Raman Spectroscopy of Covalently Bonded Alkylsilane Layers on Thin Silica Films Immobilized on Silver Substrates

Results are presented which demonstrate the use of Raman spectroscopy for the characterization of alkylsilane layers covalently bound to silica surfaces. Raman spectra of molecular assemblies of octadecylsilane have been studied by use of a thin silica gel/Ag sandwich structure as the substrate. The Ag surface, used to provide electromagnetic enhancement to the octadecylsilane overlayer, is modified with a monolayer of (3-mercaptopropyl)trimethoxysilane, which is used as a molecular adhesive for anchoring the thin silica gel film. Spin coating a silica sol on the modified Ag surface results in the formation of a thin silica gel layer which contributes minimal fluorescence background to the Raman spectra. The spectral data have been interpreted in terms of the alkyl chain conformation within a layer of octadecylsilane, used as a model of a reversed-phase chromatographic stationary phase, on dry silica surfaces as well as in the presence of solvents commonly used in chromatography. The Raman data from the ν(C-C) and ν(C-H) spectral regions suggest that the alkyl chains comprising the layer are predominantly in an ordered configuration. These data represent the first Raman spectra reported for alkylsilane layers on oxide surfaces and demonstrate the feasibility of acquiring conformational information from alkylsilane layers covalently bound to silica surfaces.