ADSORPTION-ISOTHERMS OF ALKANES ON SILICA - THE ROLE OF PREADSORBED LAYERS

Adsorption isotherms of alkanes on oxidized silicon wafers have been r ecorded by ellipsometry at room temperature. A first series of experim ents has been performed on wafers heated for increasing times at 850-d egrees-C. Heating under normal atmosphere increases the thickness of t he silica layer and decreases the amount of silanol groups at the sili ca surface. The short-range interaction between alkanes and surface is controlled by the amount of silanols, while the long-range interactio n depends on the silica thickness. A second series of experiments has been performed on wafers bearing an adsorbed hydrophobic layer. If the alkane does not penetrate this layer, a decrease in wettability is ob tained, as expected. This is the case for layers of trimethylsilyl gro ups. Thicker layers are penetrated by alkanes, and the interpretation of adsorption results is not straightforward. An extreme situation is an adsorbed polymer layer swollen by the alkane. The aim of these expe riments is to get a better understanding of wettability of model syste ms, and ultimately of reservoir rocks.