Aqueous silicate solutions formation

Sakka, S., unpublished data.)- In the solutions, a number of methylsilsesquioxane species, formed by the hydrolysis of methyltriethoxysilane, with different structures are present even under the conditions where the cubic octamer is dominant in the aqueous silicate solutions. This indicates that the use of a silica source with tetra-functionality is required for the selective structure formation with the aid of organic quaternary ammonium ions. 

P.W.J.G. Wijnen, A spectroscopic study of silica gel formation from aqueous silicate solutions, Ph.D. Thesis, TU Eindhoven, 1990. 

Figure 52.5 shows that the solubility of amorphous silica is independent of pH between 4 and 9 above pH 9 the solubility increases because of the formation of monosilicate, disilicate, and multimer ions. DS coatings are deposited at 90°C — equilibrium constants are not available at this temperature. Silica solubility data are available [24], and hence it is possible to represent the insolubility-solubility domains of silica at this temperature (if not the details of individual speciation), as in Figure 52.6. The predominant reason behind Her proposing that coatings be deposited at ca. 90°C would be the increase by about a factor of 2.5 available in solubility compared to 25°C, as well as possible dehydration and porosity aspects of resultant films [25]. Included in Figure 52.6 are dashed lines that describe the trajectory of successive aliquots of aqueous silicate solution (pH ca. 13.5, point S) added to the titania slurry (at pH ca. 10). The steadily increasing silica concentration in the slurry is depicted by the solid vertical arrow in Figure 52.6.

Wijnen, P.W. J.G. A Spectroscopic Study of Silica Gel Formation form Aqueous Silicate solution , Ph.D. Thesis, Eindhoven University, Holland (1990). 

In Figure 18-lc, the formation of cyclic trimeric silicic acid (0 3) is observed. Probably due to the high ionization of silicic acid species in the aqueous silicate solution, oligomerization of the d eric silicic acid proceeds via the cyclic trimeric isomer of trimeric silicic acid. In a later stage of the dissolution process, the formation of the linear isomer of trimeric silicic acid is encountered in the silicon-29 nmr spectrum (Figure 18-Id). Preferential formation of cyclic trimeric sUicic acid is attributed to charge stabilization of the trimeric species. Ab initio quantum mechanic calculations of cyclic structures of silicate units (Van Beest, Verbeek, and Van Santen... 

Silicon. Ionization of Cl from [0(CMe = NNMe2)2SiClPh] has been observed using Si NMR spectroscopy. Si NMR spectroscopy has been used to study the formation of [(MeO)Si(OH)3] in methanolic alkaline silicate solutions. The role of Na+ in the interaction mechanism of sodium silicate melt has been probed by and Na NMR spectroscopy. Si NMR spectroscopy has provided evidence of pentaoxosilicon complexes in dilute neutral aqueous silicate solutions. ... 

The field of mesoporous materials has developed rapidly since the first reports on these materials in 1992, as these last examples show. The trend is to utilize inexpensive, multifunctional micelle- or aggregate-forming surfactants or templates which may adopt many different liquid crystal-like configurations in aqueous solution. Formation of a silicate structure with well-defined pore dimensions and connectivity may then be accomplished by the appropriate choice of the synthetic conditions. Additional microporous and macroporosity may be incorporated by using macroporous host materials, as in the case of Stucky of the work by and coworkers, who created mesophases with unprecedented architecture.[47]... 

Effect of Temperature. The temperature of a silicate solution also affects the polymerization of silicate anions in the solution. The distribution of silicate anions in an organic quaternary ammonium silicate solution at a fixed N/Si ratio and SiC concentration varies with the temperature of the solution (7,8,13,14,16). Ray and Plaisted (8) reported the temperature dependence of the distribution of silicate anions in the tetramethylammonium silicate aqueous solution at a N/Si ratio of 2/3 and a SiC>2 concentration of 1.0 mol dm. The amount of the cubic octamer in the solution decreases with increasing temperature, and the cubic octamer practically disappears above 50 °C, indicating that the cubic octamer is unstable at higher temperatures. However, Groenen et al. (14) found that the cubic octamer remained in a significant concentration even at 85 °C, which was close to the temperature of actual zeolite formation, in the tetramethylammonium silicate aqueous solution at a N/Si ratio of 1.0 and a Si02 concentration of 1.3 mol dm-. ... 

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