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 \ensuremath{\Delta}T are explained exclusively in terms of the nonretarded van der Waals forces for films 10--120 \AA{} thick. Wafers coated with methyl terminated alkylsiloxane monolayers wet incompletely, with a microscopic film 1--3 \AA{} thick adsorbing on the surface. Changing the alkylsiloxane terminal group from -${\mathrm{CH}}_{3}$ to -${\mathrm{CH}}_{2}$OH 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.