AN ELLIPSOMETRIC STUDY OF ADSORPTION-ISOTHERMS

Adsorption isotherms of alkanes or silicone oil oligomers on silicon wafers covered with natural oxide, or bearing hydrophobic grafted layers, have been studied at room temperature. At large thicknesses, information on the long-range interactions is extracted, and plausible values of the Hamaker constants, i.e. of the strength of the van der Waals interaction, are obtained for alkanes. At molecular thicknesses, the amount adsorbed depends on the specific short-range interactions with the surface layer. The presence of an adsorbed layer of water on the bare silica surfaces must be taken into account in the interpretation of data.