Closed loops of liquid–liquid immiscibility predicted by semi-empirical cubic equations of state and classical mixing rules

Abstract

The present study demonstrates that semi-empirical cubic EOSs, which include temperature functionalities such as that of Soave and van der Waals classical mixing rules, predict the behavior around the mathematical double points of the second kind. For the first time these results allow one to fit Type VI behavior in real water solutions, that include both first and second hypercritical points, or the absence of each of these points. Although the predicted data match the experimental data, their genesis is explained by fundamental numerical pitfalls, namely non-physical and non-differentiable breaking points that are characteristic for several empirical temperature functionalities. Hence, such results cannot represent the physical nature of the real systems. Although these pitfalls may take place outside the range of practical significance for pure compounds, they may strongly affect the predictions of the mixtures even at ordinary conditions. Therefore it is shown once again that all parts of the thermodynamic phase space predicted by the EOSs are closely inter-related.