Occurrence of a Double Azeotrope in the Binary System Hexafluorobenzene+Benzene

Abstract

RECENTLY there has been much interest in the thermo-dynamic properties of binary mixtures of aromatic fluorocarbons and aromatic hydrocarbons. Experimental studies of the phase diagrams1,2, excess volumes3 and excess enthalpies4 of mixing can be interpreted by assuming that intermolecular complexing exists between the two components. This complexing is believed to result from charge–transfer interaction, with the fluorocarbon acting as the electron acceptor. The presence of a specific interaction, of this nature has the effect of reducing the magnitude of all the thermodynamic excess functions from their “normal” large, positive values. For example, in the system hexafluorobenzene + cyclohexane where there is little possibility of charge–transfer interaction, the excess free energy, enthalpy, entropy and volume are all large and positive, whereas these functions are all negative in the system hexafluorobenzene + mesitylene where the hydrocarbon is a strong electron donor. The system hexafluorobenzene + benzene is found to occupy an intermediate position.