Hydrolysis and condensation for

FIGURE 8J.8 Schanatic comparison between the rates of hydrolysis and condensation for silicon ethoxide solutions. Redrawn fiom Schaeffer [43],... 

Figure 1. General effect of pH on the relative rates of hydrolysis and condensation for TEOS. (Reproduced with permission from reference 31. Copyright 1987 American Institute of Physics.)...

The mechanism of chemical adhesion is probably best studied and demonstrated by the use of silanes as adhesion promoters. However, it must be emphasized that the formation of chemical bonds may not be the sole mechanism leading to adhesion. Details of the chemical bonding theory along with other more complex theories that particularly apply to silanes have been reviewed [48,63]. These are the Deformable Layer Hypothesis where the interfacial region allows stress relaxation to occur, the Restrained Layer Hypothesis in which an interphase of intermediate modulus is required for stress transfer, the Reversible Hydrolytic Bonding mechanism which combines the chemical bonding concept with stress relaxation through reversible hydrolysis and condensation reactions. 

NLO active molecules can be embedded in or chemically anchored to a sol-gel-matrix without changing the optical absorption spectrum. Disperse Red 1, a very efficient molecule for NLO applications, was embedded in a sol-gel-matrix, synthesized by hydrolysis and condensation of 3-glycidoxypropyltrimethoxysilane in the presence of N-methylimidazole. The dye-doped gel was applied to glass substrates and thermally cured to form a layer of perfect optical transparency. Currently, poling experiments and NLO measurements with these layers are being performed. 

Manipulation of the factors noted above may be used to control the characteristics of the hydrolysis and condensation reactions, which will dictate precursor nature. These reactions have been studied extensively for silica,1,63 and some similarity in chemical behavior to electronic material systems (PZT, etc.) has been noted because of the alkoxide reagents used in the synthesis of these material systems.52,70,71... 

In this instance, the resulting species possess both alkoxide and acetate ligands. Because the acetate species may be (and typically are) bidentate in nature,77 as well as sterically larger than short chain alkoxy groups, they are less susceptible to attack by water, and the hydrolysis and condensation reactions are slowed. Because of the reduced sensitivity toward hydrolysis, the nature of the precursor species resulting from these processes is desirable for film formation (to be discussed in Sections 2.5 and 2.6). 

Consider a trialkoxysilane as co-precursor for an initial 1 1 mixture of alkoxysilane and organoalkoxysilane, and recall that in the case of ORMOSIL condensation takes place only through elimination of water27 and not by alcohol formation. The mechanism involves hydrolysis and condensation reactions ... 

In general, the structure of sol gel materials evolves sequentially as the product of successive and/or simultaneous hydrolysis and condensation and their reverse reactions (esterification and depolymerization). Thus, in principle, by chemical control of the mechanisms and kinetics of these reactions, namely the catalytic conditions, it is possible to tailor the structure (and properties) of the gels over a wide range. For example, stable silica xerogels of tailored particle dimensions, pore morphology, density and porosity, from relatively... 

Anodic EOF for CEC has also been obtained on a silica hybrid synthesized with the two coprecursors TEOS and 3-aminopropyltriethoxysilane (APTES) [97]. Here, no catalyst was added to accelerate the hydrolysis and condensation reactions ... 

Si(OC2H5). In the two-step process of hydrolysis of the silane to the silanol species and the condensation (silanols to siloxane), the reaction-determining step is the rate of hydrolysis [54]. However, for polyfunctional silanes, hydrolysis and condensation overlap. Under defined reaction conditions, the reaction between the surface hydroxyl groups of the sihca and the silane compound follows a distinct stochiometry, which can be expressed by the number of surface hydroxyl groups that react with the organosilane. For monofunctional silanes this ratio is unity. 

Traditionally, the sol-gel process has been used for the preparation of silica nanoparticles via the hydrolysis of alkoxides in organic solvents [52,53]. Similar hydrolysis and condensation carried out in w/o microemulsion offers robust control over the synthesis process. W/o emiflsion-mediated sol-gel synthesis is currently used for the fabrication of pure sihca, as well as inorganic and organic dye-doped silica nanoparticles. The synthesis of sihca and dye-doped nanoparticles is classified in the following sections on the basis of the classification of the head group fimctionahty of the major surfactant used. 

Several growth and formation mechanisms have been proposed for the formation of monodispersed Stober silica particles. Silica in general is an extremely well-studied system, and there are numerous publications with respect to the hydrolysis and condensation reaction. At present there are two major formation mechanisms that have been used to explain the formation of Stober silica particles. 

In a typical sol-gel synthesis, the metal or the compounds of main group elements undergo hydrolysis and condensation reactions giving gel materials with extended three-dimensional structures. As shown in Equation 5.32 for silicon, addition of an acid or base catalyst to a solution of an alkoxysilane reagent such as tetrame-thoxysilane (TMOS), water and methanol leads to the hydrolysis of the Si-OMe bonds to form Si-OH functional groups. 

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