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

Abstract

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.

Surface Phase Transitions at Liquid-Mixture/Solid Interfaces

“…Sometimes, either two parallel steps or one curve with steps and another one without any step can be drawn. We previously pointed out 1,7 that this metastability is one of the best indications of surface phase changes. Moreover, one should note that for some specific compositions of the solution the successive points lie on a vertical line as illustrated in Figure 5.…”

“…2.5-DMP was supplied by Aldrich and purified as previously described. 7 Water was purified on a Milli Q device. Silica was Degussa Aerosil 200 prepared by pyrogenation.…”

“…Considering the multiplicity of phases potentially exhibited by a binary mixture, one may wonder what kind of phases will be seen on surfaces at room temperature. Previous experiments 1,7 showed that, for the water-2,5-DMP-on-silica system, prewetting can be seen as a liquidliquid demixing identical to the bulk phase change close to the one where prewetting occurs whereas layering could be attributed to a kind of liquid-solid demixing identical to the possible bulk phase change that occurs at lower temperature. This process is observed in surface phase…”

“…Looking for the evidence of such transitions in dilute solutions in contact with a solid was thus quite logical. Such an approach constitutes a new attempt to show the generality of surface phase transitions and will add to the results already found in liquid mixtures at liquid vapor interfaces, 6 in prewetting phenomena 7,8 and in other phenomena associated with wetting, 9 in electrodeposition of metal, and in many other fields. 10 In the present paper, we first recall how surface transitions appear on adsorption isotherms, then we describe the experimental procedure, and we finally present and discuss our experimental results.…”

Impact of Bulk Phase Transitions and Wetting Transitions on Adsorption Isotherms in Binary Systems in Contact with Solid

“…Experimental surface phase diagram constituted from x 2 ϭ f( x 2 ) adsorption isotherms (5,11), corresponding to a liquid-liquid surface demixing. x 2 is calculated according to a layering model for ⌫ 2 (5); values have been selected on the second vertical segment appearing on isotherms at 40, 42, 44, and 46°C (Fig.…”