Amorphous hydrogels

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... 

In another model, Harland and Peppas [159] considered the diffusion of solutes through semicrystalline hydrogel membranes. These types of membranes were assumed to consist of a crosslinked, swollen (amorphous) phase through which solute diffusion occurred and an impermeable, crystalline phase. A simplified form of the model assumes uniform amorphous regions. With this assumption, the diffusion coefficient through a semi-crystalline membrane, Dc, was written as... 

Some of the most useful polyphosphazenes are fluoroalkoxy derivatives and amorphous copolymers (11.27) that are practicable as flame-retardant, hydrocarbon solvent- and oil-resistant elastomers, which have found aerospace and automotive applications. Polymers such as the amorphous comb polymer poly[bis(methoxyethoxyethoxy)phosphazene] (11.28) weakly coordinate Li " ions and are of substantial interest as components of polymeric electrolytes in battery technology. Polyphosphazenes are also of interest as biomedical materials and bioinert, bioactive, membrane-forming and bioerodable materials and hydrogels have been prepared. 

It is not know what is the precise function of polymer crystallinity in the complex release mechanism where there are several time and release dependent changing parameters but the release, particularly in the early stages, from crystalline PEO hydrogels is more constant than the release from amorphous PEO hydrogels. However, the degree of crystallinity is related to the... 

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... 

Cupric hydroxide, Cu(OH)2.—The hydroxide has been prepared in crystalline form by the action of a solution of caustic alkali on a basic cupric nitrate 2 and a basic cupric sulphate,3 and also by other methods.4 A hydrogel of varying composition is precipitated by addition of alkali to solutions of cupric salts.5 Unlike the colloidal form, the blue crystalline variety is stable at 100° C. A solid, colloidal variety has been obtained 6 as blackish-blue, brittle lamellae which dissolve in water to form the original solution. An amorphous modification is precipitated from ammoniaeal copper solutions by the action of alkali-metal hydroxides.7... 

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. 

---