Aqueous silica sols

Healy, T. W. 1994. Stability of aqueous silica sols. In Bergna, H. E. (ed) The Colloid Chemistry of Silica, Advances in Chemistry Series 234. American Chemical Society, Washington, 147-159. 

Si02 gel DDAB/toluene/water (48.7% DDAB, 19.5% decane, 31.8% aqueous silica sol) TMOS (partially hydroiyzed)/H20 (+ 0.4-10 wt%HF) Bicontinuous p,Es used as templates for microporous silica gels monodisperse pores (2 nm pore radius) large specific surface area (—103 m2/g) (51)... 

Healy, T.W., Stability of aqueous silica sols, Adv. Chem. Ser., 234, 147, 1994. Brown, G.T. and Darwent, J.R., Zeta potential and interfacial electron transfer in colloidal TiO, 7. Chem. Soc. Chem. Commun., 98, 1985. 

The coagulation-dispersion behavior of aqueous silica sols is central to almost all processes requiring their unique adsorption, dispersion, gelation, and sol-gel properties. Aqueous silica sols are of particular interest in colloid science because their coagulation-dispersion behavior is said to be anomalous , that is, their stability in terms of electrolyte-pH control does not follow the pattern followed by almost all other oxide and latex colloidal materials. This chapter examines aqueous silica sol coagulation effects in light of studies of macroscopic silica-water interfaces and in particular the electrical double layer at such interfaces. 

It is difficult to define precisely the term aqueous silica sols and thereby contrast them with other forms of silica (colloidal silica, colloidal quartz, pyrogenic silica, and so forth). Bulk chemical distinctions are not very useful. The definition chosen here follows Iler s terminology (I). Aqueous silica sols are characteristically composed of spherical particles nucleated and grown by alkaline hydrolysis of sodium silicate solutions. They are often monodisperse systems and have particle diameters in the range 1-100 nm (density, —2.2 g/cm3) that lead to sols that vary from optically transparent to opalescent. 

The colloidal behavior referred to in the preceding text as anomalous helps to further define the term aqueous silica sols . Iler (2) noted,... 

Kolosentsev and Belotserkovsky (59-61) used the ion-exchange methods proposed by Bird (68) and Bechtold and Snyder (69) in preparing stable aqueous silica sols containing —50 wt% Si02, with particle size ranging from 65 to 380 nm. 

Considerable progress achieved in the low-temperature synthesis of zeolites is connected with the use of highly reactive materials in the colloid-dispersed state (aluminosilica gels, aqueous silica sols, silica, alumina gels, etc.). The use of these materials in the majority of cases facilitates synthesis, but at the same time it involves certain difficulties. In many such cases, not only the kinetics of the process but also the actual final results appear to be dependent on the complicated chemical behavior of these materials during their preparation, in the course of... 

Small samples of these hybrid resins can be prepared by simply mixing the alkoxysilane and aqueous silica sol using the water present in the sols as the hydrolysis medium. We have found that maintaining the pH on the acid side results in an adequate hydrolysis rate (lA) without an accompanying acceleration of silanol condensation (IB) that could result in the formation of highly crosslinked resin and gel particles. 

Concentrated aqueous silica sols of small silica particles about 10 nm or less in diameter and uniformly sized are prepared by a process for maintaining a constant number of silica nuclei particles prior to the deposition process. The process for maintaining a constant number of silica nuclei... 

Amorphous sUica-sodium sUicate-polyvinyl acetate of Si02/Na20 molar ratio 5 1 formed directly on the sand by addition of a uniform, stable mixture of aqueous silica sol of uniform particle diameter about 12 nm to the sand, mixing and then adding sodium silicate Si02/ Na20 molar ratio 3.25 1 and mixing for 2 min. 

Figure 4.1. Maximum concentration versus particle size in stable aqueous silica sols at about pH 9.S. A, concentration (wt. %) B, concenlration (grams SiOi per 100 ml) C, volume fraction of SiOi (xlOO).

Silica does not conform to the DLVO theory because it is apparently stabilized by a layer of adsorbed water that prevents coagulation even at the lEP. This form of stabilization is possible because of the unusually small Hamaker constant of silica. To destabilize an aqueous silica sol, it is necessary to reduce the degree of hydration. Allen and Matijevic [25] showed that adding salt to the sol would produce ion exchange,... 

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