Synthetic silica precipitated silicic acids

The complex morphology of the nanopatterned siUca diatom cell walls has been found to be related to species-specific sets of polycationic peptides, so-called silaffins, which were isolated from diatom cell walls [82], The morphologies of precipitated sihca can be controlled by changing the chain lengths of the polyamines as well as by a synergistic action of long-chain polyamines and silaffins [83,84]. It has been proposed that the delicate pattern formation in diatom shells can be explained by phase separation of silica solutions in the presence of these polyamines [85]. Various linear synthetic analogs of the natural active polyamines in biosilica formation can accelerate the silicic acid condensation even more than the above mentioned... 

Synthetic silica (silicon dioxide) are prepared either by pyrogenation of silica tetrachloride or by precipitation from a solution of alkaline silicates through the action of acids or metal salts (see next section). Synthetic silica exhibit specific surface area in the 100-200 m /g range and are therefore active fillers. Table 4.7 gives a comparison of some physical properties and the chenucal... 

Amorphous silica (natural and synthetic) may exist in several different forms including silica gels, nonporous precipitates, and hydrogenic silicas. These can all be regarded as polycondensation products of orthosilicic acid. The methods of preparation are either by acid precipitation from silicate solutions, or by the hydrolysis of silicon derivatives, silicon tetra-halides, or alkoxides (Mitchell [1966]). To follow the development of the properties of silica gel, it is necessary to consider polymerization of silicic acid from the sol stage. 

Industrially produced synthetic precipitated amorphous silicas are still made today through precipitation from diluted sodium- or potassium-silicate-solutions with the aid of mineral acids as sulfuric acid, hydrochloric acid or with the aid of carbon dioxide. In case of the use of sodium silicate and sulfuric acid the reaction follows Equation (43.1)... 

The first synthetic materials with memory for a template were obtained by Dickey in 1949 using a silica gel matrix. Imprinted silica materials were produced by acid precipitation of aqueous solution of sodium silicate in the presence of dyes as templates (e.g., methyl orange). In the following years research on... 

Acid-resisting Cement. The principal types are as follows 1. Silicate an inert filler bonded with silica gel that has been precipitated in situ from Na- or K- silicate in the presence of Na2SiFg, or from silicon ester. 2. Rubber Latex essentially cement sand mixes impregnated with rubber. 3. Synthetic Resin with an inert filler. 4. Sulphur Cements usually with sand as filler. 5. Bituminous Cements. ASTM specifications for these materials are listed in Vol 4-05 of the Annual Book of ASTM Standards, and number some 25, including specifications for mortars and grouts. 

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