WETTING AND PREWETTING IN A BINARY-FLUID MIXTURE

We present an ellipsometric study of wetting and adsorption at the liquid-vapour interface of the cyclohexane-methanol system both at two-phase coexistence and in the one-phase region of the phase diagram. At bulk coexistence, it is observed that either a very thin or a thick film of the heavy phase intrudes between the lighter phase and the vapour in a certain temperature range. The observed hysteresis is consistent with the assumption that the wetting transition if a first-order phase transition. Also, experimental evidence is presented for the existence of a prewetting. transition in the mixed region of the phase diagram. For certain compositions, a first-order thin-to-thick-film transition can be observed as the temperature is decreased towards the separation temperature. The prewetting critical point, where the distinction between the thin and the thick film vanishes, is found to display two-dimensional Ising criticality.