Wetting transition and pretransitional thin films in binary liquids: alcohol/perfluoromethylcyclohexane mixtures studied by x-ray reflectivity

In this study the wetting transition at the liquid-vapour interface of binary organic liquid mixtures has been investigated by x-ray reflectivity. Mixtures of various isomeric alcohols with perfluoromethylcyclohexane (PFMC) served as model systems, with alcohol carbon numbers ranging from 1 to 4. Remarkably different pretransitional behaviour of the thin films below the wetting temperature was observed, which could be classified according to the carbon number. At two-phase coexistence, no pretransitional thin films could be detected for methanol and ethanol, whereas thin-to-thick-film transitions were found for propanol and butanol and their isomers. For l-propanol and 2-propanol, the surface of the upper, alcohol-rich phase of the gravity-separated mixture displays a wetting transition at T-w = 31.5 degreesC and 38.3 degreesC, respectively, where the thickness of a PFMC-rich film jumps from less than 25 Angstrom to values exceeding the experimental resolution of about 1 200 Angstrom. For 1-butanol, 2-butanol and -butanol, we found pretransitional film thicknesses increasing up to 100 Angstrom, with wetting transitions at T-w = 45.0 degreesC, 34.2 degreesC and 40.1 degreesC, respectively. In the single-phase region, the study of adsorption isotherms above T-w revealed novel behaviour of the adsorbed PFMC-rich him. We observed both a growing film thickness and a significantly changing composition as the coexistence line was approached. Nevertheless, the variation of the excess adsorption with distance from coexistence could still be described by a power law.