WETTING AND PREWETTING IN A BINARY-FLUID MIXTURE

We present an ellipsometric study of wetting and adsorption at the liq uid-vapour interface of the cyclohexane-methanol system both at two-ph ase coexistence and in the one-phase region of the phase diagram. At b ulk coexistence, it is observed that either a very thin or a thick fil m 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 phas e transition. Also, experimental evidence is presented for the existen ce of a prewetting. transition in the mixed region of the phase diagra m. For certain compositions, a first-order thin-to-thick-film transiti on can be observed as the temperature is decreased towards the separat ion temperature. The prewetting critical point, where the distinction between the thin and the thick film vanishes, is found to display two- dimensional Ising criticality.