Layering in a two-component liquid system undergoing a phase separation

The phase change possibilities in the surface phase formed in a binary liquid system (water 2,5dimethylpyridine) in contact with a solid (silica) have been examined on the basis of adsorption isotherms determined at ten different temperatures. The liquid mixture undergoes a liquid-liquid demixing but also, some 20 degrees below the critical point, a solid phase separation. On the l.h.s. of a liquid-liquid coexistence curve 2,5dimethylpyridine, diluted in water, adsorbs step by step, and on the r.h.s. the relative adsorption is wave shaped. Both behaviors can be explained by a layering process, i.e., a solid-liquid surface demixing, which unexpectedly does not exclude a separate liquid-liquid surface demixing, which could constitute the prewetting process. This analysis is compared with known theories and its consistency is carefully checked on a thermodynamical basis.(C) 1997 American Institute of Physics.