Chemistry of hydrolysis and condensation

Chapter 48 The Chemistry of Hydrolysis and Condensation of Silica Sol-Gel Precursors. 637... 

The chemistry of hydrolysis and condensation of silicon alkoxides is now understood in considerable detail, as indicated in the chapter by Coltrain and Kelts (Eastman Kodak Co.). Extensive use of nuclear magnetic resonance has revealed the influence of factors such as pH on the kinetics of the competing reactions. With this information it is possible to rationalize the structures of the aggregates, as revealed by studies of small-angle scattering of X-rays and neutrons. This level of understanding opens the possibility for deliberate control of gel structure and properties. Nonsilicate systems have received less... 

The chemistry involved in the formation of mesoporous silica thin films is qualitatively well understood. However, specific reaction mechanisms of the individual steps are still debated. In addition, owing to the complexity of the sol-gel reaction pathways and cooperative self-assembly, full kinetic models have not been developed. From the time of mixing, hydrolysis reactions, condensation reactions, protonation and deprotonation, dynamic exchange with solution nucleophiles, complexation with solution ions and surfactants, and self-assembly, all occur in parallel and are discussed here. Although the sol-gel reactions involved may be acid or base catalyzed, mesoporous silica film formation is carried out under acidic conditions, as silica species are metastable and the relative rates of hydrolysis and condensation reactions lead to interconnected structures as opposed to the stable sols produced at higher pH. Silicon alkoxides are the primary silica source (tetramethyl orthosilicate, tetraethyl orthosilicate, tetrapropyl orthosilicate, etc.) and are abbreviated TMOS, TEOS, and TPOS, respectively. Starting from the alkoxide, Si(OR)4, in ROH and H2O solution, some of the general reactions are ... 

We will start with the sol-gel chemistry of silicon alkoxide, Si(OR)4 (R= alkyl), since this chemical is the most popular in the sol-gel method, and its reaction scheme has been extensively investigated. The sol-gel reaction ofSi(OR)4 consists of hydrolysis and condensation steps to form siloxane polymers having =Si-0-Si= bonds. The hydrolysis of Si(OR)4 proceeds via the replacement of OR groups by OH. Then, nominally, four water molecules are required for the completion of hydrolysis ofSi(OR)4 to form an Si(OH)4 molecule as follows. 

The +4 oxidation state (z = 4) is the only important one in the chemistry of silicon in naturally occurring systems [10], and the coordination number of silicon, N, is most often four. Compared to transition metals discussed in the previous chapter, silicon is generally less electropositive, e.g., the partial positive charge on silicon nucleophilic attack, and since N = z, coordination expansion does not spontaneously occur with nucleophilic reagents. These factors make the kinetics of hydrolysis and condensation considerably slower than observed in transition metal systems or in Group III systems. 

It is believed that the macroporosity is exactly dependent on the hydrolysis and condensation rates of the metal alkoxides. However, in sol-gel chemistry, the rates of hydrolysis and condensation strongly depend on the pH values as well. It was found that the hierarchically porous materials with different macroporosities could be obtained via a spontaneous self-formation process carried out at different pH values. As a result of these opposing effects, acid and base catalysts... 

Silicone polymer technology rests in practice upon the preparation of reactive substituted silanes from silicon metal and the subsequent conversion of these reactive substances, usually through stepwise hydrolysis and condensation reactions, into polysiloxanes. Thus the hydrolysis of these reactive intermediates is a fundamental process, the nature and implications of which have demanded increasing attention as organo-silicon chemistry and technology have developed. 

The sol-gel chemistry has also been used to prepare inorganic inhomogeneities in an organic matrix. Silane end-capped macrodiols can be used. Hydrolysis and condensation of alkoxy silane groups lead to inorganic hard clusters (Fig. 7.6b). Intramolecular reactions and the miscibility of the soft-segment chains with the relatively polar crosslinks determine the size distribution of the clusters (nanofillers). 

Figure 7.7 SAXS profiles for two hydroxyl-terminated oligomers crosslinked by alkoxysilane sol-gel chemistry. First, 1 mole of macrodiol, SS (hydrogenated polybutadiene, HPBD or polycaprolactone, PCL, Mn= 2 kg mol-1), was reacted at 80°C with 2 mole of dicyclohexylmethane diisocyanate, H12 MDI. After complete reaction, the prepolymer was dissolved in tetrahydro-furan and the y-aminosilane, yAPS was added dropwise at room temperature. After 1 h of reaction, the solvent was removed under pressure. The final network was obtained in the absence of a solvent by hydrolysis and condensation of the ethoxysilane groups by the addition of 0.1 mol% TFA, trifluor-oacetic acid. After stirring at room temperature, the mixture was cast into a mold and cured for 24 h at 100°C under pressure, and then postcured at 150°C for 12 h. (Cuney et al., 1997 - Copyright 2001, Reprinted by permission of John Wiley Sons, Inc.)...

In the sol-gel synthetic method, the gel graduates from inert background material to product. The underlying chemistry remains the same. The silicon-dioxide network is often generated from tetraethyl orthosilicate (Si(OCH2CH3)4), commonly abbreviated TEOS. An alcoholic solution of TEOS undergoes hydrolysis and condensation when added to water to produce Si02( ) and ethanol ... 

The chemistry of these hydrolysis and condensation reactions is very complicated. Both reactions are pH sensitive. Acids and bases catalyze the hydrolysis reactions, although each to a different extent (i). The condensation reaction is also highly pH dependent. Water is consumed in the hydrolysis reaction but liberated in the condensation reaction. To fully activate all of the silanols initially requires twice as much water as is consumed in the net reaction. To use less water means the reactions will be closely coupled. The reactions are usually conducted in a solvent, because the alkoxides are immiscible with the water required for hydrolysis. The usual solvent is the alcohol corresponding to the alkoxide group of the silicon source, which is also a byproduct of the reactions. 

The formation of colloidal particles can be obtained from hydrolysis and condensation of metal salts in aqueous media. The aqueous chemistry of metal salts is quite complicated owing to the occurrence of hydrolysis reactions which convert the ions to new ionic species or to precipitates. When dissolved in water a metal cation becomes solvated by the surrounding water molecules according to... 

This chapter is a brief overview of silicon alkoxide hydrolysis and condensation and the resulting structures, emphasizing recent studies and unpublished work. Schmidt et al. (I) and more recently Brinker (2) have published excellent overviews of this chemistry this chapter attempts to amplify and complement these reports. 

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