THERMODYNAMICS OF TERNARY MIXTURES EXHIBITING TUNNEL PHASE-BEHAVIOR .1. CYCLOHEXANE-PERDEUTEROCYCLOHEXANE-PERFLUOROHEXANE

The liquid-liquid phase separation temperatures T-sep for the binary m ixtures cyclohexane or perdeuterocyclohexane-perfluorohexane and for t he ternary mixture (equivolume cyclohexane-perdeuterocyclohexane)-perf luorohexane have been determined by visual observation. Perdeuteriatio n 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, althoug h the maximum separation temperature of the ternary mixture T-sep,T-ma x is little different from T-UCS of cyclohexane-perfluorohexane, the s hape of the T-sep vs. perfluorohexane mole fraction curve is substanti ally different. The results cast some doubt on the assumption that the thermodynamics of a solution, and hence the surface structure, in a c ontrast-matched solvent in reflectivity measurement is the same as wou ld be obtained in the pure solvent.