Aqueous silicates polymerization

A fascinating area is micellar autocatalysis reactions in which surfactant micelles catalyse the reaction by which the surfactant itself is synthesized. Thus synthesis of dimethyldoceylamino oxide (reaction between dimethyl dodecyl amine and H2O2) benefits from this strategy. Here an aqueous phase can be used and an organic solvent can be avoided. Synthesis of mesoporous molecular sieves benefit through micellar catalysis and silicate polymerization rates have been increased by a factor 2000 in the presence of cetyltrimethyl ammonium chloride (Rathman, 1996). 

The general theory of nucleation and polymerization in aqueous systems, in which silica shows some solubility, is discussed in detail in Iler s book (3). However, very little was known at the time the book was published (1979) about the polymerization of silica when Si(OH)4 is formed in nonaqueous systems. Progress made up to 1990 in the understanding of the hydrolysis and condensation of silicon alkoxides that leads to silica gels or to silica sols of large particle diameter are lucidly discussed by Brinker and Scherer (8). Brinker s chapter in this book (Chapter 18) includes a clear explanation of the difference between hydrolysis and condensation of aqueous silicates and silicon alkoxides. 

Figure 2. Polymerization pathway of aqueous silicates according to Iler (2). Stages of growth recognized by Iler polymerization of monomer to form particles, growth of particles, and linking of particles together into branched chains, networks, and, finally, gels. (Reproduced with permission from reference 2. Copyright 1978.)...

As shown in Figure 2, polymerization of aqueous silicates at pH 7-10 in the absence of salt leads to the formation of unaggregated particles that... 

An explanation of the anomalous stability of Iler s silica sols in terms of steric stabilization effects requires that oligomeric or polymeric silicate species are present at the silica-water interface and that steric repulsion results during overlap of such layers. This mechanism is appealing in that soluble silicates, usually sodium silicates, are universal dispersants of many electrostatic colloids. Again, well-hydrated silicas [2] and other colloids exposed to aqueous silicate [18] acquire high adsorption densities of aqueous silica. 

Because polymerization of the water-glass solution gives rise to a broad distribution of silicate anions and thus results in poorly resolved Si NMR fines, the application of Si NMR spectroscopy in this type of reaction provides minor information about the different oligomerization steps and the reaction mechanism of monomeric silicic acid. Therefore a different approach to the study of aqueous silicate solutions was apphed. Because of the... 

The aqueous silicic acid solution that is useful in this invention contains silica of the proper particle size, that is, no greater than 5 nm, and provides a solution of sufficient stability to allow the formation of the slurry and subsequent spray drying. The silicic acid can be in the form of a monomer or in the form of low molecular weight polymeric units. For a review of the characteristics of silicic acid, see R. K. Her, The Chemistry of Silica, John Wiley and Sons, NY., 1979. Monomeric silicic acid Si(OH)4 has never been isolated. It is a very weak acid... 

Kinrade, S.D., and Swaddle, T.W. (1988) SiUcon-29 NMR Studies of Aqueous Silicate Solutions. 2. Transverse 29si Relaxation and the Kinetics and Mechanism of SiUcate Polymerization, Inorg. Chem., 27,4259-4264. 

Rao NZ, Gelb LD (2004) Molecular dynamics simulations of the polymerization of aqueous silicic acid and analysis of the effects of concentration tm silica polymorph distributions, growth mechanisms, and reactitm... 

In aqueous silicate chemistry, the term polymerization is commonly used in its broadest sense to include reactions that result in an increase in molecular weight of silica. It includes the condensation of silanol groups... 

Euclidean objects (dense spherical particles) are most likely to form in systems (e.g., aqueous silicates) in which the particle is slightly soluble in the solvent. In this case, monomers can dissolve and reprecipitate until the equilibrium structure (having a minimum surface area) is obtained. In nonaqueous systems (e.g., silicon alkoxide-alcohol-water solutions), the solubility of the solid phase is so limited that condensation reactions are virtually irreversible. Bonds form at random and cannot convert to the equilibrium configuration, thereby leading to fractal polymeric clusters. 

Rgure 1.11 Porod plots of scattering data of silicates polymerized under a variety of conditions (from a study by Schaefer and Keefer ) (a) two-step acid-catalyzed tetraethoxysilane (TEOS) system (b) two-step acid-and base-catalyzed TEOS system (c) one-step base-catalyzed system TEOS system (W = 1) (d) one-step base-catalyzed system TEOS system (VP = 2) (e) aqueous silicate system. LUDOX . VP is the water/ silica ratio. 

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... 

Bechtold, M.F. (1955) Polymerization and properties of dilute aqueous silicic acid from cation exchange. J. Phys. Chem., 59, 532-541. 

The fate of silicic acid is of some interest. Silicic acid polymerizes, by condensation, and finally a silica gel is formed (Wilson Mesley, 1968). The insolubilization of silicic acid has been observed to parallel closely the precipitation of phosphate (Wilson Batchelor, 1967b) and is related to an increase of pH within the cement (Kent Wilson, 1969). A low concentration of silicic acid must remain in the matrix. All this is in accord with the known aqueous chemistry of silica. 

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