Silica colloid gels from

One difference between gels made from stabilized colloidal silica and gels from silicic acid is that the commercial colloids are made in hot, slightly alkaline conditions that remove water and micropores from the interior of the particles. On the other hand, the particles made by polymerization of silicic acid, usually below pH 7 and at less than 80 C, and often in the presence of sodium salts, may be microporous. This point has been discussed by Vysotskii et al. (221), who suggest that the pores may be accessible to water but not large molecules. 

Properties. Silica gel (see Fig. 8) is a coherent, rigid, continuous three-dimensional network of spherical particles of colloidal silica. Both siloxane, — Si— O—Si—, and silanol, — Si— O—H, bonds are present in the gel structure. The pores are interconnected and filled with water and/or alcohol from the hydrolysis and condensation reactions (40). A hydrogel is a gel in which the pores are filled with water. A xerogel is a gel from which the liquid medium... 

Antireflective (AR) coatings are required on optics to reduce the reflective surface losses. Vitreous silica coatings in the form of porous or multilayer films are used extensively in this application. Antireflective coatings have been developed which employ colloidal fused silica sol—gel particles made from organometallic materials (253). 

Colloidal metal NCs can be directly introduced to the sol precursors to fashion metal NC-sol-gel composites. Silica glasses fabricated from TMOS with Au NCs as the dopant and ammonia as the catalyst contained NCs with diameters... 

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. 

Si-VPI-5(2) was prepared according to a synthesis procedure reported by Davis et al. [71 using pseudobeuhmite (Vista), orthophosphoric acid (85%, from Janssen Chimica), colloidal silica (AS-40 from DuPont)) and dipropylamine (DPA) (Janssen Chimica) as reagents. The gel composition was ... 

J. Turkevich (Princeton University, Princeton, N. J. 08540) In connection with Figure 3, I would like to know how the silicon-aluminum ratio is determined. The synthetic zeolites and maybe natural ones are obtained from silica-rich gels which increase in silica-alumina ratio in the mother liquid as the silicon-aluminum ratio increases in the crystals. Silica may be occluded and adsorbed as a colloidal gel on the zeolite and difficult to wash off. The silica ratio as measured will be higher and the lines asymetrically lower. 

Silica-supported various metals (M) were tested as the catalyst for the oxidation of methanol in the presence of MP. Dry gel of silica was prepared from colloidal silica and it was impregnated with an solution containing metalic components (M). The M/Si atomic ratio was 10/1000. The catalysts were then calcined at 400 °C for 4 h. Over the 2.0 g portion of each catalyst, a mixed gas consisting of MP, methanol, oxygen, and nitrogen was passed at 360 °C with feed rates of MP/methanol/oxygen/nitrogen = 50/75/10/250 mmol/h. The results obtained in the oxidation of methanol are listed in Table 2. 

In 1853, a French researcher named Fremy [7] prepared a dilute colloidal silica from silicon sulfide. By 1864, silica colloids were being prepared by the dialysis of gels and by the hydrolysis of silicate esters [8]. All products were very dilute. 

However, Kondo [3 ] is of the opinion that the hydrolysis of the glass in water occurs and the calcium ions are released initially to the liquid phase. Simultaneously on the surface of slag grains an acid, colloidal shell of silica-alumina gel is formed. This shell has low permeability and hence the further slag reaction with water is hindered. In the presence of Ca(OH)2, added as alkaline activator, the siUcon and aluminum from the shell are released to the solution (Fig. 8.2). The solubiUty of aluminum compounds becomes considerably increased in the solution of pH higher than 12.5 because in this condition the Al(OH) ions are formed [4]. The concentration of aluminum in the liquid phase is increasing because the calcium aluminates crystallize a httle later, primarily the C-S-H (1) is formed. Simultaneously the solubility of hydrates formed in this condition is reduced. 

Figure 16.11. Photographs of a silica wet gel immersed in a colloidal suspension of gold nanoparticles at increasing times A-C. and of the Au-Si02 composite aerogel D. (reproduced from [57] by permission of Springer).

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