| Abstract
| - Raman spectroscopy was used to investigate the state of conformational and packing order foroctadecyltrichlorosilane (OTS) attached to high surface area fumed silica as a function of the hydrationstate of the silica. The fumed silica had a surface area of 50 m2/g and a primary particle size of ∼40 nm.Room temperature and temperature-dependent spectra were obtained for OTS attached to the fumed silicaand compared with those of polyoctadecyl siloxane (POS). Both the skeletal optical region, between 1000and 1150 cm-1, and the high-frequency CH2 stretching region, between 2800 and 3000 cm-1, were monitored.The intensity ratios I(1060)/I(1080), which is indicative of the ratio of trans/gauche bonds, and I(2850)/I(2885), which is indicative of lateral packing order, were plotted as a function of temperature. For OTSattached to completely dehydrated and dehydroxylated surfaces, only disordered structures of alkyl chains,similar to those of molten alkane chains, were observed. With increasing hydration of the silica surface,there was a marked increase in trans bonds and lateral packing. However, for approximately monolayercoverage of OTS on the silica surface, the packing order of the chains at room temperature never reachedthe level obtained for POS. POS formed an hexagonal closely packed structure with a differential scanningcalorimetry (DSC) melting point at 63 °C, which corresponded to the inflection points of the plots ofI(2850)/I(2885) or I(1060)/I(1080) versus temperature. A very much broader and shallower endotherm wasobserved for OTS attached to “superhydrated” fumed silica, and the ordered structures disappeared by60 °C. In addition, at room temperature, the packing order of the chains on this “superhydrated” surfacewas similar to that observed for small unilamellar vesicles of lipid bilayers. The decreased packing orderof OTS on “superhydrated” fumed silica compared with POS may be due to the constraints imposed bythe underlying silica substrate or to the curvature of the silica particles.
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