| Abstract
| - A characterization of the bulk-phase diagram from literature data and new NMR and DSC measurements providedus with valuable elements that are helpful for gaining, from aqueous solution, better insight into the surface behaviorof polyethylene glycol on Aerosil 200. Adsorption isotherms built further to measurements by a depletion methodshowed a strong and temperature-dependent variation of the isotherm shape in agreement with the variations ofinteractions 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 reportedin 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 oscillationssuggesting very different surface behavior. Our data analysis evidenced the presence of a much greater amount ofwater 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 PEGmolecules to move closer to the surface and become helix-shaped to rearrange in a monolayer. The surface phasediagram confirmed this analysis in a very satisfying way.
|
