Ian A. McLure, Beryl Edmonds and Ahmad Mokhtari
Molar excess volumes VmE 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. TUCS=296.95 and 316.4 K, respectively and the molar excess enthalpy HmE at T=298.15 K for hexamethyldisiloxane–tetradecafluorohexane has been determined by dilution calorimetry. VmE is large and positive (ca. 67 cm3 mol-1) as is the fractional excess volume (ca. 3%). Both increase with temperature but, in contrast, VmE 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 VmE 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. HmE 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 HmE, the related mixture for hexane–tetradecafluorohexane at the same temperature. An analysis of the derived excess energy at constant volume UV,mE 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.