Thermodynamics of mixtures containing a fluorinated benzene and a hydrocarbon

Abstract

Fluorobenzene, or 1,4-difluorobenzene or hexafluorobenzene + alkane mixtures and hexafluorobenzene + benzene, or + toluene, or + 1,4-dimethylbenzene systems have been studied using thermodynamic properties from the literature and through the application of the DISQUAC and UNIFAC (Dortmund) models and the concentration-concentration structure factor ($S_{\text{CC}}(0)$). Interaction parameters for the contacts F/alkane and F/aromatic have been determined for DISQUAC, and they have been taken from the literature for UNIFAC. Both models predict double azeotropy for the C$_6$F$_6$ + C$_6$H$_6$ system, although in different temperature ranges. Excess molar enthalpies ($H_{\text{m}}^{\text{E}}$) of the fluorobenzene, or 1,4-difluorobenzene + n-alkane systems are positive and are accurately described by the models using interaction parameters independent of the n-alkane, discarding Patterson's effect in such mixtures. DISQUAC calculations confirm that conclusion for C$_6$F$_6$ + n-alkane mixtures. DISQUAC provides better results than UNIFAC on excess molar isobaric heat capacities ($C_{p\text{m}}^{\text{E}}$) of solutions involving n-alkanes, or on $H_{\text{m}}^{\text{E}}$ of C$_6$F$_6$ + aromatic hydrocarbon systems. For mixtures with a given n-alkane, the relative variation of excess molar internal energies at constant volume ($U_{V\text{m}}^{\text{E}}$) and $H_{\text{m}}^{\text{E}}$ and with the fluorohydrocarbons is different, due to structural effects. C$_6$F$_6$ + aromatic hydrocarbon mixtures are characterized by interactions between unlike molecules, as seen from their negative $H_{\text{m}}^{\text{E}}$ values. The $S_{\text{CC}}(0)$ formalism reveals that homocoordination is more important in C$_6$F$_6$ + n-alkane mixtures than in the corresponding systems with C$_6$H$_5$F, and that heterocoordination is dominant in the solutions of C$_6$F$_6$ with an aromatic hydrocarbon.