Crystalline: hydrated silica

From gillispite, BaFeSliOu, Pabst (80) obtained flakes of crystalline hydrated silica consisting of Si40io " ions linked into sheets with the composition 4 H4Si40io + 2 HjO density calculated 2.15, found 2.05 refractive index 1.45 0.01. 

Certain anhydrous crystalline silicates yield layerlike crystalline hydrated silicas with the approximate formula (HjSiiO ),. These are cited to illustrate the complexity of such apparently simple compounds. 

Few if any of these various disilicic acids, which are highly polymerized in two dimensions, are identical. The relation of these to the previously discussed hydrated crystalline silicas obtained from hydrated sodium polysilicates is not known. It is evident that a large number of crystalline hydrated silicas may exist, some more stable than others, but all obtained from crystalline silicates by ion exchange. No crystalline hydrate silica is likely to be formed directly in the silica-water system in the absence of cations to bring about the organization of a regular polysilicate structure. 

A solid emulsion is a suspension of a liquid or solid phase in a solid. For example, opals are solid emulsions formed when partly hydrated silica fills the interstices between close-packed microspheres of silica aggregates. Gelatin desserts are a type of solid emulsion called a gel, which is soft but holds its shape. Photographic emulsions are gels that also contain solid colloidal particles of light-sensitive materials such as silver bromide. Many liquid crystalline arrays can be considered colloids. Cell membranes form a two-dimensional colloidal structure (Fig. 8.44). 

Figure 1 shows the crystallization kinetics of ZSM-48. A good agreement is shown between the crystallinity evaluated by X-ray and adsorption of n-hexane. These kinetic curves confirm the metastability of ZSM-48 zeolite. Indeed the conversion of ZSM-48 into cristobalite, a dense and stable phase, occurs for long reaction times. The difference between the two curves at start reaction times is due to the presence of hydrated silica (Aerosil) that also adsorbs n-hexane. 

Owing to this activation threshold, the first precipitation product from aqueous solutions of silicic acids will be an amorphous silica of some degree of hydration, while at room temperature the growth of vitreous and crystalline forms of silica from the precipitate (and thus the approach toward the absolute equilibrium) will proceed extremely slowly. With this understanding the data in Figure 1 are said to represent, an equilibrium—i.e., the reversible equilibrium between silicic acids in aqueous solution and metastable hydrated silica or polymeric silicic acid as precipitate. 

The commercial alumina and silica gel sorbents are mesoporous, i.e., with pores mostly larger than 20 A (see Fig. 1). Activated alumina is produced by thermal dehydration or activation of aluminum trihydroxide, A1 (OH)3 (Yang, 1997), and is crystalline. Commercially, silica is prepared by mixing a sodium silicate solution with a mineral acid such as sulfuric or hydrochloric acid. The reaction produces a concentrated dispersion of finely divided particles of hydrated Si02, known as silica hydrosol or silicic acid ... 

Opal is related to the very common Si02 mineral species, quartz. Oceans are at present undersaturated with respect to opal (Broecker, 1971) possibly because of the biological formation of animals with silicified skeletons such as the diatoms. These delicate structured creatures, which proliferate in the upper photic zone, dissolve at depth. Therefore, only robust siliceous skeletons such as sponge spicules are retained in sediments that accumulate in deep waters, although some diatoms survive on the continental shelf under zones with high productivity. The initial deposition of the amorphous hydrated silica, opal, converts first to opal-CT and eventually to crystalline quartz (Kastner, 1981). 

The latter take the form of hydrated silica or deposits on the cell walls. Other inorganic components include Cl, P, S, and alkali earth materials, i.e, Mg and Ca, which are present either on the cell walls or as crystalline Ca or Mg oxalates in the cytoplasm. 

Vitreous Si02 occurs as tectites, obsidian and the rare mineral lechatelierite. Synthetic forms include keatite and W-silica. Opals are an exceedingly complex crystalline aggregate of partly hydrated silica. 

Il in, Turutina, and co-workers (Institute of Physical Chemistry, the Ukrainian S.S.R. Academy of Sciences, Kiev) (113-115) investigated the cation processes for obtaining crystalline porous silicas. The nature of the cation and the composition of the systems M20-Si02-H20 (where M is Li+, Na+, or K+) affect the rate of crystallization, the structure, and the adsorption properties of silica sorbents of a new class of microporous hydrated polysilicates (Siolit). These polysilicates are intermediate metastable products of the transformation of amorphous silica into a dense crystalline modification. The ion-exchange adsorption of alkali and alkaline earth metals by these polysilicates under acidic conditions increases with an increase in the crystallographic radius and the basicity of the cations under alkaline conditions, the selectivity has a reverse order. The polysilicates exhibit preferential sorption of alkali cations in the presence of which the hydrothermal synthesis of silica was carried out. This phenomenon is known as the memory effect. 

Channel Inclusion Compounds, p. 223 Chiral Guest Recognition, p. 236 Clathrate Hydrates, p. 274 Crown Ethers, p. 326 Crystalline Microporous Silicas, p. 380 Cyclodextrins, p. 398 Hofmann-Type Clathrates, p. 645 Hydrogen Bonding, p. 658 Hydroquinone, p. 679... 

Sodium disilicate, Na2SijO, may be converted to hydrated silica. Thus by treating the silicate with concentrated cold acid, washing out salts with water and water with acetone, and drying in vacuum at 40°C, a crystalline disilicic acid is obtained (76, 77). Such layerlike structures arc termed lepidoidal (scalelike) or phylloidal (leaflike). Liebau distinguished two types of layer structures. They are internally hydrogen bonded and exhibit only weak acidity (78). 

The number of technically interesting SiOi materials has been significantly complemented and enriched over the last three decades. A large number of synthetic, crystalline high-silica compounds with framework or layer structure have been described, including microporous silicalite (an example of a zeolite), crystalline silicic acids, and their salts, the layered metal silicate hydrates. 

Synthetics - Synthetic fillers, other than Ti02, are the silicas and aluminas. The silicas are either amorphous or crystalline, and are formed by precipitating various forms of silicon dioxide fi om acidified soluble silicates. The synthetics can be hydrated silica, simple alkaline earth metal silicates, or almninum silicates. 

Amorphous silica, ie, silicon dioxide [7631-86-9] Si02, does not have a crystalline stmcture as defined by x-ray diffraction measurements. Amorphous silica, which can be naturally occurring or synthetic, can be either surface-hydrated or anhydrous. Synthetic amorphous silica can be broadly divided into two categories of stable materials (1) vitreous silica or glass (qv), which is made by fusing quart2 at temperatures greater than approximately 1700°C (see Silica, vitreous silica), and microamorphous silica, which is discussed herein. 

Although hydration under hydrothermal conditions may be rapid, metastable iatermediate phases tend to form, and final equiUbria may not be reached for months at 100—200°C, or weeks at even higher temperatures. Hence, the temperatures of formation given ia Table 6 iadicate the conditions under saturated steam pressure that may be expected to yield appreciable quantities of the compound, although it may not be the most stable phase at the given temperature. The compounds are Hsted ia order of decreasiag basicity, or lime/siHca ratio. Reaction mixtures having ratios C S = 1 yield xonotHte at 150—400°C. Intermediate phases of C—S—H (I), C—S—H (II), and crystalline tobermorite ate formed ia succession. Tobermorite (1.13 nm) appears to persist indefinitely under hydrothermal conditions at 110—140°C it is a principal part of the biader ia many autoclaved cement—silica and lime—silica products. 

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