Hydrate amorphous

Microscopic sheets of amorphous silica have been prepared in the laboratory by either (/) hydrolysis of gaseous SiCl or SiF to form monosilicic acid [10193-36-9] (orthosihcic acid), Si(OH)4, with simultaneous polymerisation in water of the monosilicic acid that is formed (7) (2) freesing of colloidal silica or polysilicic acid (8—10) (J) hydrolysis of HSiCl in ether, followed by solvent evaporation (11) or (4) coagulation of silica in the presence of cationic surfactants (12). Amorphous silica fibers are prepared by drying thin films of sols or oxidising silicon monoxide (13). Hydrated amorphous silica differs in solubility from anhydrous or surface-hydrated amorphous sdica forms (1) in that the former is generally stable up to 60°C, and water is not lost by evaporation at room temperature. Hydrated sdica gel can be prepared by reaction of hydrated sodium siUcate crystals and anhydrous acid, followed by polymerisation of the monosilicic acid that is formed into a dense state (14). This process can result in a water content of approximately one molecule of H2O for each sdanol group present. 

Hydrated amorphous silica dissolves more rapidly than does the anhydrous amorphous silica. The solubility in neutral dilute aqueous salt solutions is only slighdy less than in pure water. The presence of dissolved salts increases the rate of dissolution in neutral solution. Trace amounts of impurities, especially aluminum or iron (24,25), cause a decrease in solubility. Acid cleaning of impure silica to remove metal ions increases its solubility. The dissolution of amorphous silica is significantly accelerated by hydroxyl ion at high pH values and by hydrofluoric acid at low pH values (1). Dissolution follows first-order kinetic behavior and is dependent on the equilibria shown in equations 2 and 3. Below a pH value of 9, the solubility of amorphous silica is independent of pH. Above pH 9, the solubility of amorphous silica increases because of increased ionization of monosilicic acid. 

Silica has 22 polymorphs, although only some of them are of geochemical interest—namely, the crystalline polymorphs quartz, tridymite, cristobahte, coesite, and stishovite (in their structural modifications of low and high T, usually designated, respectively, as a and jS forms) and the amorphous phases chalcedony and opal (hydrated amorphous silica). The crystalline polymorphs of silica are tectosilicates (dimensionality = 3). Table 5.68 reports their structural properties, after the synthesis of Smyth and Bish (1988). Note that the number of formula units per unit cell varies conspicuously from phase to phase. Also noteworthy is the high density of the stishovite polymorph. 

The most important property of sodium and potassium silicate glasses and hydrated amorphous powders is their solubility in water. The dissolution of vitreous alkali is a two-stage process. In an ion-exchange process between the alkali-metal ions in the glass and the hydrogen ions in the aqueous phase, the aqueous phase becomes alkaline, due to the excess of hydroxyl ions produced while a protective layer of silanol groups is formed in the surface of the glass. In the second phase, a nucleophilic depolymerization similar to the base-catalyzed depolymerization of silicate micelles in water takes place. 

The fact that silanol persistence can be favored by equilibrium conditions rather than control of condensation kinetics by steric or electronic factors is usually not considered. The phase separation which results from highly condensed systems continuously removes material from deposition solutions, depleting soluble silane species. While condensed silanols or siloxanes are typically not regarded as participating in a reversible reaction with water or alcohol, they do indeed participate in an equilibrium reaction. Iler [16] has shown that even hydrated amorphous silicon dioxide has an equilibrium solubility in methanol, which implies the formation of soluble low molecular... 

At the onset of biomineralization the mechanism of phosphate and silica deposition is essentially the same. Both start with a highly hydrated amorphous phase having glass-like physical-chemical properties. The kinetics of crystallisation of the two differs. ACP will rapidly alter in the direction of apatite in hours or days, whereas amorphous silica requires thousands of years or higher temperatures to yield quartz. 

Figure 12.1 FTIR ATR spectrum of (A) anhydrous crystalline glucose and (B) hydrated amorphous glucose.

Re-immersion of the ordered oxide films into HC104 or HC1 solutions led to the disappearance of the LEED beams of Type 2. That is, the CrO phase was not stable in acid solutions. This is an indication that acidic electrolytes, particularly HC1, attacked the passive layer at the comparatively thin CrO regions, replacing or covering those regions with a thin, hydrated amorphous iron oxide layer. 

The hydrated amorphous mineral silica (opal) is widely used by many plants and animals for structural purposes. Most skeletons are formed by unicellular organisms (diatoms Figure 2(a), radiolarians Figure 2(b)), but silica is also present within multicellular organisms (sponge spicules, plant... 

Ili-Sil [PPG]. TM for a group of hydrated, amorphous silicas used as reinforcing pigments in elastomers, as fillers and brightening agents in paper and paints, and as flow conditioners. 

In this overall process two sub-processes operate which partly overlap. At relatively low temperature, usually up to about 300 00°C, the strongly hydrated amorphous gel particles are transformed to more crystalline, mainly dehydrated particles. This step is usually called calcination. In this step organic additives also have to be burned out. Usually considerable volume changes of the constituting crystallites occur. 

An old example (1992) of Le Bail fit for structure factor amplitude extraction, prior to structure determination by powder difiractometry of the X AIF3 polymorph (conventional Cu X ray data). No isostruc tural phase is known for that metastable compound based on [A1F6] octahedra exclusively connected by corners in a completely new 3D framework, synthesized only in fine powder form, either from organo metallic or hydrated amorphous precursors. The structure was solved by applying the direct methods (no heavy atom), revealing totally the 11 independent atom sites. 

Borde, B., Bizot, H., Vigier, G., and Buleon, A. Calorimetric analysis of the structural relaxation in partially hydrated amorphous polysaccharides, n. Phenomenological study of physical ageing, Carbohydr. Polym., 48, 111, 2002. 

It is of worthy of note that the International Conference on Harmonization (ICH, Guideline Q6A of the October of1999) includes under the heading of polymorphs single entity polymorphs molecular adducts (solvates, hydrates), amorphous forms . The FDA currently requires that pharmaceutical manufacturers investigate the polymorphism of the active ingredients before clinical tests and that polymorphism is continuously monitored during scale-up and production processes [42], The... 

Voronkov, Klyuchnikov, and co-workers (Institute of Organic Chemistry, Siberian Division of the U.S.S.R. Academy of Sciences, Irkutsk) (376, 377) developed a precise method of combusting organosilicon compounds in a calorimetric bomb to prepare hydrated amorphous silica and measure its standard heat of formation. For samples of Si02 that had specific surface areas varied from 140 to 300 m2/g, the heat of formation was... 

Titanium oxides. These include anatase and rutile (1102), leucoxene (hydrated, amorphous titanium oxide), and ilmenite (FeTi03>, plus zircon (Z1O2) and corundum (AI2O3). Titanium and zirconium are so immobile in soils that members of mineral group are used as indicators of the amount of parent material that has weathered to produce a given volume of soil. 

The mineral silica, or more specifically hydrated amorphous silica, often referred to as opal, is the second most abundant mineral type formed by organisms, with only the carbonate minerals exceeding it in abundance and distribution. Much of the biogenic silica produced is formed at temperatures of 4 C or lower in the polar oceans. 

In the following sections, the solid-state physicochemical characteristics of drug substances and their physical changes related to polymorphism, hydrates, amorphous form, and the effect of water will be discussed. 

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