Search

Surface state changes described as phase transitions or simple molecular rearrangements have become a key issue in modern science. Indeed, they have an impact on the development of numerous (nano)technologies; they are also involved in biochemical and chemical mechanisms at the molecular level and also in environmental phenomena. At last, they have been at the origin of flourishing statistical descriptions that have illuminated new and very interesting aspects of surface behaviors. Here, to obtain still lacking coherent sets of experimental data on systems in which molecular interactions and thermodynamic properties are different, the adsorption behaviors of three aqueous mixtures in contact with a dense and homogeneous silica were studied versus concentrations and temperatures. Of course, these mixtures displayed very different bulk phase properties. Their stairlike isotherms are interpreted through surface phase diagrams; each of them is very similar to the corresponding bulk phase diagram. Their comparison gives new insights into the different surface states, the role of solvent in the surface, and the probable molecular mobility.

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 π-system is proposed; it is consistent with both thermodynamic measurements and stacking of substituted pyridines within the adsorbed layer.

In an adsorption study from a liquid mixture, the choice of a moderately hydrophilic silica and of a solute like 2,5-dimethylpyridine (2,5-DMP), known for its layering ability in dilute solution in water, allows the observation of several adsorption gaps on the isotherms established at different temperatures. These gaps may be attributed to surface phase transitions. Collected on a (T−Γ21) diagram, they constitute a surface phase diagram highly similar to the bulk water−2,5 DMP liquid−solid diagram at low temperature. It is the first example of a surface phase diagram at a liquid-mixture/solid interface.

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.