Phase equilibria in perfluoroalkane+alkane binary systems from PC-SAFT equation of state

Phase equilibria of perfluoroalkane + alkane binary mixtures have been studied according to the perturbed chain-statistical associated fluid theory (PC-SAFT) molecular-based equation of state together with a simple monoparametric mixing rule. These systems, for a fixed perfluoroalkane, show transitions from type II phase behaviour, according to the van Konynenburg classification, for the short members of each homologous series, to type III for the largest ones which are properly predicted by the model. These transitions are described by the model via a type IV zone which is studied according to a quasi-binary approach inferring a double critical endpoint (DCEP) and a tricritical point (TCP) in the intermediate transitional zone. The barotropic inversion phenomena for the type III mixtures are also analyzed with the studied equation of state. The general shape of the global phase diagrams of these mixtures is correctly predicted in a qualitative way by the model with a binary interaction parameter temperature independent of the whole temperature range, but the simultaneous correlation of critical loci, three-phase equilibria and equilibrium compositions is not possible with just a single parameter in a quantitative way. The accurate correlation of the complex phase behaviour of these mixtures would require temperature-dependent binary interaction parameters or more complex mixing rules that should take account of the complex weak intermolecular forces rising in these mixtures.