Thermodynamics of linear dimethylsiloxane–perfluoroalkane mixtures

Part 1.—Liquid–liquid coexistence curves for hexamethyldisiloxane–, octamethyltrisiloxane– or decamethyltetrasiloxane–tetradecafluorohexane near the upper critical endpoint and upper coexistence temperatures for 21 other dimethylsiloxane–perfluoroalkane mixtures

Abstract

Measurements of the liquidliquid coexistence curve for hexamethyldisiloxane, octamethyltrisiloxane or decamethyltetrasiloxanetetradecafluorohexane near the upper critical endpoint and of the upper coexistence temperatures T_UCX for equivolume mixtures of 21 other mixtures of this kind confirm the basic similarity of the thermodynamics of such mixtures to those of alkaneperfluoroalkane mixtures. The upper critical solution temperatures T_UCS increase linearly within the range studied with perfluoroalkane chain length and nearly so with dimethylsiloxane chain length. The coexistence curves have been fitted to Wegner extended-scaling expressions in order to investigate the universality of the shape. The temperaturevolume-fraction (T−φ) representation is more symmetrical than that in mole fraction and with a reduced temperature T̃=(T/T_UCS), withthe upper critical solution temperature T_UCS as the reduction factor, the (T̃−φ) representation is universal for all threemixtures as well as nearly so for the comparison mixture C₆H₁₄C₆F₁₄. T_UCS(C₆H₁₄C₆ F₁₄)=295.80 K and T_UCS(hexamethyldisiloxaneC₆ F₁₄)=296.95 K are unsurprisingly close, in view of the near-equal temperature reduction factors of hexamethyldisiloxane and hexane. The effect of this correspondence is extended to the analysis of the T_UCX of the other mixtures. The results are discussed in terms of the solubility parameter version of the regular solution theory incorporating the Flory–Huggins entropy of mixing term. With a correction of nearly 10% on the geometric-mean combining rule for the cohesive energy density T_UCS is reasonably well predicted. For these mixtures, the critical mole fraction is also surprisingly well predicted from the ratio of component molar volumes.