Thermodynamics of ternary mixtures exhibiting tunnel phase behaviour

Part 3.—Hexane–hexamethyldisiloxane–perfluorohexane

Abstract

The phase diagram for the ternary mixture hexane–hexamethyldisiloxane–perfluorohexane has been determined by visual observation of the liquid–liquid phase separation temperatures T_sep for the two partially miscible binary mixtures containing perfluorohexane and for three quasibinary temperature-composition slices of volume ratios of hexamethyldisiloxane:hexane of 1:3, 1:1 and 3:1. The upper critical solution temperatures T_UCS for the binary mixtures of hexane or hexamethyldisiloxane–perfluorohexane are separated by less than 1 K, being 295.76 K and 296.65 K, respectively. For the quasibinary slices, the maximum separation temperatures T_sep,max are lower than T_UCS for either binary mixture, for the central ternary slice T_sep,max = 294.64 K. The projections of the ternary liquid–liquid quasibinary slices map well on to the binary liquid–liquid coexistence curves. A simplified version of the statistical associating fluid theory (SAFT-HS) has been used to predict the phase equilibrium for this system and is in good qualitative agreement with the experimental results. In particular, the decrease in T_sep,max for the ternary mixtures is reproduced.