Silica condensation

D. retaining capacity of the mix. These include very fine clays (bentonites), pyrogenic silicas, condensed silica fiune, milled asbestos and other fibrous materials. 

Keefer Structure and Growth of Silica Condensation Polymers... 

Keywords silica condensation, polyamines, biomineralization, silica... 

In contrast to extracts obtained from nature, which usually consist of a mixture of substances and are available only in very limited amounts, synthetic model systems have the advantages of controlled purity and good availability. Such systems enable the scientist to vary reaction parameters in a systematic fashion and to adjust individual functions to analyze their influence on the system. However, model systems should be as close as possible to the natural system. Therefore, we have undertaken a study to elucidate the chemical behavior of polyamine-silica systems, including not only polyamines which can be purchased, but also polyamines specifically synthesized, with structures as close as possible to the naturally occurring ones [11], Here, we report on investigations on the kinetics of silica condensation in the presence of various polyamines... 

In the acidic route (with pH < 2), both kinetic and thermodynamic controlling factors need to be considered. First, the acid catalysis speeds up the hydrolysis of silicon alkoxides. Second, the silica species in solution are positively charged as =SiOH2 (denoted as I+). Finally, the siloxane bond condensation rate is kinetically promoted near the micelle surface. The surfactant (S+)-silica interaction in S+X 11 is mediated by the counterion X-. The micelle-counterion interaction is in thermodynamic equilibrium. Thus the factors involved in determining the total rate of nanostructure formation are the counterion adsorption equilibrium of X on the micellar surface, surface-enhanced concentration of I+, and proton-catalysed silica condensation near the micellar surface. From consideration of the surfactant, the surfactants first form micelles as a combination of the S+X assemblies, which then form a liquid crystal with molecular silicate species, and finally the mesoporous material is formed through inorganic polymerization and condensation of the silicate species. In the S+X I+ model, the surfactant-to-counteranion... 

Once those parameters which have more influence on mesophase stability are known, the new synthetic pathways utilizing structural rearrangements to create new materials will become a reality. An investigation of phase transformation11581 under hydrothermal conditions indicated the most mildly basic conditions utilized (pH 9), which favor silica condensation, best inhibit the phase transformation, and thus produce the most kinetically... 

Films are typically allowed to dry immediately after deposition to drive the silica condensation reaction toward completion. However, the film may not be fully condensed and stable after drying, and postsynthesis treatments are often employed to improve film stability, reduce shrinkage upon template removal, and drive the condensation reaction toward completion. 

Figure 3. Illustration of the pH dependence of the hydrolysis, condensation, and dissolution rates of silica. Condensation rates are judged by the reciprocal of gel times. RLCA denotes reaction-limited cluster aggregation. (Reproduced with permission from reference 30. Copyright 1988.)...

Synthesis of pure and iron-containing mesoporous silicas are carried out from CTMA mixtures containing different salts and hydrochloric acid. Ferric salts decrease the pH of the synthesis batch, except when complexed by fluoride ions. Samples obtained from neutral batches in the presence of fluoride are stable when washed and calcined. For syntheses at acidic pH and low temperature, a low degree of silica condensation is responsible for thin unstable silica walls. When surfactant molecules are extracted by washing or calcination, the porosity shrinks and disordered microporous solids are formed from ordered mesophases with 40 A micelles. 

Hollow silica tubes 0.3 to 3 pm in diameter and exhibiting mesopore channels along the tubular axis were described in 1996 (284). Such an organization is produced by careful control of the surfactant to water ratio and the rate of silica condensation at elevated pH. However, more typical sol-gel methods used for preparing silica gel fibers by... 

The silica condensation reaction can be done under acidic, alkaline or neutral conditions and all three have been used to synthesise mesoporous materials. Under neutral conditions often the surfactant itself acts as a catalyst for silica condensation as formation of templated materials occurs much more quickly than precipitation or gel formation in silica-only solutions at the same Under neutral conditions addition of F, which... 

The use of UV/ozone treatment to break down the surfactant at room temperature in air has also been shown to completely remove surfactant from the pores both for mesoporous powders, and for thin films. Silica condensation is promoted by the UV light, simultaneously with surfactant removal. UV light in conjunction with dilute hydrogen... 

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