Silicic acid polymerization rate

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. 

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 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. ... 

Figure 9 gives some results obtained with vitreous silica. Three runs were made with 5, 15, and 45 sq. meters of total surface area in suspension. Apparently, the data indicate again the approach toward the two-phase equilibrium for polymeric silicic acid—i.e., independent of the total amount of solid material, the equilibrium concentration of about 110 //grams Si02/ml., as indicated in Figure 1, is approached in all three cases. Only the rate of dissolution is different, being determined by the size of the exposed surface area. 

These cements are formed by a similar process to the silicate minerals described in Chapter 1, the difference being the rate. Silicate minerals are formed at a rate lower by orders of magnitude compared with dental cements. In the case of dental cements, the phosphoric acid releases protons in the solution and lowers its pH. This decomposes the glass and releases silicon in the solution, and silicic acid forms as an intermediate product [26,27]. Simultaneously, cations such as Ap, Ca " ", and Na" " and the anion F are also released [28]. The cations and anions are attracted to each other, and neutral bonding phases form. Such a bonding network, especially that of aluminum, results in gelation and subsequent polymerization of a hard product. 

The rate of silica dissolution is proportional to its surface area. Silicic acid being passed into solution undergoes polymerization giving water and the molecules of polysilicic acids of variable complexity. At pH... 

Considering FA and TFS as consolidation additives, it should be noted that penetration of the SPC compositions modified by FA strongly depends on concentration of an aggressive environment. The polymerization rate of FA in the silicate media under the action of aggressive environments depends on pH, especially in the first reaction stages. Corrosion of silica by hydrofluoric acid occurs rather quickly and leads to loosening of the SPC composition and an increase of diffusive penetration. Consequently, application of the consolidation additive FA is desirable at pH < 5. 

Extensive research on sol—gel processing of silicic acid esters, eg, tetraethoxysilane (TEOS), Si(OC2H5)4, in alcohol—water mixtures has ducidated silica polymerization in nonaqueous solvents (54,55). The relative rates of hydrolysis and condensation depend on the substrate, water, and catalyst (acid or base) concentrations these rates determine the polymer structure. For example, acid-catalyzed hydrolysis of TEOS at low water concentrations produces linear polymers. These solutions yield fibers upon spinning. Conversely, high H20 /TEOS ratios favor the formation of highly cross-linked polymers under... 

Thilo (25) classified oxo acids of various elements into three groups on the basis of the stability of the oxo acid in aqueous solutions, as shown in Table I. Although Thilo placed the elements in order of their ionic radii, covalent radii of the elements are used in Table I. The most stable phosphate in aqueous solutions is the monomer, orthophosphate. However, isopoly acid anions of phosphorus, i.e., condensed phosphates, are generally stable in an approximately neutral aqueous solution at room temperature. The rate of hydrolysis of the condensed phosphates is very low under these conditions. On the other hand, polysulfates and polyarsenates are very rapidly hydrolyzed into their monomers in aqueous solutions. In an alkaline solution, vanadate anions are present as monomer, i.e., orthovanadate. When the pH of the solution is de- creased, the orthovanadate anions are successively polymerized to form polymers with medium degrees of polymerization. Although silicate anions behave similarly, a highly polymerized form of silicate anions,... 

Fig. 14 shows the elution curves for silicic acid in solutions of pH 9.5 on a Sephadex G-lOO column. The peaks on the right are due to monosilicic acid and those on the left to polysilicic acid. The elution curves for the polysilicic acids indicate a symmetrical distribution of particle sizes, and the elution volume of the polymers obtained after standing the sample solution for 250 h is almost the same as that obtained after 100 h. This suggests that when the concentration of monosilicic acid is close to the solubility of amorphous silica, the growth rate of the particles of the polymers becomes very low. It was thus assumed that the growth of the particles is mainly due to the polymerization between monomer and polymer, and the polymerization between polymer species... 

