Silicic acid, polycondensation

The research of Wolter et al. (2004) was related to the development of semiperme-able membranes comprising organically modihed silicic acid polycondensates and a process for preparing them for their use in gas exchange and in separation techniques, especially in gas separation, dialysis, and PV. The membranes of their invention can be flat or tubular. The membranes are cured by addition polymerization or polyaddition of the C=C double bonds. 

Figure 3.15. Formation of silica particle by polycondensation of silicic acid molecules (after LePage, 1987).

The sol-gel processes are based on the polycondensation reactions that proceed with participation of silicic acid. There are three main opportunities to perform them. They are considered below. 

Segmented gas-liquid (Taylor) flow was used for particle synthesis within the liquid slugs. Tetraethylorthosilicate in ethanol was hydrolyzed by a solution of ammonia, water and ethanol (Stober synthesis) [329]. The resulting silicic acid monomer Si (OH)4 is then converted by polycondensation to colloidal monodisperse silica nanoparticles. These particles have industrial application, for example, in pigments, catalysts, sensors, health care, antireflective coatings and chromatography. 

Diatoms are unicellular, photosynthetic microalgae that are abundant in the world s oceans and fresh waters. It is estimated that several tens of thousands of different species exist sizes typically range from ca 5 to 400 pm, and most contain an outer wall of amorphous hydrated silica. These outer walls (named frustules ) are intricately shaped and fenestrated in species-specific (genetically inherited) patterns5,6. The intricacy of these structures in many cases exceeds our present capability for nanoscale structural control. In this respect, the diatoms resemble another group of armored unicellular microalgae, the coccolithophorids, that produce intricately structured shells of calcium carbonate. The silica wall of each diatom is formed in sections by polycondensation of silicic acid or as-yet unidentified derivatives (see below) within a membrane-enclosed silica deposition vesicle 1,7,8. In this vesicle, the silica is coated with specific proteins that act like a coat of varnish to protect the silica from dissolution (see below). The silica is then extruded through the cell membrane and cell wall (lipid- and polysaccharide-based boundary layers, respectively) to the periphery of the cell. 

FIGURE 2. Glassy silica needles (2 mm x 30 gm) produced in copious amount by a marine sponge. Each needle contains an occluded axial filament comprised of silicateins, enzyme-like proteins that catalyze and spatially direct the polycondensation of silicon alkoxides and silicic acid at neutral pjj41,42 Reprinted from Reference 3, copyright (1999), with permission from Elsevier Science... 

Silica gel is a polymer of silicic acid and has the general formula Si02 x H20. There are many ways to prepare silica gel. Let us consider only one preparation of silica gel to demonstrate how and where differences or variations in the silica may contribute to a different bonded (final) phase. A common way of making silica gel particles for LC is to use acidic hydrolysis of sodium silicate, followed by polycondensation dehydration of orthosilicic acid and grinding of the silica to make granular particles (e.g., 10 jam). If the reaction is not controlled (buffer, pH, temperature, etc.) the hydrogel will... 

Frolov, Shabanova, and co-workers (37-39) studied the transition of a sol into a gel and the aggregate stability of colloidal silica. Their aim was to develop a technology for the production of highly-concentrated silica sols and to use them as binders, catalyst supports, polymer fillers, adsorbents, and so forth. Kinetic studies were made of polycondensation and gel formation in aqueous solutions of silicic acids. At the stage of particle growth, poly condensation proceeds in the diffusion-kinetic region. With changes in pH, temperature, concentration, and the nature of electrolytes,... 

The sol-gel process for the production of silica is an industrially widely apphed procedure. The process is usually conducted in homogeneous solution, and it is possible to manufacture nano- or micro-sized particles, clear macroscopic silica objects (monoliths), such as fibres or lenses [17]. Sol-gel processing is commonly conducted at quiescent temperature, and no increased pressure is necessary, which makes this method suitable for LLC templating. The starting material for the sol-gel preparation of silica is an orthoester (alkoxide) of the general structure Si(OR)4, which is hydrolysed in order to formally yield silicic acid. The latter undergoes polycondensation into a three-dimensional network of silicon... 

Bhattacharya S, Kieffer J (2008) Molecular dynamics simulation study of growth regimes during polycondensation of silicic acid from silica nanoparticles to porous gels. J Phys Chem C 112 1764-1771. 

A general theory of polycondensation equilibrium in silicic acid solutions was proposed by Stbber (94). From it he deduced the concentration of monomer in equilibrium with polymers of different degrees of condensation. Further data are needed to check the validity of the complex equations that were developed. 

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