Surface Vibrational Spectroscopy of Alkylsilane Layers Covalently Bonded to Monolayers of (3-Mercaptopropyl)trimethoxysilane on Ag Substrates

Fourier transform IR and Raman spectroscopy have been used to characterize the structure and conformational order of octadecyltrichlorosilane (OTS) and dimethylchlorooctadecylsilane (DOS) covalently bonded to Ag surfaces modified with a hydrolyzed and condensed self-assembled monolayer of (3-mercaptopropyl)trimethoxysilane (3MPT). This 3MPT layer is used as a model silica surface with a very low surface silanol density and, hence, much less surface-confined water than normal silica surfaces. OTS bonds to this surface at the silanol sites in heterogeneous polymeric islands possessing both highly crystalline and disordered domains. DOS bonds in submonolayer coverages with its alkyl chains in a disordered state. The results from both FTIR and Raman spectroscopies confirm the critical role of surface water in forming highly crystalline assemblies of cross-linked alkylsilanes.