Polymerization of silicic acid

The reaction results in the formation of a Si O—Si siloxane bond in which two silicon atoms are bridged by oxygen. The remaining three Si—OH sylanol groups can also be involved in condensation reactions. This will lead to the polymerization of silicic acid with the formation of branched inorganic polymers. 

A single technique that preserves all three nutrients (N03, P04, and Si(OH)4) in a single matrix would be most desirable. If this proves impossible, it will be necessary to use different approaches to preserve and store different nutrients. For example, glass containers cannot be used to store a reference material for Si(OH)4 due to the slow dissolution of silica (Zhang et al., 1999), while the polymerization of silicic acid upon freezing eliminates that option for preserving stable Si(OH)4 levels (Zhang and Ortner, 1998). 

Opal (or opaline silica) An amorphous silicate formed through the polymerization of silicic acid molecules. Though most is biogenic in origin, some forms as a result of diagenesis. 

Monosilicic acid is stable in aqueous solution only at low pH and very low concentration (Her, 1979, p. 209). The rate of condensation-polymerization of silicic acid is dependent on pH, concentration and temperature. At a certain stage, the polysilicic acid sol is converted into either a precipitate (i.e. a flocculated system) or a hydrogel. 

The sorption of silicic acid (added 60 hours before arsenic) also decreased the rate and the total amount of arsenic sorbed (Waltham and Eick, 2002 Table 5.4). The amount of As(lll) sorbed decreased as the surface concentration of silicic acid increased. Furthermore, the inhibition of arsenite sorbed ranged from about 4% at a pH of 6 and 0.1 mM silicic acid up to 40% at a pH of 8 and 1 mol silicic acid. In all experiments except at a pH value of 8 and 1 mM silicic acid, the molar ratio of As(lll) sorbed to sihcic acid desorbed was greater than 1 (Table 5.4), indicating a greater quantity of arsenite sorbed compared with silicic acid desorbed. In contrast, silicic acid reduced the rate of As(V) sorption, which decreased by increasing pH and silicic acid concentration, but the total quantity of As(V) sorbed remained nearly constant, indicating that arsenate was able to replace silicate. Swendlund and Webster (1999) observed a reduction in As(V) sorption onto ferrihydrite at pH > 6 and attributed it to the polymerization of silicic acid. 

Scheme I. Polymerization of silicic acid by dehydrating condensation.

Strelko and co-workers (71-73) investigated the properties of the siloxane bond and the polymerization mechanism for silicic acid on the basis of the difference in the electronic structure of the Si-O bonds in silanol and the siloxane groups of silica. A kinetic equation was proposed to describe the polymerization of silicic acid throughout the entire pH range. The authors believe that the molecular mechanisms for the formation of globular skeletons in silica gels are based on the polymerization and depolymerization of silicic acids. 

T. Yokoyama and T. Tarutani, Flow Injection Analysis of Silicic Acid Application to Study of the Polymerization of Silicic Acid [in Japanese]. J. Flow Injection Anal., 2(1) (1985) 30. 

PJ Swedlund, JG Webster. Adsorption and polymerization of silicic acid on ferrihy-drite and its effect on arsenic adsorption. Water Res 33 3414-3422, 1999. Methods for Evaluating Solid Wastes. SW-846, U.S. Environmental Protection Agency, Cincinnati, OH, 1997. 

In the freshly made solutions the state of polymerization of silicic acid corresponds to that in the silicate used and led to the following conclusions ... 

There are three types of silica gels, of which the first results from neutralizing dilute aqueous solutions of a silicate and is formed by the polymerization of silicic acid. 

An important part of the success of the use of a 3-body term is related to its stiffness, or how quickly the 3-body term rises from zero below the cut-off distance, r°. If this is too stiff (3-body repulsion rises too quickly), then non-equilibrium structures would be inhibited and possible reaction states would be precluded from forming in the simulations. An example of this is the formation of the 5-coordinated Si that is believed to form during the reaction of a water molecule with silica or the polymerization of silicic acid (H4Si04) molecules. 

Garofahni SH, Martin G (1994) Molecular simulations of the polymerization of silicic acid molecules and network formatioa J Phys Chem 98 1311-1316... 

Monosilicic acid polymerizes in neutral or acidic solution, with a characteristic rate depending the pH [ref. 118-120]. Tarutani [ref. 118] suggested that exclusion chromatography is useful for studying the polymerization of silicic acid. The mechanism of the growth of the polymer particles was discussed on the basis of changes in the elution curves for polysilicic acids with time [ref. 118-121]. The polymerization of silicic acid is slowest at pH 2 in aqueous media [ref. 118, 119]. The eluent adjusted to pH 2 was used throughout the experiments. 

