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

1997 
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° 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.
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