29 Si-NMR Studies of the Sol-Gel Hybrid Solution Containing Octadecyltriethoxysilane and Tetraethoxysilane Used to Fabricate a Stationary Phase for Open Tubular Capillary Electrochromatography

Abstract

n-Octadecyltriethoxysilane (C₁₈-TEOS) and tetraethoxysilane (TEOS) were used as the precursors to fabricate an organic-inorganic hybrid composite by sol-gel processing. The hydrolysis and initial condensation reactions of TEOS and C₁₈-TEOS/TEOS mixtures were studied by ²⁹Si-NMR spectroscopy. The rate of disappearance of the silane precursors and the evolution of the various silicon species during the reaction indicate that TEOS hydrolyzes and condenses faster than C₁₈-TEOS. The rate of hydrolysis for C₁₈-TEOS, however, increases when reacting in the C₁₈-TEOS/TEOS hybrid mixture, indicating that the condensation reactions involving TEOS species drive the reaction of C₁₈-TEOS. The formation of cross-condensed products leads to the composite with octadecyl groups incorporated into the final glass material. The maximum degree of condensation in the C₁₈-TEOS/TEOS mixture was observed after 2 h of reaction. Fused-silica capillary columns were coated with the C₁₈-TEOS/TEOS mixture while in its liquid-like stage. After drying, the columns were tested for open tubular capillary electrochromatography (OT-CEC). Coating of the columns was performed after the maximum degree of condensation was achieved, which has been shown to correlate with maximum chromatographic retention.