Silica stability

Colloidal silica as it is manufactured is almost always stabilized with sodium hydroxide. The purpose of the alkali is to react with a fraction of the weak acid —OH groups on the particle surface and form water and leave behind a charge site  

The symbols represent the relative number of hydroxyl units present at each pH 

Removing hydroxyl ions decreases the pH, and the charge density decreases as the number of surface silanol groups which are ionized, decreases. The gel rate increases with the decrease in charge density. With the decrease in charge density, there is less repulsion between particles 

Hydroxide ions have reacted with three acid sites producing water and leaving behind three negative sites which are counter-balanced by the three sodium ions originally associated with the hydroxide ions 

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Increasing the molecular weight of a copolymer containing 5% methyl acrylate (MA) from 100,000 to 1,000,000 daltons had little effect on silica stabilization effectiveness (see Table IX). Increasing the methyl acrylate content from 5% to 30% had also little effect on silica fines stabilization effectiveness. Acidizing substantially reduced the effectiveness of this class of copolymer. Results for the injection of 10,000 pore volumes of water indicated that silica fines elution from the test column was substantially reduced on a long-term basis. 

ICI Co. uses a sol-gel method to produce silica-stabilized alumina (Saffil) and calcia-stabilized zirconia fibre [15], The saffil fibre is a 8-alumina short staple fibre that has about 4% Si02 and a very fine diameter (3 pm). 

Leitner et al. have synthesized the PEG-modified silica stabilized and immobilized palladium nanoparticles for aerobic alcohol oxidation in combination with scC02 as reaction medium under mild conditions, which show high activity and excellent stability under continuous-flow operation [68], ScC02 could diffuse the substrates and products from the active nanoparticles in a gas-like manner. This allows rapid chemical transformation at the active center, ensures efficient removal of the products from the surface, and minimizes the mobility of solid-supported catalytically active species [69]. In this way, catalysts based on palladium nanoparticles together with PEG as stabilizing matrix could avoid aggregating and forming less active and selective Pd-black [20, 60, 70]. 

To produce Fe(oxide)/Si02 particles one could use a solution of Fe(II), in which case air has to be excluded to prevent its oxidation to Fe(III). Iron (II) starts to react markedly above pH = 4.8 (urea, 90°C). In this case, the reaction is not limited to Eqn. 9.14, but a bulk hydrosilicate is formed. Upon performing an injection experiment at 45°C, it is observed that the reaction with the support is less extensive at that temperature. As the slightly higher pH at the injection point brings about the formation of a less reactive iron species, attack of the support is less marked than in the urea case even at 90°C. The structures obtained in the three different experiments are indicated in Fig. 9.11. The different extents of hydrosilicate formation are reflected in the temperature-programmed reduction (TPR) experiments, as can be seen in Fig. 9.11. Previous air-drying partially oxidizes the Fe(II). As interaction with silica stabilizes Fe(II), the supported Fe samples show a separate reduction step to Fe(II), which is not displayed by bulk Fe oxide. The iron hydrosilicate obtained in urea precipitation at 90°C is fairly stable and is reduced only above 650°C. The Fe(II) precipitated at 45°C is more... 

We will next consider the case of a lew silica content co-gel. A 5% silica-content silica-alumina was prepared by precipitation of aluminum isdsutoxide and tetraethoxv-silane as described for the silica-free gel. After gelation water was added just sufficient to fill the pore voids of the gel. The added water led to formation of a boehmite-rich hase during recrystallization. After drying at 120 and calcination at 500 0 for 16 hours, a transitional alumina hase is formed with a surface area of 410 m /g and a pore volume of 1.9 oc/g. This silica-alumina had an average pore diameter of 18 nm, similar to the silica-free material discussed previously. Steam treatment of this 18 nm pore diameter silica-alumina at 870°C (1600 ) in 90% H20-10% N2 for 16 hours resulted in a material with surface area of 196 m /g. This surface area is much hi er than expected for an amori ous gel and is consistent with silica enrichment of the outer surface during the recrystallization step vhere water was added to the pores of the amoridious gel. Silica stabilization of bodunite alumina by formation of a surface Aiase complex has been reported in recent work (9). ESCA analysis also indicates silica surface enrichment vhen compared to the amori ous gel. 

Homola, A.M., and Lorenz, M.R., Novel magnetic dispersions using silica stabilized particles, IEEE Trans. Magn., 22, 716, 1986. 

FIGURE 2.6. Dependence of silica stability on the radius of curvature of particles. After Dove and Rimstidt. ... 

A detailed description of a chromia-on-alumina catalyst prepared by impregnation has been given elsewhere . Another supported nonmetallic catalyst widely used commercially is cobalt molybdate-on-alumina. The preparation of this catalyst using an alumina support with controlled pore-size distribution is as follows. Silica-stabilized alumina, with greater than 50% of its surface area in 3-8 nm pores and at least 3% of the total pore volume in pores greater than 200 nm in diameter, is impregnated with an aqueous solution of cobalt and molybdenum. The finished oxysulfide catalyst was tested for hydrodesulfurization of petroleum residuum at 370°C and 100 atm for 28 days and compared with a convential cobalt-molybdate catalyst having a major portion of the surface area in 3-7 nm pores. The latter catalyst and controlled pore catalyst maintained 57 and 80% activity, respectively. 

Antislip 239. [Piedrnont Chem. Industries] Colloidal silica stabilizer for yarns and frd>rk preventing fiber shifting and seam slippi e. 

Chem. Descrip. Colloidal silica, stabilized EINECS/ELINCS 231-545-4... 

Chemical analysis. Silica, stabilizing base, carbon including carbon dioxide, organic carbon, soluble salts of alkali metals, total solids and non-siliceous ash, metals including aluminum and iron. 

Inorganic paint for asbestos board can be made with silica as a binder. Hard, weather-resistant paint is made with a combination of colloidal silica, lithium hydroxide and potassium silicate or alkali metal phosphate, clay, and pigment, and baked on in the presence of steam (629). Collidal silica stabilized with tetraethanol ammonium silicate was used as a binder for iron oxide and clay pigments (630). A heatproof coating for asbestos or metals consisted of colloidal silica and magnesium hydrogen phosphate (631) and was hardened at 200 C. 

Particularly for microanalytical investigations, it is favorable to adsorb 1 to 10 pL of liquid or a few milligrams of a material dissolved in the same volume of water or another suitable solvent on thin-layers (0.2 mm) of alumina or silica stabilized on sheets of polystyrene, polyethylene, or aluminum [35]. 

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