Gelation of silica sol

Figure 19. Schematic representation of the reaction between hydroxy groups on different silica particles leading to gelation of silica sols (left-hand side) and of the hydroxy group(s) on partially hydroxylated metal ions with surface hydroxy groups of silica.

Silica gel could be prepared via the gelation of silica sol. The process for the formation of water-containing uniform gel from spherical silical colloidal particles is very fast. It is known that there is adhesive force on the surface of spherical silica collodial particles, which could lead to the aggregation of these particles. This process could be described as below. 

Silica-based monolithic columns (Figure 9) are generally prepared using sol-gel technology. This involves the preparation of a sol solution and the gelation of the sol to form a network in a continuous liquid phase within the capillary. The precursors for the synthesis of these monoliths are normally metal alkoxides that react readily with water. The most widely used are alkoxysilanes such as tetramethoxysilane (TMOS) and TEOS. 

Effect of TFPS/TEOS molar ratio on the gelation time of silica sols (weight ratio of sol to NH4OH solution is 50 1)... 

For several decades, carbon black and fumed silica have been used in tires, tubings, sealants and so on.[162,163] More recently, functionalized silica nanoparticles were reported to copolymerize with MMA to produce transparent films with good surface finish and controllable refractive index. [162] The sol-gel process is another research topic to prepare the polymer nanocomposites. The principal sol-gel process includes two steps the formation of a colloidal suspension, i.e. sol, and the gelation of the sol to formulate a network in a continuous liquid phase, i.e. gel. For example, the hydrolysis and poly condensation of silicon alkoxides in the presence of MMA will produce a polymer nanocomposite with superior optical and mechanical properties.[163]... 

FIGURE 5.4 Stages in sol-gel processing are captured by a new electron microscopy technique. (1) Spherical particles tens of nanometers across can be seen in a colloidal silica sol. (2) Addition of a concentrated salt solution initiates gelation. (3) The gelled sample, after drying under the electron beam of the microscope, shows a highly porous structure. Courtesy, J. R. Bellare, J. K. Bailey, and M. L. Mecartney, University of Minnesota. 

For example, a two-step processing method can be used in which the hydrolysis is performed under acidic condition and then a buffered solution containing the protein of interest is added to the hydrolyzed silica sol to initiate gelation under conditions that are protein compatible [121],... 

The sol-gel process for the preparation of silica-based monoliths involves heat treatment at lower temperatures for gelation and aging and the formation of mesopores by heating at high temperatures. 

These reactions are responsive to both acid and base catalysis, and can be manipulated to give a variety of silica products, e.g., discrete particles, monolithic gels, films, and fibers. This technique of materials synthesis via alkoxide hydrolysis has become known as sol-gel processing (17). It should be noted, however, that under certain conditions, gelation may be confined only to the interior of discrete particles (base-catalyzed systems), while the sol may consist of polymeric networks rather than individual particles (acid-catalyzed systems). 

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