ORTHOBARIC SURFACE-TENSION OF (METHYLCYCLOHEXANE PLUS TETRADECAFLUOROMETHYLCYCLOHEXANE) IN THE CRITICAL REGION AT TEMPERATURES FROM 320-K TO 335-K (CORRECTED VERSION OF PC241)

With a view to investigating further the surface tensions of (a hydroc arbon + a perfluorocarbon) in which neither component is aliphatic-and thus open to chain-flexibility effects-or aromatic-and thus dominated ,by specific interactions, we have studied the orthobaric (gas + liqui d) surface tension of methylcyclohexane at temperatures from 293 K to 338 K, of tetradecafluoromethylcyclohexane from 298 K to 338 K, and of their mixture across the full composition range at temperatures from 320 K to 335 K close to the upper critical solution temperature, T(UCS ) = 319.262 K. The mixture is normal in the sense that the more volati le component, tetradecafluoromethylcyclohexane, has the lower surface tension. Reflecting the occurrence of marked positive deviations from bulk-phase ideality, emphasized by the occurrence of positive azeotrop y, the mixture displays marked negative deviations from surface ideali ty, just sufficient to yield aneotropy or surface azeotropy a few kelv ins above T(UCS). Near T(UCS) the isotherms display the inflection cha racteristic of the prediction of Widom's theory of surface tensions at the non-critical interface of a mixture close to an upper critical en d point. The total surface segregation, although positive across the e ntire mole-fraction range, shows the signature of both the critical en d point and the azeotrope.