal solid solubility and solid vapor pressure.

A mixing rule for cubic equations of state (CEoS) applicable to all types of system asymmetriess referred to hereafter as the universal mixing rule (UMR)sis proposed. For the cohesion parameter of the CEoS, the mixing rule involves the Staverman-Guggenheim part of the combinatorial term and the residual term of the original UNIFAC Gibbs free energy expression. For the covolume parameter of the CEoS, the quadratic concentration-dependent mixing rule is used with the combining rule for the cross parameter b ij ) [ 1 / 2 (b i 1/2 + b j 1/2 )] 2 . This UMR is applied to the volume-translated and modified version of the Peng-Robinson equation of state of Magoulas and Tassios (Fluid Phase Equilib. 1990, 56, 119), leading to what is referred to as the UMR-PR model. Very satisfactory results are obtained using the existing interaction parameters of the original UNIFAC model for vapor-liquid equilibrium predictions at low and high pressures for a wide range of system asymmetries including mixtures containing polymers. Satisfactory liquid-liquid equilibrium predictions are also obtained with the UMR-PR model.

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