Thermodynamics of ternary mixtures exhibiting tunnel phase behaviour. Part 1.—Cyclohexane–perdeuterocyclohexane–perfluorohexane

Abstract

The liquid–liquid phase separation temperatures T_sep for the binary mixtures cyclohexane or perdeuterocyclohexane–perfluorohexane and for the ternary mixture (equivolume cyclohexane–perdeuterocyclohexane)–perfluorohexane have been determined by visual observation. Perdeuteriation of cyclohexane lowers the upper critical solution temperature T_UCS from 346.16 to 337.69 K but the shape of the phase diagram in terms of the Wegner extended scaling series is unaltered. In contrast, although the maximum separation temperature of the ternary mixture T_{sep, max} is little different from T_UCS of cyclohexane–perfluorohexane, the shape of the T_sepvs. perfluorohexane mole fraction curve is substantially different. The results cast some doubt on the assumption that the thermodynamics of a solution, and hence the surface structure, in a contrast-matched solvent in reflectivity measurement is the same as would be obtained in the pure solvent.