Silicon alkoxides determination

Abstract—The effects of metal alkoxide type and relative humidity on the durability of alkoxide-primed, adhesively bonded steel wedge crack specimens have been determined. Aluminum tri-sec-butoxide, aluminum tri-tert-butoxide, tetrabutyl orthosilicate, and titanium(IV) butoxide were used as alkoxide primers. Grit-blasted, acetone-rinsed mild steel adherends were the substrates bonded with epoxy and polyethersulfone. The two aluminum alkoxides significantly enhanced the durability of the adhesively bonded steel, while the titanium alkoxide showed no improvement in durability over a nonprimed control. The silicon alkoxide-primed samples gave an intermediate response. The failure plane in the adhesively bonded samples varied with the relative humidity during the priming process. 

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... 

As discussed above, the reaction mechanisms for acid or base catalysis are very different. Furthermore, the reaction rates for hydrolysis and condensation of silicon alkoxides have different pH dependence (Figure 1.3). The minimal reaction rate for hydrolysis is at pH 7, and for condensation at around pH 4.5. The latter corresponds to the PZC of silica At pH <5, hydrolysis is feivored, and condensation is the rate-determining step. A large number of monomers or small oligomers with reactive Si—OH groups are simultaneously formed. In contrast, hydrolysis is the rate-determining step at pH >5, and hydrolyzed species are immediately consumed because of the faster condensation. Catalysis by fluoride ions is similar to that of hydroxide ions (basic conditions). 

Yold has determined by IR spectroscopy that the degree of silicon oxide network connectivity increases with increasing H O/TEOS mole ratio for add-catalyzed polymerizations in bulk solutions (21). These results, if applicable to the membrane in situ add-catalyzed polymerizations described herein, serve to reinforce our conclusion that a more highly-coordinated silicon oxide structure exists within microcomposites produced with short immersion times according to procedure A, or according to the slow stepwise TEOS addition in procedure B. In either case, more initial hydrolysis water molecules per alkoxide molecule are available to promote this situation. General Conclusions... 

The hydrolysis mechanisms involve first a nucleophilic attack of oxygen lone pairs of the H2O molecule on the Si atoms [33]. Because of the polarized Si-O bonds, the silicon atoms hold a partial positive electrcMiic charge which in turn determines the kinetics of the nucleophilic attack and hence of the overall hydrolysis reaction. In alkoxides, the Si atoms carry a relatively moderate partial positive charge (e.g., 8" 0.32 in Si(OEt)4 by... 

---