Acids monosilicic

The solution in equihbrium with amorphous sihca at ordinary temperatures contains monomeric monosilicic acid, Si(OH)4. The acid is dibasic, dissociating in two steps (36) ... 

The possibihty of six-coordinate sihcon species in aqueous solution has been suggested. Raman studies have indicated, however, that monosilicic acid in solution contains a tetracoordinate sihcon species (37). 

Solutions of monosilicic acid may also be obtained by carehil hydrolysis of tetrahalo-, tetraalkoxy-, or tetraacyloxysilanes by electrolysis or acidification of alkah sihcate solutions or by ion exchange (qv). By operating under carefully controlled conditions at low temperature and pH, solutions may be obtained that remain supersaturated with respect to amorphous sihca for hours at temperatures near 0°C. Eventually, however, polymerization reactions involving the formation of siloxane linkages occur, leading ultimately to the formation of coUoidal particles and further aggregation or gel... 

Microscopic sheets of amorphous silica have been prepared in the laboratory by either (/) hydrolysis of gaseous SiCl or SiF to form monosilicic acid [10193-36-9] (orthosihcic acid), Si(OH)4, with simultaneous polymerisation in water of the monosilicic acid that is formed (7) (2) freesing of colloidal silica or polysilicic acid (8—10) (J) hydrolysis of HSiCl in ether, followed by solvent evaporation (11) or (4) coagulation of silica in the presence of cationic surfactants (12). Amorphous silica fibers are prepared by drying thin films of sols or oxidising silicon monoxide (13). Hydrated amorphous silica differs in solubility from anhydrous or surface-hydrated amorphous sdica forms (1) in that the former is generally stable up to 60°C, and water is not lost by evaporation at room temperature. Hydrated sdica gel can be prepared by reaction of hydrated sodium siUcate crystals and anhydrous acid, followed by polymerisation of the monosilicic acid that is formed into a dense state (14). This process can result in a water content of approximately one molecule of H2O for each sdanol group present. 

Hydrated amorphous silica dissolves more rapidly than does the anhydrous amorphous silica. The solubility in neutral dilute aqueous salt solutions is only slighdy less than in pure water. The presence of dissolved salts increases the rate of dissolution in neutral solution. Trace amounts of impurities, especially aluminum or iron (24,25), cause a decrease in solubility. Acid cleaning of impure silica to remove metal ions increases its solubility. The dissolution of amorphous silica is significantly accelerated by hydroxyl ion at high pH values and by hydrofluoric acid at low pH values (1). Dissolution follows first-order kinetic behavior and is dependent on the equilibria shown in equations 2 and 3. Below a pH value of 9, the solubility of amorphous silica is independent of pH. Above pH 9, the solubility of amorphous silica increases because of increased ionization of monosilicic acid. 

In the absence of a suitable soHd phase for deposition and in supersaturated solutions of pH values from 7 to 10, monosilicic acid polymerizes to form discrete particles. Electrostatic repulsion of the particles prevents aggregation if the concentration of electrolyte is below ca 0.2 N. The particle size that can be attained is dependent on the temperature. Particle size increases significantly with increasing temperature. For example, particles of 4—8 nm in diameter are obtained at 50—100°C, whereas particles of up to 150 nm in diameter are formed at 350°C in an autoclave. However, the size of the particles obtained in an autoclave is limited by the conversion of amorphous siUca to quartz at high temperatures. Particle size influences the stabiUty of the sol because particles <7 nm in diameter tend to grow spontaneously in storage, which may affect the sol properties. However, sols can be stabilized by the addition of sufficient alkaU (1,33). 

Vitreous sihca does not react significantly with water under ambient conditions. The solution process involves the formation of monosilicic acid, Si(OH)4. Solubihty is fairly constant at low pH but increases rapidly when the pH exceeds 9 (84—86). Above a pH of 10.7 sihca dissolves mainly as soluble sihcates. Solubihty also increases with higher temperatures and pressures. At 200—400°C and 1—30 MPa (<10 300 atm), for example, the solubihty, S, of Si02 in g/kg H2O can be expressed as foUows, where d ls the density of the vapor phase and T is the absolute temperature in Kelvin. 

Condensation occurs most readily at a pH value equal to the piC of the participating silanol group. This representation becomes less vaUd at pH values above 10, where the rate constant of the depolymerization reaction k 2 ) becomes significant and at very low pH values where acids exert a catalytic influence on polymerization. The piC of monosilicic acid is 9.91 0.04 (51). The piC value of Si—OH decreases to 6.5 in higher order sihcate polymers (52), which is consistent with piC values of 6.8 0.2 reported for the surface silanol groups of sihca gel (53). Thus, the acidity of silanol functionahties increases as the degree of polymerization of the anion increases. However, the exact relationship between the connectivity of the silanol sihcon and SiOH acidity is not known. 

