WETTING AND LAYERING IN CRITICAL BINARY-FLUID MIXTURES

We present an ellipsometric study of wetting and adsorption at the liquid-vapor interface of cyclohexane-methanol and cyclohexane-deuterated methanol systems. Due to the first-order character of the wetting transition 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 effects are observed. As the hydrogen bonds in the latter system are somewhat weaker, this suggests that hydrogen bonding plays a role in the observed layering effects. Measurements above the critical temperature 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.