Phase Equilibrium of Cross-Associating Mixtures Using Association Theory-Based Equation of State

The polar and perturbed chain form of statistical associating fluid theory (Polar PC-SAFT) and the universal quasi-chemical (UNIQUAC) excess Gibbs free energy model were used to model the VLE and excess properties of solvating mixtures. Mixtures of chloroform and halothane with oxygenated solvents were selected for which good quality data were found in the literature. The phase behavior of these systems is characterized by a strong negative deviation from Raoult's law due to cross-association between the oxygenated solvents' lone pairs of electrons and the hydrogen bond donor haloalkane molecule. The impact of such cross-association is reflected on the activity coefficients of oxygenated solvents in chloroform and halothane at low concentrations. Results show that UNIQUAC was unable to properly capture the highly nonlinear function of the activity coefficient curves which resulted in overall poor infinite dilution estimates. Moreover, the model was unable to simultaneously fit the VLE and excess enthalpy for many of these systems accurately. On the contrary, and through the explicit modeling of association using Wertheim's first order thermodynamic perturbation theory (TPT1), Polar PC-SAFT was able to capture well the distribution of hydrogen bonds found in these systems which resulted in good prediction of both VLE and excess properties. Furthermore, the symmetry and asymmetry found in the excess enthalpy of mixtures forming 1:1 and 1:2 complexes, respectively, was successfully reproduced by the molecular model.