Silica hydrated form

As a result of its unique chemical and physical properties, silica gel is probably the most important single substance involved in liquid chromatography today. Without silica gel, it is doubtful whether HPLC could have evolved at all. Silica gel is an amorphous, highly porous, partially hydrated form of silica which is a substance made from the two most abundant elements in the earth s crust, silicon and oxygen. Silica, from which silica gel is manufactured, occurs naturally, either in conjunction with metal oxides in the form of silicates, such as clay or shale, or as free silica in the form of quartz, cristobalite or tridymite crystals. Quartz is sometimes found clear and colorless, but more often in an opaque form, frequently colored... 

Another very well known fibrous variety of silica is the gem called opal. Opal is actually a hydrated form of cristobalite whose fiery primary colors come from the diffraction of light by aligned water and siliea fibers. Lus-... 

FIGURE 19 Flint tools. Axes, scrapers, and knives. Flint is a hydrated form of ciyp-tociystalline silica that occurs naturally as irregular nodules in chalk deposits. It is colorless and translucent when pure, but opaque and often colored when it contains impurities. When struck, flint breaks with a conchoidal fracture and the fragments formed have smooth, sharp edges. It is for this reason that prehistoric humans used flint to make tools. Since it also sparks when struck, flint was also used, until the eighteenth century, for lighting Are. 

Polymer crystals grown from silica gel at room temperature contain 1-D chains of [Er(TMA)(H20)5]n,[4] The use of a highly controlled layer diffusion method gives 2-D [ErfTMA)(H20)3]n. In this material loss of two ancillary aqua groups allows pendant carboxylates of adjacent 1-D polymer strands to connect and create a sheet. Finally the use of hydrothermal conditions (180°C, autogenous pressure. 3 days) allows formation of anhydrous [Er(TMA)]n. This has a 3-D network with no simple topological relationship to the hydrated forms.[5]... 

With regard to a solubility equilibrium, the fact that vitreous silica behaves like a precipitate of polymeric silicic acid must be caused by the similarity between polymeric silicic acid and the hydrated surface of vitreous silica. Both forms can release silicic acid by hydrolysis and desorption, and likewise both forms are able to adsorb and condense silicic acid by means of silanol groups randomly distributed on their surfaces. Thus, in order to explain equal final states, the only assumption necessary is that the condensates will not attain the degree of dehydration of the bulk of the vitreous silica. The resulting equilibrium then relates to the two-phase system silicic acid—polymeric precipitate, and strictly speaking, this system is in a supersaturated state with respect to vitreous silica, which can be considered as an aged form of silica gel. 

Silica gel is an amorphous, highly porous, partially hydrated form of silica, which is a substance made from the two most abundant elements in the Earth s crust silicon and oxygen. Silica gel powder is widely used in industries for catalyst support as adsorbents,... 

Normal-phase silica packing requires only drying at a uniform temperature to be ready for packing. At 250°C, the fully hydrated silica is produced, while at 300°C water is lost between adjacent silica molecules forming the anhydride form normally packed in normal-phase columns. 

The way in which salts such as calcium chloride and calcium formate operate is not fully understood, but it is clear that the mechanism involves an acceleration of the C S and C S hydration. It has been proposed [23] that the initial products of cement hydration form a sort of membrane which acts as a restraint to the diffusion process which in turn leads to the dormancy period . It seems likely that the chloride ion, by virtue of its small size and high mobility, is able more easily to penetrate the pores of the restraining layer allowing the diffusion process to proceed more rapidly. The resultant tobermorite gel has a higher lime-silica ratio and a more open, accessible structure, based on a crumpled foil morphology rather than the usual spicular. The considerable reaction with,... 

The anhydrous form of a compound that has a strong tendency to absorb water can be used as a desiccant. Desiccants scavenge the last traces of water from a system. One of the most commonly used desiccants is silica gel (Si02). Addition of water to a desiccant is a reversible process, so hydrated forms of a desiccant can be used as moisturizers. 

Natural and oxides of iron are mined either as the mineral hematite (Fe203) or as hematite in its hydrated form. Indian red is a naturally occurring mineral whose ferric oxide content may vary from 80 to 95 percent, the remainder being clay and silica. It is made by grinding hematite and floating off the fines for use. 

Silica biomineralization is the incorporation of the extremely fine-grained, and virtually amorphous varieties of opal, a hydrated form of the common, but highly insoluble species that has the formula Si02 H20 (see Section 8.05.2.2.1). 

Another aspect of heating soybeans in particular is the impact on the phospholipase enzyme. The phospholipase enzyme is activated at approximately 55°C and remains activated up to approximately 100°C. In this temperature range, and with sufficient exposed surface area and time, the phospholipase enzyme modifies a portion of the phospatides in the oil fraction by splitting off the non-fatty acid moiety (16). The resultant calcium and magnesium salts of phosphatidic acids that are formed tend to be more oil-soluble than water-soluble, thereby converting phospatides from a hydratable form to a nonhydratable form (16). This has a resultant impact on the quantities of acid, caustic and silica needed to reduce the phosphorus content of the soybean oil in the downstream degumming and refining unit operations. 

Depending on the pH of the silica, metal ions can exist in either nonhydrated or hydrated forms. 

Fig. 5 Three-phase equilibrium curves showing incipient hydrate forming conditions for methane and propane hydrates formed in 70-A silica gel pores. Points are the data from Handa and Stupin. (From Ref. l) (View this art in color at www.dekker.com.)...

Recent studies have shown the formation of altered surface layers thicker than one or two unit cell layers on feldspar surfaces, in apparent support of the leached layer theory. Thick (> 100 nm), silica-rich surface layers were detected by XPS on feldspar samples weathered in solutions having pH < 3 (Casey et at, 1988b) or >9 (Heilman et al., 1990). At these extreme pH values, the rate of release of Al and charge-balancing cations to solution is much faster than the rate of hydrolysis of silica. Under these conditions, oversaturation with respect to amorphous silica could occur, and a highly hydrated, residual leached or precipitated layer of silica could form. However, this layer is probably too porous and discontinuous to be a diffusion-limiting mechanism, but would still account for incongruence under these conditions (Hellmann et al., 1990). Whether the altered layer formed by a leached layer process or by simple precipitation from oversaturated solutions was not determined. 

As the examples of non-crystalline condensation disperse structures, one can name silicates and aluminosilicates (silica and aluminosilica gels, both hydrated and dehydrated). Silica gels form in the course of chemical reaction between sodium silicate and acid, namely [16,33] ... 

The source of catalyst activity is considered to be the aluminum atom held by oxygen to adjacent silica tetrahedra. Whether the aluminum is 4- or 6-coordinated as it originally forms in an aqueous medium seems relatively unimportant, since it is the dehydrated form rather than the hydrated form which acts as the catalyst. Either form dehydrates to the same struc-... 

Sandlime bricks, also called calcium silicate bricks, are produced by moulding, under high pressure, a moist mixture of silica sand (or crushed siliceous stone, or flint) and hydrated lime. The green bricks are then autoclaved using steam pressures of at least 11 atmospheres. Under these conditions, the hydrated time reacts with the silica to form hydrated calcium silicate, which bonds the aggregate particles into a strong and durable brick [26.47]. Other shapes, such as blocks and building elements are also produced. 

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