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 DELTA-T are explained exclusively in terms of the nonretarded van der Waals forces for films 10-120 angstrom thick. Wafers coated with methyl terminated alkylsiloxane monolayers wet incompletely, with a microscopic film 1-3 angstrom thick adsorbing on the surface. Changing the alkylsiloxane terminal group from -CH3 to -CH2OH converts the surface from incompletely to completely wet. Surfaces coated with partial monolayers of methyl terminated alkylsiloxane of greater than 50% coverage are incompletely wet by cyclohexane, 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 50%. 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.