FORMATION OF CAGE-LIKE INTERMEDIATES FROM NONRANDOM CYCLIZATION DURING ACID-CATALYZED SOL-GEL POLYMERIZATION OF TETRAETHYL ORTHOSILICATE

In the sol-gel synthesis of silica by the acid-catalyzed hydrolysis and condensation of tetraethyl orthosilicate (TEOS), the fractal dimension, density, and homogeneity of the gel depend strongly on reaction conditions such as the acid and water concentrations. However, Si-29 nuclear magnetic resonance shows that gel conversions of at least 80% are observed over a wide range of initial acid (10(-5)-10(-1) mol/L), water (4-22 mol/L), and TEOS (1.3-2.5 mol/L) concentrations. This is very high compared to the value predicted by random branching theory even under the worst possible negative first-shell substitution effect. Moreover, nonrandom cyclization is commonly evidenced in these systems by Si-29 NMR, Raman spectroscopy, and gas chromatography with mass spectroscopy. The gel conversions suggest that nonrandom cyclization reactions are strongly favored, and the development of conversions with time is consistent with the formation of compact three-dimensional cagelike intermediates. These are probably the same nanometer-scale objects shown by recent scattering and microscopy observations.