The phase change possibilities in the surface phase formed in a binary
liquid system (water 2,5dimethylpyridine) in contact with a solid (si
lica) have been examined on the basis of adsorption isotherms determin
ed at ten different temperatures. The liquid mixture undergoes a liqui
d-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, an
d 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 caref
ully checked on a thermodynamical basis.(C) 1997 American Institute of
Physics.