Chromatographic and NMR Investigations of the Solvation and Mobility of Bonded Alkyl Surfaces

Abstract

The scope of the present work was to investigate in depth surface solvation and dynamics. Bonded materials were prepared incorporating ¹³C tags into the attached alkyl chains. Using a combination of both liquid chromatography (LC) and nuclear magnetic resonance (NMR) experiments, new information about the surface in mixed mobile-phase systems was obtained. Significant differences in NMR line shape were observed, depending on the solvent or combination of solvents. By shifting the label to different positions along the chain, relative alkyl movement was studied. As expected, labeling at or very near the point of attachment resulted in extremely broad line spectra. However, tags further from the surface produced either broad or narrow resonance lines. These data were interpreted to indicate that a preferentially rich organic layer forms at the surface. Additionally, the results showed at least two different kinds of bonded attachment for hydrocarbon chains prepared from trichlorosilane monomers: this is the first direct spectroscopic evidence to support the widely-held idea of multiple-type polymerization when tri-reactive silanes are employed. At mid-range coverages, the loosely bound material was found to be small and may be reduced further by a protic wash followed by thermal treatment.