Surface Phase Diagrams:  Evidence of Molecular Arrangements at the Aqueous-Solution/Solid Interface

Abstract

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.

Polyethylene Glycol Adsorption on Silica: From Bulk Phase Behavior to Surface Phase Diagram

“…The first vertical part suggests the occurrence of a change in the surface layer at the origin of a massive entry of water. The first surface transition on the silica-hold surface layer has often been attributed to the penetration of a solute into the water layer attached to silica surface sites by hydrogen bonds; , as shown in the next paragraph, this assumption was confirmed by the analysis of the surface phase diagram of the system under study. Let us consider Figure a and assume that the first vertical part, denoted 1, results from such a penetration accompanied by water intake.…”

“…First, the measured values are relative ones with respect to water where Γ PEG and Γ W are the Gibbs excesses of PEG and water, respectively, and x PEG and x W are the mole fractions in the bulk.…”