Silica glasses, covalent properties

Oxides of Silicon Silicates Silica, Si02, is the only stable oxide of silicon. Silica is a network covalent solid (not a molecular solid, like CO2). In silica, each Si atom is bonded to four O atoms and each O atom to txvo Si atoms. The structure is that of a network covalent solid, as suggested by Figure 21-31 (a). This structure is reminiscent of the diamond structure, and silica has certain properties that resemble those of diamond. For example, quartz, a form of silica, is fairly hard (with a Mohs hardness of 7) but not as hard as diamond (with a Mohs hardness of 10), has a high melting point (about 1700 °C), and is a nonconductor of electricity. Silica is the basic raw material of the glass and ceramics industries. 

Physical adsorption and covalent attachment of both native and reconibinant OPH onto various supports such as nylon membranes, porous glass and nanometer size silica beads have been enqiloyed (3,21,22).. Unfortunately, physical adsorption offers poor and nonspecific binding, vdiile coval modifications to OPH often results in reduction of enzyme activity and kinetic properties (22,23). In addition to reducing catalytic activity, there is no controlled orientation of the immobilized enzymes, leading to inaccessibility of the substrate to the enzyme active site. In the case of covalent bondii, the immobilization support is not reusable since the formed covalent bond is irreversible. In addition, the tedious and costly protocol for purification of OPH limits its use in large-scale enzymatic degradation. 

Sol-gel is basically a synthetic glass with ceramiclike properties. The processing consists of hydrolysis and condensation of a metal alkoxide (i.e., tetraethoxysilane) to form a glassy material at room temperature. Further modification of this material with a polymer (stationary phase) is used to prepare phases for capillary columns there has been keen interest in this process (118-123). The final sol-gel product retains the properties of the polymer as well as the properties of the sol-gel component. The sol-gel material is able to covalently bond to fused silica. 

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