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Experimental evidence is brought for layering transitions at solid (silica) surfaces in contact with liquid mixtures, i.e., first-order transitions whereby the coverage of the surface jumps by an amount equivalent to one extra monolayer of 2,5-dimethylpyridine (2,5-DMP) adsorbed from a liquid mixture with water. These jumps are similar to surface phase changes whose aspects are all shown on the adsorption isotherms:  coexistence of two values of adsorption for the same bulk equilibrium composition, adsorption plateaus, persistent metastability lines, characteristic evolution of the plateau's length with temperature, and the proximity of bulk demixing. Known for gas adsorption on a solid both experimentally and theoretically, this behavior is seen for the first time at a solid−liquid interface.

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A characterization of the bulk-phase diagram from literature data and new NMR and DSC measurements provided us with valuable elements that are helpful for gaining, from aqueous solution, better insight into the surface behavior of polyethylene glycol on Aerosil 200. Adsorption isotherms built further to measurements by a depletion method showed a strong and temperature-dependent variation of the isotherm shape in agreement with the variations of interactions already evidenced in the bulk. In temperature-concentration areas, where water is behaving as a helix-promoting solvent, the finding of positive PEG adsorptions and stairlike isotherms agrees with observations reported in the literature. We identified some of the vertical parts as corresponding to the formation of monolayers of helix-shaped PEG molecules. In poor-solvent zones, adsorptions were null or negative, and the isotherms exhibited oscillations suggesting very different surface behavior. Our data analysis evidenced the presence of a much greater amount of water than in the previous surface states; however, the similar analysis of PEG behavior remains relevant. Indeed, the occurrence of first-order transitions in the surface layer implies some water reorganization, permitting the PEG molecules to move closer to the surface and become helix-shaped to rearrange in a monolayer. The surface phase diagram confirmed this analysis in a very satisfying way.

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Adsorption isotherms of 2,5-dimethylpyridine (2,5-DMP) on Aerosil 200 silica from water-2,5-DMP binary mixtures are known to exhibit special features indicative of surface phase transitions in the adsorbed layer. We have made similar observations on another substituted pyridine, 2,4,6-trimethylpyridine (2,4,6-TMP). By using 1D and 2D NMR spectroscopy, we investigated adsorbed layers on silica in suspension in water/substituted-pyridine mixtures and demonstrated the existence of adsorbed species specific signals. Comparison of signals with those displayed on NMR spectra of liquid binary mixtures under various pH conditions rules out adsorption via interaction of the surface silanol group and the lone electron pair of the nitrogen atom. A mechanism of adsorption through the interaction of surface siloxane oxygen and the aromatic pi-system is proposed; it is consistent with both thermodynamic measurements and stacking of substituted pyridines within the adsorbed layer.

Articles you may be interested inThe isotropic-nematic and nematic-nematic phase transition of binary mixtures of tangent hard-sphere chain fluids: An analytical equation of stateThe prewetting transition in the silica-water-2,5-dimethylpyridine system has been studied through the adsorption isotherms. Steep wave shapes for these isotherms lead to the conclusion of alternate solid and liquid surface demixing very close to bulk coexistence and wetting temperature. Liquid demixing has the generic aspects of prewetting. An empirical model for demixings, surface phase diagrams in several representations, and critical end points are discussed, as well as the phenomena universality.