Wetting films on chemically modified surfaces: An x-ray study
- Department of Physics and Division of Applied Sciences, Harvard University, Cambridge, Massachusetts 02138 (United States)
- Department of Chemistry, Harvard University, Cambridge, Massachusetts 02138 (United States)
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}{ital 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 1--3 A thick adsorbing on the surface. Changing the alkylsiloxane terminal group from -CH{sub 3} to -CH{sub 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.
- DOE Contract Number:
- AC02-76CH00016
- OSTI ID:
- 5942628
- Journal Information:
- Physical Review, B: Condensed Matter; (United States), Journal Name: Physical Review, B: Condensed Matter; (United States) Vol. 44:19; ISSN 0163-1829; ISSN PRBMD
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
360606 -- Other Materials-- Physical Properties-- (1992-)
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201* -- Chemical & Physicochemical Properties
ALCOHOLS
ALKANES
CYCLOALKANES
CYCLOHEXANE
ELECTROMAGNETIC RADIATION
ELEMENTS
HYDROCARBONS
HYDROXY COMPOUNDS
IONIZING RADIATIONS
METHANOL
ORGANIC COMPOUNDS
RADIATIONS
REFLECTION
SEMIMETALS
SILICON
VAN DER WAALS FORCES
WETTABILITY
X RADIATION