NMR and Theoretical Investigation of Cation Binding with Sol-Gel Silicates

Previous reports suggest that the choice of base catalyst affects both the kinetics and structural evolution of SiO₂ gelation from tetraethoxysilane (TEOS). Furthermore, these effects have been attributed to the interaction of the alkaline cations with charged silicate fragments. In this study, NMR analysis of alkali-metal nuclei and of ²⁹Si directly shows that during the condensation of silicon alkoxides in basic alcohol solutions, cations form ion-pair complexes with negatively charged silicate groups. It is determined that the ion pair is formed most favorably with potassium. In seeking a predictive model of complex stability, we found it most successful to use a bond orbital electronegativity equalization model developed by Bergman and Hinze.