Silica network

Fine silica network is visible in the in situ silica-filled rubber composite synthesized from 50 wt% of TEOS, producing almost 15 wt% of sdica. On the other hand, addition of only 10 wt% of precipitated silica externally gives distinct aggregations. 

Figure 6.2. (a). Colloidal silica network on the surface of spores from Isoetes pantii (quill wort). Scale = 20 pm. (b). Polystyrene networks and foams produced as a biproduct of colloidal latex formation. Both types of colloidal system are typical of the diversity of patterns that can be derived from the interactions of minute particles. Scale (in (a)) = 50pm. 

More than 20 different forms of silica exist, because the bonds and lone pairs around the oxygen atoms can be arranged in various ways. Each arrangement creates a different stmctural form for the silica network. Quartz, the most common form of silica, is found in granite, sandstone, and beach sand. 

We find that this solid state reaction is very slow, even at 800 °C., and occurs at the interface of the two types of particles. The reaction is slow because it is diffusion-limited. What is happening is that since the silica-network is three-dimensionally bound, the only reaction that occurs is caused by the difiusion of Ba2+ atoms within the network, as shown in the following ... 

Similarly to the above-mentioned entrapment of proteins by biomimetic routes, the sol-gel procedure is a useful method for the encapsulation of enzymes and other biological material due to the mild conditions required for the preparation of the silica networks [54,55]. The confinement of the enzyme in the pores of the silica matrix preserves its catalytic activity, since it prevents irreversible structural deformations in the biomolecule. The silica matrix may exert a protective effect against enzyme denaturation even under harsh conditions, as recently reported by Frenkel-Mullerad and Avnir [56] for physically trapped phosphatase enzymes within silica matrices (Figure 1.3). A wide number of organoalkoxy- and alkoxy-silanes have been employed for this purpose, as extensively reviewed by Gill and Ballesteros [57], and the resulting materials have been applied in the construction of optical and electrochemical biosensor devices. Optimization of the sol-gel process is required to prevent denaturation of encapsulated enzymes. Alcohol released during the... 

Interpenetrating PCL-silica network is formed as in the scheme shown in Figure 12.9. 

Figure 2 (Left) shows the 27Al NMR spectra for the aluminosilicates. All of them displayed a tetrahedral incorporation of aluminum inside the silica network. That is corroborated by the signal at 55 ppm [9, 10] which also become more intense with the decreasing of Si/Al ratio. Octahedral aluminum was observed just for the samples with the lowest Si/Al ratio. Tetrahedral aluminum gives place to strong Bronsted acid sites, which were identified by the interaction of these groups with pyridine that generates a...

The hybrids generated by the sol-gel process combine the flexibility and mechanical strength of the organic constituent with the hardness, stiffness and transparency of the inorganic silica network. Hardness is often further conferred by the employment of other inorganic oxide particles (such as alumina or titania). This first started in the 1980s when... 

Equation (5.1) represents the hydrolysis of the salt of a weak acid. The sodium ion migrates into the solution, and the hydroxyl ion released in the glass can then attack the otherwise stable silica network, converting a bridging oxygen into a non-bridging site, thus disrupting the network ... 

The silica networks of mesoporous sihcas are terminated at the surfaces of the amorphous pore walls, thus resulting in terminal silanol groups on the walls. The density of the silanol groups of all mesoporous sihcas (1 to 3 nm 2) is somewhat lower than usually found for other typical silica materials (4 to 6 nm 2) [27], The specific surface areas of the different mesoporous silicas vary depending on their pore sizes, the thicknesses of their pore walls, and the density of their sihca networks. For some MCM-41- and MCM-48-type materials, surface areas of about 1000 to 1400 m2 g 1 have been reported. The surface areas of SBA-15-type materials can be > 600 m2 g 1 [27],... 

---