Vibrational spectroscopy of dimethylchlorooctadecylsilane covalently bonded to ultrathin silica films immobilized on Ag surfaces

FTIR and Raman spectroscopies have been used to characterize the structure and conformational order of dimethylchlorooctadecylsilane (DOS) covalently bonded to ultrathin silica films supported on Ag substrates. Ultrathin silica films of ca. 30 Å thickness prepared from sol-gel methods are immobilized on Ag surfaces modified with a self-assembled monolayer of (3-mercaptopropyl)trimethoxysilane (3MPT). This layered structure provides a unique opportunity for acquiring complementary spectral data from both FTIR and Raman spectroscopies, which are useful in elucidating alkylsilane conformation pertaining to stationary phases for reversed-phase liquid chromatography (RPLC). Characterization of octadecyltrichlorosilane (OTS) layers on thin silica films of ca. 800 Å thickness on 3MPT-modified Ag surfaces has been reported previously. Differences between the ultrathin silica films used in this study and the thin silica films used in this previous study are considered. The results from both FTIR and Raman spectroscopy presented here suggest that bonded DOS alkyl chains are in a disordered, liquid-like state with close to monolayer surface coverage.