In the sol-gel synthesis of silica, a silicon-containing raw material, a solvent, water, and a catalyst are utilized. By varying the catalyst, dramatic effects on gelation time, porosity, bulk and apparent density, and volume shrinkage on drying were observed. For example, porosities ranging from two to sixty-eight per cent can be obtained for dried and fired gels. Mechanisms for catalysis have been proposed for the catalysts considered.

Sol-gel processing of silica. II: The role of the catalyst

Studies of the kinetics of the precipitation of uniform silica particles through the hydrolysis and condensation of silicon alkoxides

http://phoenix12345.persiangig.com/1-s2.0-S0021979798959859-main.pdf

Synthesis of nanosize silica in a nonionic water-in-oil microemulsion: Effects of the water/surfactant molar ratio and ammonia concentration

Kinetics and mechanisms of the hydrolysis-condensation reactions, starting with tetraethyl orthosilicate (TEOS), have been studied by 29 Si NMR. A standard experimental procedure for the sol/gel process was used: acid-catalyzed reactions and different concentration ratios of alkoxide and water. The inorganic polymerization was influenced by the following experimenta) parameters: — The starting alkoxides which may exhibit weakly reactive “Si-OR groups”. — The use of gel precursors with different chain lengths which lead to parasite ester exchange reactions. — The variations of pH which modify hydrolysis and condensation rates. — The H 2 O content where an excess of water was required to fully separate hydrolysis and condensation domains, and to eliminate “OR groups” in the gel.

NMR study of the sol/gel polymerization

The NH3-catalyzed formation of colloidal silica particles from tetra-ethyl-orthosilicate (TEOS) in methanol and ethanol is studied by means of light scattering and Raman spectroscopy. We find that the growth is characterized by an incubation period after which no significant nucleation takes place. The particles have uniform, non-fractal structure and show low polydispersity. In the presence of excess water, the rate-limiting step is the hydrolysis, which is a first-order process in the orthosilicate concentration. © 1988 Academic Press, Inc.

/pdf/dynamics-of-growth-of-silica-particles-from-ammonia-qq4u6yae1b.pdf

Dynamics of Growth of Silica Particles from Ammonia-Catalyzed Hydrolysis of Tetra-ethyl-orthosilicate

Donnees theoriques et cas de l'hydrolyse de tetramethylorthosilicate, Si(OCH 3 ) 4 en solution dans du methanol ou du formamide

NMR and Raman study of the hydrolysis reaction in sol-gel processes

The Raman bands of the C–H ν3 and C–O ν8 stretching modes of liquid methanol have been measured at temperatures ranging from 273 to 363 K and pressures from 10 bar to 4 kbar. The effects of density and temperature on the isotropic linewidth, peak frequency of the isotropic band, ν0 (ISO), and the difference δν between anisotropic ν0(VH) and ν0(ISO) band frequencies are reported and discussed qualitatively in terms of available theoretical models. It appears that repulsive interactions are responsible for the observed C–H line changes, and the Schweizer–Chandler (SC) model is used to evaluate frequency shift and bandwidth of the ν3 C–H mode after the effect of Fermi resonance coupling to the 2ν4 overtone was removed. In particular, the anomalous temperature and density dependence of the Raman band of the C–O stretching mode is noted and compared to the relatively typical behavior observed for the Raman line shape of the C–H mode.

M. Bradley

Papers

NMR and Raman study of the hydrolysis reaction in sol-gel processes

High pressure isotropic bandwidths and frequency shifts of the C–H and C–O modes of liquid methanol

Fermi resonance and vibrational lineshapes of the CH3 group in liquid methanol

Raman study of the sol to gel transformation under normal and high pressure

Raman study of the temperature and pressure effects on the Fermi resonance and hydrogen bonding in liquid ammonia