The formation of allophane clays and silica gels seems however to be different a leaching mechanism for the allophane and the polymerization of silicic acid for the silica gels. However, in both cases, the network is obtained by the aggregation of colloidal... 

In order to explain the mechanism of the polymerization of silicic acid, we would have to start with the fact that the tetravalent silicon is still unsaturated coordinaiively. Both of the strongly defined secondary valences which are active in the fluoride complexes of silicon must also play a role in the hydrated oxfde. 

The foregoing description of the polymerization of silicic acid was based on a uniform interpretation of the data of about 20 investigators who studied polymerization under different coiiditipns by different methods and interpreted their results in different ways. Froth the following separate discussions of each investigation it will be seen that the data are consistent with the view that at an early stage in the polymerization, the silicic acid is converted to spheroidal particles about 1-2 nm (10-20 A) in diameter. The data of several earlier investigators confirm the most recent work that... 

Table 3.9. Polymerization of Silicic Acid at pH 1-5—Oligomer Species...

Goto (157) was one of the first to examine the polymerization of silicic acid in the absence of salts and in the pH range 7-10, where colloidal particles are formed and no aggregation or gelling can occur as at lower pH. He used the equation... 

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. 

It had been otherwise reported (193b) that four times the normal amount of aluminum is present ih the nuclei of neurons in such diseased brains but the increase in silicon now appears to be far more significant. The aluminum may simply be adsorbed on the silica in view of the great affinity between silica and alumina. The highly localized silicon in plaques may only be the result of polymerization of silicic acid released from the debris of the deteriorating surrounding tissues in which a very low concentration of silicon may be bound when the brain is in the normal state (see following section). 

In our laboratory, we have recently conducted gelation studies of silica nanopardcles in microgravity during the STS-95 space shuttle mission (28). Stable silica nanoparticle dispersions may be form either by polymerization of silicic acids in an aqueous system or through hydrolysis and condensation of silicon alkoxides (the sol-gel or Stober route). Comparison of small-angle x-ray scattering (SAXS) measurements of Ludox, a commercial aqueous silicate with acid- and base-catalyzed alkoxides shows that only aqueous silicate sols are uniform, whereas alkoxides generate fractal particles. As Brinker and Scherer point out (29), these results illustrate that sols derived from aqueous silicates are... 

Greenberg, S.A., Sinclair, D., 1955. The polymerization of silicic acid. J. Phys. Chem. 59,... 

Silica, in its various forms, can cause serious problems for an RO system. Silica scaling and fouling can occur via a number of ways and is not well understood due to the number of different mechanisms that can take place. The concentration of silica, the speciation of silica, and temperature, pH, and the general chemistry of the water all affect the chemistry of silica scaling and fouling. In general, the silica issues that affect RO systems can be summarized as deposition of silicates, polymerization of silicic acid to amorphous silica, and the accumulation of amorphous colloidal particles. To understand the potential problems, it is first necessary to understand a little about the chemistry of silica. 

Tarutani, Toshikazu, Polymerization of Silicic Acid, a Review, Analytical Sciences, 5, 245-252 (1989). 

The rate of polymerization of silicic acid produced by the hydrolysis of tetramethoxysilane (followed by silicon-29 nmr) has turned out to be slow. Indeed at pH 3.5. 0 monosilicic acid may still be detected after several weeks. There is no direct relationship between disappearance of monomeric silicic acid and production of silica gel. ... 

Inorganic microparticles Silica particles Polymerization of silicic acids by interfacial reaction Nakashima (1991a)... 

Amorphous silica (natural and synthetic) may exist in several different forms including silica gels, nonporous precipitates, and hydrogenic silicas. These can all be regarded as polycondensation products of orthosilicic acid. The methods of preparation are either by acid precipitation from silicate solutions, or by the hydrolysis of silicon derivatives, silicon tetra-halides, or alkoxides (Mitchell [1966]). To follow the development of the properties of silica gel, it is necessary to consider polymerization of silicic acid from the sol stage. 

Baumann, H., 1958. Polymerization of silicic acid. Wiss. Forschungsber., Naturw. Reihe 66 99. Bazilevich, N. I., R. A. Kalashnikova, and E. A. Yarilova, 1954. The accumulation of amorphous silicic acid in soils. Trudy-pochv. Inst. Dokuchaeva 44 262. 

Kitahara, S., 1960. Polymerization of silicic acid obtained by the hydrothermal treatment of quartz and the solubility of amorphous silica. Rev. Phys. Chem. (Japan) 30 131. 

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