Raman spectroscopy was used to investigate the state of conformational and packing order for octadecyltrichlorosilane (OTS) attached to high surface area fumed silica as a function of the hydration state of the silica. The fumed silica had a surface area of 50 m(2)/g and a primary particle size of similar to 40 nm. Room temperature and temperature-dependent spectra were obtained for OTS attached to the fumed silica and compared with those of polyoctadecyl siloxane (POS). Both the skeletal optical region, between 1000 and 1150 cm-l, 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 OTS attached 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 monolayer coverage of OTS on the silica surface, the packing order of the chains at room temperature never reached the level obtained for POS. POS formed an hexagonal closely packed structure with a differential scanning calorimetry (DSC) melting point at 63 degrees C, which corresponded to the inflection points of the plots of I(2850)/I(2885) orI(1060)/I(1080) versus temperature. A very much broader and shallower endotherm was observed for OTS attached to "superhydrated" fumed silica, and the ordered structures disappeared by 60 degrees C. In addition, at room temperature, the packing order of the chains on this "superhydrated" surface was similar to that observed for small unilamellar vesicles of lipid bilayers. The decreased packing order of OTS on "superhydrated" fumed silica compared with POS may be due to the constraints imposed by the underlying silica substrate or to the curvature of the silica particles.