Silica sols and gels

As condensation proceeds, small three-dimensional siloxane networks are gradually formed. The condensation reaction may be influenced by the addition of an electrolyte 

A more profound survey of the sol-gel process will be given below. 

NATURAL SILICAS Have a specific surface area, similar to their geometric surface. 

SYNTHETIC SILICAS (mostly amorphous) Surface area, pore volume, pore size and particle size are to some extent independently controllable (commercially interesting). 

Colloidal silica Silica sols Stable dispersions or sols of discrete particles of amorphous silica. 

---

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. 

Sodium silicate solutions are neutralized using sulphuric acid to obtain the Si(OH)4. The sodium silicate solution characteristics are dependent on the Si02 Na20 ratio. For production of precipitated silicas, sols and gels, a sodium silicate ratio 10 3 is usually employed rather than lower ratios, since less acid is required for neutralisation. Furthermore this ratio is available at low cost, since large quantities are produced. 

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. 

One of the most important developments in the 1980s in the search for novel concepts to characterize colloidal systems such as silica sols and gels was the advent of the fractal approach. Another very important development in the past 10 years was the application of 29Si CP MAS NMR methods to the study of the silica surface. This technique made it possible, for example, to identify without ambiguity the presence of silanediol groups on the silica surface (55). 

Raw materials for the production of precipitated forms of silica, sols and gels from solutions with ratios equal to 3.3 or greater. 

Commercial aqueous silica sols and gels [4] Base TMOS I121J Base NaBSi [121]... 

Saeki T, Ishida M (2011) Production of acid silica sols and gels by using a Y-shaped reactor and dilution technique. Int J Chem Reactor Eng 9 1-11... 

The synthesis of silica membranes has only recently been described. Silica forms sols and gels very easily both by the colloidal suspension and by the polymeric gel route. Its chemical resistance and its thermal stability in the presence of water vapor or metal impurities are not very good however. Larbot et al. (1989) have described the synthesis of silica membranes starting with a commercially available silica sol (Cecasol Sobret) in an aqueous solution at pH 8. 

The rate of silicate sol and gel formation is pH and water-alcohol-sensitive as is the solubility of the amorphous silica that is formed. Silica networks are based on (Si04) " tetrahedra modified by (O3 Si-O, M+) units and often addition of boron oxide, aluminum oxide, titanium IV oxide, or zirconium IV oxide. 

Silica is one of the most abundant chemical substances on earth. It can be both crystalline or amorphous. The crystalline forms of silica are quartz, cristobalite, and tridymite [51,52]. The amorphous forms, which are normally porous [149] are precipitated silica, silica gel, colloidal silica sols, and pyrogenic silica [150-156], According to the definition of the International Union of Pure and Applied Chemistry (IUPAC), porous materials can be classified as follows microporous materials are those with pore diameters from 3 to 20 A mesoporous materials are those that have pore diameters between 20 and 500 A and macroporous materials are those with pores bigger than 500 A [149],... 

One of the most exciting developments of the past decade in the study of colloidal silica is the application of the fractal approach to the study of sols and gels. Fractals are disordered systems for which disorder can be described in terms of nonintegral dimension. The concept of fractal geometry, developed by Mandelbrot (II) in the early 1980s, provides a means of quantitatively describing the average structure of certain random objects. The fractal dimension of an object of mass M and radius r is defined by the relation... 

The following fundamental aspects of the colloid chemistry of silica are briefly reviewed in this chapter nucleation, polymerization, and preparation stability of sols surface structure characterization methods sol-gel science gels and powders and uses of silica sols and powders. Silica in biology is not within the scope of this book. Scientists working in this area should soon put together a protocol covering progress done since the publication of Iler s book. 

Frolov, Shabanova, and co-workers (37-39) studied the transition of a sol into a gel and the aggregate stability of colloidal silica. Their aim was to develop a technology for the production of highly-concentrated silica sols and to use them as binders, catalyst supports, polymer fillers, adsorbents, and so forth. Kinetic studies were made of polycondensation and gel formation in aqueous solutions of silicic acids. At the stage of particle growth, poly condensation proceeds in the diffusion-kinetic region. With changes in pH, temperature, concentration, and the nature of electrolytes,... 

Zeolite P-B (B or P Structure Type). Zeolite P-B was crystallized from coprecipitated gels using NaOH solution as the titrant and Ludox colloidal silica sol. The gel was crystallized at 150°-200°C for 45-72 hours. Phosphorus contents up to 23 wt % P203 were achieved in P-B zeolites. 

At pH less than 7 or 8, silica sols are not stable. Agglomerates of colloidal particles form and the silica sol can gel. This effect grows more pronounced as the concentration of the sol increases. In very dilute sols, low pH can be tolerated but in general should be avoided. 

Aluminosilicate gels are prepared by the mixing of reactive sources of alumina and silica. These include fumed silica, silica sols and alkoxides of silica, and aluminium, aluminates, aluminium salts and alkoxides of aluminium. Other synthetic routes have been explored that use structured aluminosilicate precursors, such as zeolites themselves, to supply aluminate and silicate species to... 

Figure 4.15. Evaporating film of silica sol to gel and drying schematic cross-section, (a) sol (A) concentrated sol—beginning of aggregation (c) gel compressed by surface tension (J) fracturing of gel by shrinkage (e) dried loose gel fragments. W, water surface S. solid substrate.

---