Ralf K. Heilmann, Masafumi Fukuto, and Peter S. Pershan

We report x-ray reflectivity measurements of the internal structure of thin wetting films formed on a silicon substrate from the vapor of a binary mixture of methylcyclohexane (MC) and perfluoromethylcyclohexane (PF). At T = 30 °C (below the bulk consolute temperature of T_c46 °C) variation in the difference in temperature between the substrate and the vapor induces changes in film thickness (25 to 135 Å) that are consistent with complete wetting of both surfaces, with a MC-rich phase wetting the substrate and a PF-rich phase wetting the free surface. The width of the internal liquid/liquid interface, however, is found to be noticeably smaller than values predicted from the convolution of the intrinsic interfacial profile between bulk liquids with the capillary wave roughness as modified by dispersion interactions with the substrate. The difference is attributed to the effects of confinement. Although the width of the liquid/liquid interface is less well defined above the consolute point (T = 60 °C), we still find PF enrichment at the free surface and MC enrichment at the substrate.

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