The reaction of silica with catechol, pyrocatechol, and 2,3-naphthalenediol has been studied by several investigators (158-162), but Bartels and Erlenmeyer appear to have been the only ones to use this reaction to characterize the rate of depolymerization of silica (163a). For example, monomeric silicic acid from ethyl silicate in a standard solution of catechol in 0.8 N HCl reacted rapidly and could be titrated to a constant pH of 8.5 with an equivalent amount of standard NaOH solution in a few minutes. An equivalent amount of silica gel required 2.5 hr, but ignited gel reacted only slightly in 5 hr. The rate of reaction, followed by a constant pH titration, provides a way to estimate the relative degree of polymerization of silica or possibly the specific surface area. 

Silicic Acid Degree of Polymerization Type of Polymer Rate Constant, K (min ) Author... 

The value of the pH for optimum stability of silicic acid depends on what impurities are present in the solution. Traces of aluminum ions and to a lesser extent, iron, thorium, and beryllium ions tend to offset the effect of fluoride ion by forming complex fluorides and thus retard polymerization in this pH range. Depending on the purity of the solution, the pH of optimum stability may range from 1 to as high as 3-3.5. In silicic acid. solutions free from aluminum impurity, as little as 1 ppm of fluorine has a marked effect on the rate of polymerization in acid solution. 

In the silicic acid system, the formation of gel has usually been ascribed to the condensation of Si(OH)< into siloxane chains, then branching and cross-linking to form a three-dimensional molecular network. Measurement of gel time has often been used as an indication of rate of polymerization (90, 110-113). 

In 5 min, 40% of the silica had polymerized to dimer. By chromatography it was shown that after I hr, perhaps 40% of the monomer and dimer had polymerized to form cyclotetrasilicic acid as well as some he.xameric silicic acid and higher polymeric acid. After 4 hr, about 50% of the polymers up to tetramer were converted to cyclopolysilicic acids with an average molybdate reaction rate constant of 0.103 min. ... 

Greenberg (1S6) also studied polymerization of silica in a mixture of sodium metasilicate and ammonium acetate which served as an acid to lower the pH below about 10.7 so that the silicic acid was liberated and polymerized. He followed polymerization by small changes in the refractive index of the solution which bear a significant but not entirely understood relation to the condensation of SiOH groups to siloxane (Si-O-Si) linkages. No polymerization occurred at pH 11.4 but rates were measured at pH 10.0 and 10.8. It was concluded that the rate of disappearance of SiOH groups in the system followed the first-order equation ... 

Rate of Gel Formation. The time required to gel the type of silicic acid sol made from acid and silicate has been studied for many years. The gel time is assumed to be the inverse of the rate of polymerization or gel formation. In a series of papers by Hurd and associates practically all the factors influencing the gel time of silicic acid sols made from acid and 3.3 Si03 Na30 silicate were reported (216a). Reference should also be made to the studies of Sen and Ghosh (216b), who... 

In the alkali dissolution experiments of Holt and King [1955], quartz powder after etching with dilute sodium hydroxide was treated with a solution of monosilicic acid in borate buffer at pH 8, and the results indicate that monosilicic acid is absorbed on the etched quartz with a layer of silicic acids on the quartz surface. The tendency of the silicic acid to condense and polymerize is thought to be inhibited by its coordination to the quartz surface. It is interesting to speculate that the initial high dissolution rate of silica polymorphs is caused by the desorption of silicic acid. However, in comparable experiments (Bergman and Paterson... 

The dissociation of water coordinated to exchangeable cations of clays results in Brtfnsted acidity. At low moisture content, the Brrfnsted sites may produce extreme acidities at the clay surface-As a result, acid-catalyzed reactions, such as hydrolysis, addition, elimination, and hydrogen exchange, are promoted. Base-catalyzed reactions are inhibited and neutral reactions are not influenced. Metal oxides and primary minerals can promote the oxidative polymerization of some substituted phenols to humic acid-like products, probably through OH radicals formed from the reaction between dissolved oxygen and Fe + sites in silicates. In general, clay minerals promote many of the reactions that also occur in homogenous acid or oxidant solutions. However, rates and selectivity may be different and difficult to predict under environmental conditions. This problem merits further study. 

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