Thermodynamics of linear dimethylsiloxane-perfluoroalkane mixtures .3. Orthobaric surface tensions of hexamethyldisiloxane-, octamethyltrisiloxane- or decamethyltetrasiloxane-tetradecafluorohexane near the upper critical endpoint

The orthobaric surface tensions of three dimethylsiloxane-perfluoroalkane mixtures: hexamethyldisiloxane-, octamethyltrisiloxane- and decamethyltetrasiloxane-tetradecafluorohexane have been measured at temperatures close to the upper critical solution temperatures. All three mixtures are normal in the sense that the more volatile component has the lower surface tension. The surface tension isotherm closest to the critical endpoint displays a characteristic horizontal inflection at a composition very close to the critical composition previously determined from the liquid-liquid coexistence curve. The incidence of negative aneotropy is distinct in the hexamethyldisiloxane-tetradecafluorohexane mixture and wholly absent in the decamethyltetrasiloxane-tetradecafluorohexane mixture with the mixture containing octamethyltrisiloxane intermediate in behaviour. The first mixture displays both azeotropy (predicted via the application of the regular solution theory of mixtures) and aneotropy, the second azeotropy and rather indistinct aneotropy and the third mixture neither azeotropy nor aneotropy; these observations emphasise that although vapour pressure and surface tension extrema often occur in mixtures simultaneously there is no theoretical necessity that they should do so. As in many other mixture properties there are great similarities between dimethylsiloxane-perfluoroalkane and alkane-perfluoroalkane mixtures, particularly those containing hexamethyldisiloxane or hexane.