Characterization of Octadecylsilane and Stearic acid Layers on AI2O3 Surfaces by Raman Spectroscopy

Results are presented which demonstrate the use of Raman spectroscopy for the characterization of octadecylsilane, octadecyldimethylsilane,and stearic acid layers on AI₂O₃ surfaces without the use of surface enhancement. Unlike previous attempts to study adsorbates at AI₂O₃ surfaces,an enhancing Ag adlayer was not used. These results represent the first report of the use of Raman spectroscopy for the characterization of alkylsilane and stearic acid layers on AI₂O₃ surfaces. The spectral data are interpreted in terms of alkyl chain conformation within these layers. Raman data from the v(C-C) and v(C-H) spectral regions suggest that the alkyl chains comprising the octadecylsilane layer are moee disordered than similar films formed on silica. The octadecyldimethylsilane monolayers are less ordered, suggestingAn even greater concentration of gauche conformations in the alkyl chains of these films. The observed increase in disorder in films formed from octadecyldimethylsilane is attributed to the influence of the bulky methyl groups on bonding density which prevent the alkyl chains from packing At their van der Waals radii. The Raman spectral data from self-assembled films formed from stearic acid suggest that these films are moee ordered than the similar films from alkyl chlorosilanes. The increase in order is believed to be due to the bonding orientation of the carbonyl group At the AI₂O₃ surface.