WETTING OBSERVED BY EVANESCENT-WAVE-GENERATED FLUORESCENCE SPECTROSCOPY .3. FLUORESCENCE LIFETIMES - N-HEPTANE PLUS PERFLUOROHEXANE AT THE UPPER CRITICAL END-POINT

The technique of evanescent-wave-generated-fluorescence spectroscopy E WGFS has been applied here to the investigation of the critical-point wetting in the phase-separated mixture (n-heptane + perfluoro-n-hexane ). The discrimination between the two coexisting liquid phases was ach ieved by exploiting the fluorescence lifetime difference of the fluoro phor 1,6-diphenyl-hexatriene in each of the phases. The wetting transi tion temperature T-w is clearly identified as being (31.5 +/- 0.2)degr ees C, in relation to the upper consolute temperature of ca. 42 degree s C. This version of the EWGFS technique for studying critical-point w etting may be useful for the future determination of composition and r elative thickness of the wetting layer, in addition to the elucidation of the order of the transition. Other related applications of the tec hnique include the study of critical-point wetting and critical adsorp tion at modified solid surfaces.