Amorphous silica hydrogels

As with powdered activated carbon, amorphous silica hydrogels are used in conjunction with bleaching clays in the adsorptive purification of fats and oils. Silica hydrogels possessing average pore diameters greater than about 60 A exhibit high... 

Typical Properties Typical physical properties for commercial amorphous silica hydrogels are shown in Table 4. Chemical purity is high typically >99.5% Si02, <0.1% AI2O3, <0.01% Fc203, with only traces of alkali and alkaline earth oxides on a volatile free basis. By definition, amorphous sihca is noncrystalline it contains no crystalline silica forms (i.e., quartz, ciistobahte, tridymite). 

TABLE 4. Typical Properties of Commercial Amorphous Silica Hydrogels. 

Porous silica is one of the different forms of amorphous silica. It can be prepared by acidification of basic aqueous silicate solutions, and when reaction conditions are properly adjusted, porous silica gels are obtained [150], If water is evaporated from the pores of silica hydrogels prepared in this fashion, porous xerogels are obtained [153],... 

Silica applied as a support for catalysts is an X-ray-amorphous form of silicon dioxide [37], It is manufactured in two steps. First, a silica hydrogel is formed b> means of a sol-gel process [38] (Section A.2.1.4) Secondly, the silica hydrogel is subjected to aftertreatment followed by dehydration to remove water. The product... 

Carbonates Phosphates Silica Calcite Aragonite Vaterite Monohydrocalcite Amorphous Dahllite Francolite Amorphous calcium phosphate hydrogel Amorphous ferric phosphate hydrogel Opal Iron oxides Sulfates Halides Oxalates Magnetite Goethite Lepidocrocite Amorphous hydrates Celestite Barite Gypsum Fluorite Weddellite Whewellite... 

The sorbent used the most for TLC work is silica gel. It is an amorphous, porous matrix made by the addition of acid to a sodium silicate solution. With proper control of the hydration process, the silica gel will cross-link to form a hydrogel, which is then washed and dried, yielding a product with pore sizes of 60 A and a surface area of 500m g Other pore sizes/surface area silica gels can be made by changing the conditions of the hydrolysis, but the 60 A silica gel has been the standard used in TLC for... 

Synthetic silica types are used in coatings less as filler than as thixotropic, antisettling and matting additives. The different types include pyrogenic silica, precipitated silica and silica aerogels and hydrogels. They consist of coagulated amorphous spherical particles of very fine... 

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