关键词: 动力学, ²⁹Si NMR, Stöiber反应, 溶胶-凝胶

Stöber syntheses, the ammonia-catalyzed reactions of tetraethoxysilane with water in low molecular weight (MW) alcohols, produce highly monodisperse, spherical silica nanoparticles with size ranging from 5 to 2000 nm. It is the initial hydrolysis and polycondensation process that have been investigated by liquid-state ²⁹Si NMR. We found that the intermediate species was Q₀¹; the reaction equation of the rate-determining step was r=k_h[TEOS]·[NH₃]^0.457[H₂O^]0.051; the constant value was k_h=7.32×10^-3 mol^-0.5·dm^1.5·min^-1 (T=25℃). In the meantime, the reaction rate constant values of those non-rate-determining steps by using the transition state theory were calculated, and the values of the activation energy and the Arrhenius constant were also derived. The results suggested that the reaction rates of the hydrolysis and condensation become faster with increasing the temperature, and the rate constant value of the rate-determining step increases larger than that of non-rate-determining step, indicating that high temperature is more favorable for the hydrolysis of TEOS. The molar ratios of H₂O/TEOS and NH 3/TEOS were adjusted to examine the effect of H₂O and NH 3 and we found that the reaction rates of hydrolysis and condensation of TEOS largely depended on the concentration of NH 3 but not that of H₂O.

Key words: kinetics, ²⁹Si NMR, Stöber synthesis, sol-gel