WETTING AND LAYERING IN CRITICAL BINARY-FLUID MIXTURES

We present an ellipsometric study of wetting and adsorption at the liq uid-vapor interface of cyclohexane-methanol and cyclohexane-deuterated methanol systems. Due to the first-order character of the wetting tra nsition and the hysteresis associated with it, either a very thin or a thick film of the heavy phase intrudes between the lighter phase and the vapor. In the cyclohexane-methanol mixture, the thin film grows in layers with a thickness on the order of the molecular diameter rather than continuously; a series of first-order layering transitions takes place at the interface. In the deuterated system, no clear layering e ffects are observed. As the hydrogen bonds in the latter system are so mewhat weaker, this suggests that hydrogen bonding plays a role in the observed layering effects. Measurements above the critical temperatur e T(C) allow for the conclusion that the thin film corresponds to the critical adsorption state. Residual layering effects may persist above T(C) for the cyclohexane-methanol system.