A STUDY OF THE STRUCTURE AND DYNAMICS OF DIMETHYLOCTADECYLSILYL-MODIFIED SILICA USING WIDE-LINE H-2-NMR TECHNIQUES

The structure and dynamics of dimethyloctadecylsilyl-modified silica gel (C18-silica) were studied by using solid-state H-2 NMR techniques. In order to reproduce with a theoretical model the H-2 line shapes that were observed for the C18-silica samples, it was necessary to assume that a variety of environments are present on the C18-silica surface and that this heterogeneity results in different details of the motion of silane groups that are bonded to different regions of the C18-silica surface. H-2 spectra that were taken on the C18-silica samples at temperatures below room temperature appear to be superpositions of two line shapes, indicating that some silane groups move with a simple two-site jump motion, along with others that move with a more complex distribution of motions. At -50-degrees-C, the addition of wetting liquids can affect the H-2 line shape; different liquids can have dramatically different effects on the observed H-2 line shapes. Simple surface structures for Cl8-silica are proposed that are consistent with the NMR data observed in this study.