Thermodynamics of linear dimethylsiloxane–perfluoroalkane mixtures

Part 2.—Excess volumes at 298.15 and 303.15 K and excess enthalpies at 298.15 K of hexamethyldisiloxane–tetradecafluorohexane and excess volumes at 318.15 and 323.15 K of octamethyltrisiloxane–tetradecafluorohexane

Abstract

Molar excess volumes V_m^E of hexamethyldisiloxane–tetradecafluorohexane at temperatures T=298.15 and 303.1 K and octamethyltrisiloxane–tetradecafluorohexane at T=318.15 and 323.15 K were determined in batch or continuous-dilution dilatometers near the upper critical solution temperatures. T_UCS=296.95 and 316.4 K, respectively and the molar excess enthalpy H_m^E at T=298.15 K for hexamethyldisiloxane–tetradecafluorohexane has been determined by dilution calorimetry. V_m^E is large and positive (ca. 67 cm³ mol^-1) as is the fractional excess volume (ca. 3%). Both increase with temperature but, in contrast, V_m^E increases and the fractional excess volume decreases with dimethylsiloxane chain length. Although the lower measurement T for each mixture is within 23 K of the upper critical solution temperature, the V_m^E isotherms reveal but little of the marked flattening that normally characterises such propinquity to the endpoint, however, a detailed numerical analysis reveals some evidence for this effect. The evidence for ‘flattening’ is that the second Redlich–Kister coefficient is markedly smaller than the first or higher coefficients: as can be seen our results display this property. As in other properties of mixtures with other substances, the mixture with hexamethyldisiloxane resembles that with hexane, e.g. here the fractional excess volumes are almost equal. H_m^E is positive, large (ca. 2.2 kJ mol^-1) and symmetrical in mole and volume fraction. In magnitude and composition dependence it is very similar to H_m^E, the related mixture for hexane–tetradecafluorohexane at the same temperature. An analysis of the derived excess energy at constant volume U_V,m^E for both mixtures using the solubility parameter version of the regular resolution theory shows this quantity to be well described using the same value of the parameter l₁₂, for the deviation of the cross-term in the cohesive energy density of the mixture from the geometric-mean rule value, that is required in the same treatment to describe the upper critical solution temperature. Similar values of l₁₂ are required to describe the upper critical solution temperatures of other dimethylsiloxane–perfluoroalkane mixtures. The results are also discussed in relation to the local correlation of molecular orientation treatment of Patterson.