Study of the solution in the synthesis of a sol-gel composite used as a chromatographic phase

Tetraethoxysilane (TEOS) and octyltriethoxysilane (C₈-TEOS) were used in the synthesis of an organic-inorganic hybrid glass composite by the sol - gel process. This composite was used as the stationary phase for chromatographic and electrophoretic separations when cast onto the inner walls of capillary columns. To investigate the reactions leading to the final chromatographic material, the hydrolysis and initial condensation reactions of TEOS, C₈TEOS, and the C₈-TEOS/TEOS hybrid mixture were studied by ²⁹Si and ¹H NMR spectroscopy. The reaction of methyltriethoxysilane (C₁-TEOS) and ethyltriethoxysilane (C₂TEOS) were also followed for comparison purposes. The formation of the various silicon species and the degree of substitution on the Si atom in the reactions revealed that TEOS hydrolyzes faster than C₈-TEOS when each precursor reacts individually. However, the rate of hydrolysis for C₈-TEOS increases when reacting in a C₈-TEOS/TEOS (1:1 mole ratio) mixture, driven by the condensation of hydrolyzed TEOS species with those of C₈-TEOS. The rate of hydrolysis for alkyl-monosubstituted alkoxysilanes and TEOS followed the trend C₁-TEOS > C₂-TEOS > TEOS > C₈-TEOS. The maximum degree of condensation in the C₈-TEOS/TEOS reaction mixture was observed after 7 h of reaction. The maximum chromatographic retention on the columns fabricated with the C₈-TEOS/TEOS sol-gel composite correlates with the maximum degree of condensation and with a C₈-TEOS/TEOS mole ratio of 1:1.