Wetting observed by evanescent-wave-generated fluorescence spectroscopy .3. Fluorescence lifetimes: n-Heptane plus perfluorohexane at the upper critical endpoint

The technique of evanescent-wave-generated-fluorescence spectroscopy EWGFS 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 achieved by exploiting the fluorescence lifetime difference of the fluorophor 1,6-diphenyl-hexatriene in each of the phases. The wetting transition temperature T-w is clearly identified as being (31.5 +/- 0.2)degrees C, in relation to the upper consolute temperature of ca. 42 degrees C. This version of the EWGFS technique for studying critical-point wetting may be useful for the future determination of composition and relative thickness of the wetting layer, in addition to the elucidation of the order of the transition. Other related applications of the technique include the study of critical-point wetting and critical adsorption at modified solid surfaces.