USUAL AND UNUSUAL SURFACE TENSIONS OF PERFLUOROCARBON-CONTAINING BINARY-LIQUID MIXTURES NEAR A CRITICAL END-POINT

The surface tension of binary mixtures near critical endpoints is disc ussed with detailed focus on the composition dependence. The similarit y with the corresponding composition dependence of the vapor pressure is explored. The differences between mixtures in which the more volati le component is of lower surface tension, i.e. is less tense-known as usual mixtures-and those in which the more volatile component is more tense-known as unusual mixtures-are analyzed. As a basis of discussion , a limited review of the related literature is presented along with s ome fresh results for two usual mixtures (octamethylcyclotetrasiloxane + perfluoropentane and heptane + perfluorohexane) and two unusual mix tures (pentane + perfluoroheptane and 2-methylpentane + perfluorotribu tylamine). These mixtures display a variety of horizontal inflections characteristic of the behavior of the noncritical interface near a cri tical endpoint. The form of the horizontal inflection is uniformly rel ated to the sign of the difference between the surface tensions of the pure components, i.e. with the overall negative slope towards the les s tense component. A number of the mixtures also exhibit aneotropy or surface azeotropy. All these results are shown to be well described by the Surface Regular Solution theory. (C) 1998 Academic Press.