Silica sols stability

The industrial development of silica sol manufacturing methods is reviewed. Primary attention is focused on the preparation of monodispersed sols from water glass by the ion-exchange method. Details are given for variations of manufacturing process and for the characteristics of both the processes and sols obtained. Furthermore, the following surface modifications of particles are demonstrated silica sols stabilized with ammonia, amine, and quaternary ammonium hydroxide aluminum-modified or cation-coated silica sol and lithium silicate. Finally, future trends in silica sol manufacturing are discussed from the viewpoint of not only raw materials and improvement of the procedures but also the function of the silica sols and their particle shape. 

Figure 6. Flow chart of production of silica sols stabilized with ammonia, amine, or quaternary ammonium hydroxide.

Silica Sols Stabilized with Ammonia, Amine,... 

Silica sols are often called colloidal silicas, although other amorphous forms also exhibit colloidal properties owing to high surface areas. Sols are stable dispersions of amorphous siUca particles in a Hquid, almost always water. Commercial products contain siUca particles having diameters of about 3—100 nm, specific surface areas of 50—270 m /g, and siUca contents of 15—50 wt %. These contain small (<1 wt%) amounts of stabilizers, most commonly sodium ions. The discrete particles are prevented from aggregating by mutually repulsive negative charges. 

The introduction of 2-[4-(dimethylamino)phenylazo]benzoic acid into a silica sol allows the preparation of pH-sensitive doped coatings upon glass substrates. The behavior of this system was evaluated as the function of pH changes in liquid and gas media68. Optical absorption and sensitivity against pH were monitored by Vis spectroscopy. Chemical and mechanical stability tests carried out with coatings demonstrated that they were resistant enough to be use in sensor devices for pH measurements in laboratories. 

The key to the successful preparation of this new composite is the identification of a surfactant, PE-b-PEG, that is capable of stabilizing the emulsion and promoting the dissolution of the PE. Then submicrometre particles of low-density PE silica and high-density PE silica are synthesized by carrying out a silica sol-gel polycondensation process within emulsion droplets of TEOS-dissolved PE, at elevated temperatures (78 and 130°C for low- and high-density PE, respectively). 

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. 

Stabilization of activated oxidoreductases on time scales of months to years has historically been challenging, and the lack of success in this regard has limited the industrial implementation of redox enzymes to applications that do not require long lifetimes. However, as mentioned in the Introduction, some possibility of improved stability has arisen from immobilization of enzymes in hydrophilic cages formed by silica sol—gels and aerogels, primarily for sensor applications.The tradeoff of this approach is expected to be a lowering of current density because... 

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. 

To study the effect of pH on the stability of undialyzed silica sols, solutions of these initial concentrations were used 300, 750, 1500, and 3000 mg/1 SiOj. The observations were made over several months, starting from the moment the sol was prepared. 

In an experimental study of mixed iron-silica colloids we found the opposite phenomenon, stabilization of colloids of iron hydroxide by silica colloids, which is manifested very clearly in undialyzed iron hydroxide sols of average concentration with a ratio of Fe203 SiOj = 1 3. Colloidal iron in mixed solutions proved to be more resistant to the action of electrolytes than in isolated sols of iron hydroxide. Only colloidal silica shows any stabilizing effect on sols of iron hydroxide dilute solutions undersaturated with Si(OH)4 are not stabilizers. It is characteristic that colloidal silica is capable of stabilizing colloidal iron in the same pH ranges in which pure silica sols are stable in acid (pH < 4) and alkaline (pH > 8) environments. In slightly acid environments (pH = 5-6) iron-silica sols are unstable and decompose to form mixed sediments, which sometimes are not uniform due to different rates of coagulation and deposition. 

DN-550TS = a sample prepared from Disperal type aluminum hydroxide kneaded with 1% water solution of nitric acid, calcined 4 h at 550 C, coated with TEOS and coated with silica sol. We have selected concentrations of TEOS, silica sol and cerium nitrate at the level allowing obtaining ca 1.8 wt % of the individual stabilizers in final alumina material. 

The additional impregnation with silica sol of alumina earlier stabilised by TEOS does not change noticeably good hydrothermal stability of alumina. 

A higher electrokinetic potential leads to a higher stability of silica sol. In general, the stability of acidic silica sol is better than that of basic silica sol. 

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. 

Bismuth cerium molybdates were prepared by coprecipitation using aqueous solutions of (NH ) Mo 02, (NH,)2Ce(N0 ), and Bi(NO ) 5H2O. The catalysts were supported on oiO (20% by weight) using an ammonium stabilized silica sol. Samples for diffraction analysis were unsupported. Samples were calcined in air at 290 and 425°C for three hours each followed by 16 hours at 500, 550, or 600 C. X-ray powder patterns were obtained using a Rlgaku D/Max-IIA X-ray diffractometer using Cu K radiation. 

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