Silicates. For many years, siUcates have been used to inhibit aqueous corrosion, particularly in potable water systems. Probably due to the complexity of siUcate chemistry, their mechanism of inhibition has not yet been firmly estabUshed. They are nonoxidizing and require oxygen to inhibit corrosion, so they are not passivators in the classical sense. Yet they do not form visible precipitates on the metal surface. They appear to inhibit by an adsorption mechanism. It is thought that siUca and iron corrosion products interact. However, recent work indicates that this interaction may not be necessary. SiUcates are slow-acting inhibitors in some cases, 2 or 3 weeks may be required to estabUsh protection fully. It is beheved that the polysiUcate ions or coUoidal siUca are the active species and these are formed slowly from monosilicic acid, which is the predorninant species in water at the pH levels maintained in cooling systems. 

Hydrogen peroxide (H2O2) has shown to be an excellent source of oxygen in a plasma. The reaction has several steps beginning by the formation of a gaseous monosilicic acid, followed by polymerization and the removal of H2O, as follows 0 1... 

Fig. 4.18 Formation ofhexagonal silica platelets with poly-L-lysine helical chains in the presence of monosilicic acid and phosphate. Reprinted with permission from [170], M.M.Tomczaketa/.J. Am. Chem. Soc. 2005, 727, 12577. 2005, American Chemical Society.

Silicic Adds. The behavior of silicate ions in solution, the dependence of various properties on pn, the nature of silica sols and gels, and the study of hydrated silicas constitute chapters in inorganic and colloid chemistry that go far beyond the scope of this review. Germane to the present subject, however, are certain observations on the formation of monosilicic acid and its stepwise polymerization. 

Silanetetrol (monosilicic acid) has been prepared only in solution and is stable only within a very narrow pn range (see section V). 

Hingston, F. J., and Raupach, M. (1967). The reaction between monosilicic acid and aluminum hydroxide. 1. Kinetics of adsorption of silicic acid by aluminum hydroxide. 

It is clearly established that, in these solutions, the silicon is in tetrahedral coordination (13). The simplest form of silica in solution is the monosilicic acid Si(0H)4. Polymerization results from the condensation of two silanol groups with the elimination of a water molecule. A whole series of silicate anions, of various polymerization and ionization degrees, are thus connected through dynamic equilibria. The equilibria are governed by the normal chemical parameters, namely the silica concentration, the pH and the cation type. 

Depending on the case considered, the monosilicic acid concentration is... 

Richardson and Waddams (31) found the same behavior for silica. Ground quartz particles agitated with water release some monosilicic acid molecules which form a true solution and, in addition, some very small quartz crystals which form a dispersion in water until a hydrated equilibrium surface is established. Analogous to the ferric oxide, heating the quartz to a temperature above 600° C. regenerates the anomalous solubility behavior. 

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. 

Fine crystalline quartz or amorphous substances (gel, quartz glass, etc.) are used as one of initial components for the synthesis [19]. Depending on temperature, pressure, pH of the medium and the presence of salts, silica can exist in solution both as simple ions or molecules, and as more complicated polymer particles. Under normal conditions silica passes into solution in monomer form, as silicic acid Si(OH)4 at large pH, silicate ions SiOj - are formed. Monosilicic acid is a very weak acid however, at increased temperature its dissociation constant increases substantially. The amount of monomer form also increases with temperature. The dissolution of Si02 is due to hydration as well as to depolymerization. 

If the reaction follows the first route, the interaction of monosilicic acid with calcium hydroxide molecules appears to be a direct reason of polymerization and precipitation of hydrated calcium silicates. The scheme of the reaction can be written as follows [20] ... 

Fig. 5.9. Electron density distributions in olivines (a) Experimental difference density map of part of the forsterite structure showing the residual peaks around Si. Contours are at intervals of 0.1 electrons A negative contours being broken and zero contours dotted. Numbers in decimal fractions of the a length indicate the heights of the atoms. The tetrahedron formed by oxygen atoms around Si is shown (after Fujino et al., 1981 reproduced with the publisher s permission), (b) A comparison of a theoretical difference density map (i) of the O-Si-O group in the monosilicic acid molecule [Si(OH)4] with an experimental map (ii) of the same group in the monosilicate mineral andalusite (AljSiO,). Contours are at intervals of 0.07 electrons A in (ii). The region around the nucleus of each atom in the theoretical map represents the core region, where the data are not expected to be accurate (after Gibbs, 1982 reproduced with the publisher s permission).

Roy et al. also investigated the use of dilute HF in the clean sequence. Because the solubility of monosilicic acid (Si(OH)4) de-... 

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