Hydrolysis of silicon alkoxides

Biological species such as enzymes, whole cells, antibodies and even bacteria can all be successfully entrapped in silica sol-gel matrices, often with enhancement of activity with respect to the free biologicals. In these cases, the process is adapted to eliminate toxic alcohols which are typically released in conventional sol-gel processes based on the hydrolysis of silicon alkoxides. Two such methods are the use of silicon alkoxide... 

As one would expect from the similar electronegativities of Si and O, the hydrolysis of silicon alkoxides are significantly slower than other metal analogues. For identical metal coordination spheres and reaction conditions, the general order or reactivity... 

Base-catalyzed hydrolysis of silicon alkoxides proceeds much more slowly than acid-catalyzed hydrolysis at an equivalent catalyst concentration (i), the tetraalkoxides being particularly resistant to attack (14). Hydrolysis proceeds stepwise, with each subsequent alkoxide group more easily removed from the monomer than the previous one (14). Alkoxide groups are much more diflScult to remove from polymers than from monomers. 

Inorganic silicate polymers prepared by the hydrolysis of silicon alkoxides evolve by a complicated reaction pathway. The structures can be described only in a statistical fashion by means of fractal geometry. The structures are... 

In the acidic route (with pH < 2), both kinetic and thermodynamic controlling factors need to be considered. First, the acid catalysis speeds up the hydrolysis of silicon alkoxides. Second, the silica species in solution are positively charged as =SiOH2 (denoted as I+). Finally, the siloxane bond condensation rate is kinetically promoted near the micelle surface. The surfactant (S+)-silica interaction in S+X 11 is mediated by the counterion X-. The micelle-counterion interaction is in thermodynamic equilibrium. Thus the factors involved in determining the total rate of nanostructure formation are the counterion adsorption equilibrium of X on the micellar surface, surface-enhanced concentration of I+, and proton-catalysed silica condensation near the micellar surface. From consideration of the surfactant, the surfactants first form micelles as a combination of the S+X assemblies, which then form a liquid crystal with molecular silicate species, and finally the mesoporous material is formed through inorganic polymerization and condensation of the silicate species. In the S+X I+ model, the surfactant-to-counteranion... 

Acid or base is used as a catalyst for the hydrolysis of silicon alkoxide. Acid or base with a mole ratio to alkoxide as low as 0.01 or less is usually enough for the catalytic effect. On the other hand, water with a mole ratio to alkoxide as high as 10 or more is used for the complete hydrolysis of alkoxide. We have found, however, that monolithic opaque gels composed of micrometer-sized silica particles can be prepared from tetramethoxysi-lane (TMOS) when a limited amount of water and a large amount of... 

FIGURE 22.5 Schematic showing reaction mechanisms for (a) acid- and (b) base-catalyzed hydrolysis of silicon alkoxides. 

Stober et al. (56) carried out a systematic study of the factors that control the preparation of fine, uniform Si02 particles by the hydrolysis of silicon alkoxides in the presence of NH3. The NH3 served to produce pH values in the basic range. For the hydrolysis of silicon tetraethoxide, Si(OC2H5)4, referred to commonly as TEOS, with ethanol as the solvent, the particle size of the powder was dependent... 

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