Wetting films on chemically modified surfaces: An x-ray study.

The wetting of silicon wafers and silicon wafers coated with alkylsiloxane monolayers by saturated vapors of cyclohexane and methanol were studied using x-ray specular reflection. Differentially heating the substrate surface relative to the temperature of a liquid reservoir was used to probe the disjoining pressure as a function of the film thickness and surface chemistry. Uncoated silicon wafers wet completely. The variations in film thickness with L^T are explained exclusively in terms of the nonretarded van der Waals forces for films 10-120 A thick. Wafers coated with methyl terminated alkylsiloxane monolayers wet incompletely, with a microscopic film l-3 A thick adsorbing on the surface. Changing the alkylsiloxane terminal group from -CHr to -CH2OH converts the surface from incompletely to conrpletely wet. Surfaces coated with partial monolayers of methyl terminated alkylsiloxane of greater than 507o coverage are incompletely wet by cyciohexane, with the monolayers "swelling" to a thickness close to that of fully extended alkane chains through incorporation of cyclohexane into the film structure. The data are consistent with a first-order transition to complete wetting upon reduction of the alkylsiloxane coverage below approximately 50Vo. The importance of the surface atomic layer in the promotion or suppression of complete wetting is explained in the context of van der Waals forces.