Formation of silica sols

The most important property of silicate species is their polymerization state. For a solution containing silicate, low SiC>2 concentration can result in the formation of silica sol, while high SiC>2 concentration can lead to the formation of silica gel. 

Figure 4 shows the formation of silica sols, gels, and powders—a genealogical tree of colloidal silicas. 

The classic description of the structure and mechanisms of formation of silica sols by the hydrolysis and condensation of silicates in aqueous media was given by Iler in 1979 (I). According to Iler, polymerization may occur in essentially three stages (1) the polymerization of monomers to oligomers and then to primary particles, (2) growth of particles, and (3) particle aggregation to form networks that eventually give rise to a gel... 

At the ACS National Meeting, in Washington, D.C., 1990, Paul Yates gave a talk on the Kinetics of Gel Formation of Silica Sols [24]. He described that the gellation of silica sols is kinetically quite different from that of soluble silicates although the same factors are important, that is silica concentration, pH, salt content, temperature and particle size of the sols. Expressions were derived for the quantitative prediction of the gel times of colloidal silica dispersions over a wide range of these variables. The following expose is a summary of Yates presentation. 

Figure 3.1. Schematic representation of reactions taking place during formation of silica sol gels.

The first phase in the process is the formation of the sol . A sol is a colloidal suspension of solid particles in a liquid. Colloids are solid particles with diameters of 1-100 nm. After a certain period, the colloidal particles and condensed silica species link to form a gel - an interconnected, rigid network with pores of submicrometer dimensions and polymeric chains whose average length is greater than one micrometer. After the sol-gel transition, the solvent phase is removed from the interconnected pore network. If removed by conventional drying such as evaporation, so-called xerogels are obtained, if removed via supercritical evacuation, the product is an aerogel . 

The formation of a sol-gel functional silica gel takes place by hydrolytic polycondensation of suitable precursors in the presence of a dopant in solution. An example is given in the following (unbalanced) equation ... 

FIGURE 8.15 Protein encapsulation in a silica matrix via sol-gel processing the process include (a) formation of the sol particles during hydrolysis and initial condensation, (b) addition of the protein into the sol solution, and (c) the growing silica network entraps the protein molecule. (Reprinted from M. Kato et al. Anal. Chem., 74 1915 (2002). With permission. Copyright American Chemical Society 2002.)... 

Silicic Adds. The behavior of silicate ions in solution, the dependence of various properties on pn, the nature of silica sols and gels, and the study of hydrated silicas constitute chapters in inorganic and colloid chemistry that go far beyond the scope of this review. Germane to the present subject, however, are certain observations on the formation of monosilicic acid and its stepwise polymerization. 

The data obtained do not preclude the possibility of mutual coagulation of colloids of iron and silica for different quantitative relationships in the solution or for another mechanism of formation of mixed sols. Obviously in that case colloidal silica proper would occur in the solutions and its concentration would be higher than the equilibrium concentration. 

The texture of precipitated silica-alumina depends on the texture of the silica when silica results from a sol-gel transition. The condensation of silicic acids leads to the formation of primary spherical particles (sol) which aggregate in defined conditions, forming the tridimensional network of the gel [1]. In the gel framework each primary particle of silica is connected to two or three particles [1] and the gel pores are the cavities existing between these particles [2], The size of the particles, conjugated to their connectivity, defines the surface area, the volume and diameter of the gel pores. Thus, the silica texture could be controlled by mastering the size and the packing of the silica particles [3], characteristics which depend on the conditions of preparation of silica sol and gel. 

The formation of silica from a sol-gel transition is a low activation energy reaction. Thus, it was only expected a slight effect of the preparation temperature on the gel texture (silica-alumina as well as silica). However, as shown in table 5, the effect of preparation temperature on the texture of silica and silica-alumina hydrogels is important. 

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. 

In the present study, amorphous silica-alumina nanomaterials with controlled mesoporous distribution have been synthesized by two templateless approaches (1) vacuum-sol process, and (2) ultrasonic-sol process. It is found that the preparation method affects the precursor sol properties and the specific surface area and pore volume of the final materials. Ultrasonic-sol method favors the formation of monodispersed sol particles with narrow size distribution. Because of several base-exchange cycles and absence of drying process prior to heat treatment, the gel derived fiom ultrasonic-sol method may have enough stiffiiess to protect the network fiom pore collapse by capillary force, thus, leading to produce the materials with... 

Ramsay et al. Formation and Interfacial Structure of Silica Sols... 

Formation of Silica Gels Composed of Micrometer-Sized Particles by the Sol-Gel Method... 

The Stober route (8) is a well-known method for providing submicro-meter- or micrometer-sized silica particles by hydrolysis and condensation of silicon alkoxide an excess of base and water is used in the reaction. Compared with this method, ours has quite different reaction conditions, namely, the use of a limited amount of water and a large amount of acid. In contrast to the reaction of silicon alkoxide with a large amount of water in basic conditions, Sakka and Kamiya (9) noticed from the measurement of the intrinsic viscosity of silica sols that linear particles or polymers, not round particles, are formed with acidic conditions and the addition of a small amount of water. Therefore, the reaction conditions for this method for producing round micrometer-sized particles is new, and the mechanism of formation of round particles is of interest. 

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