EVALUATION AND MODIFICATION OF SILICA GEL (NEAR-INFRARED, ISOTOPE EXCHANGE, ALKYLATION)

Evaluation methods for studying the surface functionality of silica gels have been developed using both infrared (4000-800 cm('-1)) and near infrared (8000-4000 cm('-1)) spectroscopies. Translucent silica gel samples for transmission spectra were obtained by adding carbon tetrachloride as a refractive index matching medium. A near infrared (NIR) cell has been designed which allows for convenient and reproducible sample preparation. The resulting technique has been found to be particularly useful in the quantitative and qualitative analysis of silica surface functionalities. Confirmation of this method's quantitative capabilities has been performed using a newly developed deuterium exchange method and the more established methods of tritium radioisotopic exchange and carbon analysis. The NIR technique was routinely used for the evaluation of surface modified silicas. Complementary surface information was obtained for some samples using NMR, B.E.T. adsorption, mercury intrusion porosimetry and gas chromatographic methods.;Various routes for the surface modification of chromatographically useful silica gels have been explored. Because of the importance of silanols to the chromatographic properties of siliceous materials, this work has focused on methods for the reduction of silanol coverage beyond that possible with existing reaction schemes. Observations made of initial experiments using a two-step chlorination/alkylation route found in the literature led to the discovery of the "direct alkylation" of silica gels using alkyl lithiums and Grighards. These alkylating reagents apparently break siloxane bonds on the surface to form silicon-carbon bonds directly to the surface. Adsorbents formed by this direct alkylation route were found to be less adsorptive to polar solutes by gas chromatography than conventionally modified adsorbents (i.e. silica silylated with trimethyl chlorosilane). Direct butylation (with butyl lithium) seems to be particularly promising in its application to siliceous materials for chromatography.