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

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_sep vs. 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.