Theoretical prediction of phase behaviour at high temperatures and pressures for non-polar mixtures. Part 3.—Comparison with upper critical solution temperatures for perfluoromethylcyclohexane + hydrocarbons

Abstract

The upper critical solution temperatures of perfluoromethylcyclohexane + n-alkanes (C₅ to C₁₅) have been measured. The results have been compared with the van der Waals one fluid model using the Guggenheim equation of state and employing the technique for solving the criticality conditions described previously. The energy parameter, ξ, defined by the equation ξ=T^c₁₂/(T^c₁₁T^c₂₂)^½(where T^c_ij is the energy reducing parameter characterising interactions between molecules of species i and j) needed to fit the experimental UCST data have been calculated. The value of ξ is always less than unity and decreases with an increase in the chain length of the n-alkane. Similar trends have been previously observed for values of ξ obtained from gas–liquid critical points of mixtures of hexafluorobenzene + n-alkanes and perfluoron-heptane + n-alkanes. Possible explanations of such trends are suggested.