Thermodynamic study of (perfluoroalkane + alkane) mixtures: Excess and solvation enthalpies

A newly designed calorimetric technique and calculation procedure have been used to obtain partial molar enthalpies, H¯i, and excess enthalpies, HE, for binary mixture of hexane + perfluoro-n-alkanes (C5–C8) and perfluorohexane + n-alkanes (C5–C8), + cyclohexane, and + 2-methylheptane. All mixtures are endothermic, and the heat effects increase with the size of the second component. The HE and H¯i values found are the largest ever observed for mixtures of non-polar compounds. An estimate of the excess heat capacity for (perfluorohexane + 2-methylheptane) has been obtained from H¯i at two different temperatures. From H¯i at infinite dilution and from the known enthalpies of vaporization, the enthalpies of solvation, ΔsolvH∘, have been evaluated either for alkanes and perfluoroalkanes in both hexane and perfluorohexane solvent. Solute–solvent interactions have been examined by describing the ΔsolvH∘ with an additive scheme of surface interactions and by applying the Scaled Particle Theory. The effects of chain lengthening, branching, and cyclization have been discussed. Perfluoroalkanes proved to be inert molecules that interact weakly with themselves as well as with alkanes.