Amorphous forms

Carbon and Graphite. Fluorine reacts with amorphous forms of carbon, such as wood charcoal, to form carbon tetrafluoride [75-73-0], CF, and small amounts of other perfluorocarbons. The reaction initiates at ambient conditions, but proceeds to elevated temperatures as the charcoal bums ia fluoriae. 

In addition to the semicrystalline nylons, which comprise the vast majority of commercial resins, nylon is also available in an amorphous form that gives rise to transparency and improved toughness at the expense of high temperature properties and chemical stress crack resistance. Table 2 shows the properties of some different polyamide types. 

Silica sols are often called colloidal silicas, although other amorphous forms also exhibit colloidal properties owing to high surface areas. Sols are stable dispersions of amorphous siUca particles in a Hquid, almost always water. Commercial products contain siUca particles having diameters of about 3—100 nm, specific surface areas of 50—270 m /g, and siUca contents of 15—50 wt %. These contain small (<1 wt%) amounts of stabilizers, most commonly sodium ions. The discrete particles are prevented from aggregating by mutually repulsive negative charges. 

MetaUic arsenic is stable in dry air, but when exposed to humid air the surface oxidizes, giving a superficial golden bronze tarnish that turns black upon further exposure. The amorphous form is more stable to atmospheric oxidation. Upon heating in air, both forms sublime and the vapor oxidizes to arsenic trioxide [1327-53-3] AS2O2. Although As O represents its crystalline makeup, the oxide is more commonly referred to as arsenic trioxide. A persistent garliclike odor is noted during oxidation. 

AH graphite has crystal stmcture but only certain kinds and sizes of natural graphites are commercially classified as crystalline, a term used for import duty purposes. Throughout this article reference is made separately to dake, vein (lump or high crystalline), and amorphous forms, all of which are essentially the same crystalline form of carbon. However, fine stmctured graphites (cryptocrystalline (2)) have been classified as amorphous. 

Now take another batch of sulphur flowers, but this time heat it well past its melting point. The liquid sulphur gets darker in colour and becomes more and more viscous. Just before the liquid becomes completely unpourable it is decanted into a dish of cold water, quenching it. When we test the properties of this quenched sulphur we find that we have produced a tough and rubbery substance. We have, in fact, produced an amorphous form of sulphur with radically altered properties. 

In order to obtain pure title substance, the acid 8 is suspended in 150 ml of methanol and treated while stirring with 10 ml of a 2N solution of the sodium salt of 2-ethylcaproic acid in ethyl acetate. After about 10 minutes, there results a solution which is treated with 100 ml of ethanol. The mixture is extensively concentrated at 40°C in vacuo. The sodium salt precipitates out in amorphous form after adding ethanol. This salt is filtered off under suction, washed successively with ethanol and low-boiling petroleum ether and dried at 40°C in a high vacuum. There is obtained the title substance in the form of an almost colorless amorphous powder. 

Long exposure to Si02 dust can result in fibrosis of the lung or silicosis which may eventuate in pulmonary hypertension and cor pulmonale. Susceptibility to tuberculosis is enhanced. The tolerance level for cryst forms of Si02 is calculated from the formula 250/(%SiO2 +5), and for amorph forms the tolerance level is 20 millions of particles/cu ft of air... 

PIB exists either as a low mw (about 12,000) viscous or atactic liq or as a cryst matl of about 1.00,000 mw. In this latter form the iso tactic (chain) type configuration predominates over the atactic and amorph forms (Ref 8). Since the isotactic form is the form designated by mil spec (Ref 2), its parameters are presented below ... 

In areas of the system where the heat gradient is less severe, calcium carbonate precipitates in both crystalline and amorphous forms. It may precipitate as a calcite or aragonite sludge, but more usually an aragonite scale is produced. Aragonite is hard and adherent, depositing in FW lines and various boiler surface components such as boiler tubes. 

Boron is mined as borax and kernite, Na2B407-xH20, with x = 10 and 4, respectively. Large deposits from ancient hot springs are found in volcanic regions, such as the Mojave Desert region of California. In the extraction process, the ore is converted into boron oxide with acid and then reduced with magnesium to an impure brown, amorphous form of boron ... 

Because of the conversion of orthorhombic sulfur to monoclinic form, the above values of melting points are difficult to observe, as the resulting allotropic mixture melts at only 115 C. Amorphous or plastic sulfur can be produced through the rapid cooling of molten sulfur. X-ray crystallographic studies show that the amorphous form may have a helical structure with eight atoms per turn. This form is metastable at room temperature and gradually reverts back to crystalline within hours to days but this conversion can be rapidly catalyzed. 

The infrared spectra of hevea (natural rubber), balata (or guttapercha), the latter both in the crystalline (a) and the amorphous forms, and of synthetic polyisoprene are compared in Fig. 32. The hevea and balata (amorphous) spectra offer calibrations for cfs-1,4 and irans-1,4 structures, respectively, in the synthetic polymer. Owing to the presence of the methyl substituent, however, the spectral difference between the as and trans forms is slight both absorb at about 840... 

There are other soUd states which sometimes confuse the measurement and definition of solubiUty. The dmg may crystaUize as a hydrate, i.e. under inclusion of water molecules. If the hydrate form is more stable than the pure form it may be difficult to measure the intrinsic solubility of the drug at all. Often drugs tend to precipitate in an amorphous form, often under the inclusion of impurities. As with metastable polymorphs, such amorphous precipitates may lead to erroneously high solubility measurements. CommerciaUy, drugs are often crystallized in salt form, e.g. as the hydrochloride salt, a cation with a chloride anion. In these co-crystallized salts, a much lower solubility than the intrinsic solubility will typi-... 

Subsequently, the dried ore is reduced in an electric furnace to ferronickel. Drying of the ore ensures smooth operation in the reduction furnace. As another example, reference may be drawn to the processing of kaolinite, Al2(Si05)(0H)4, for the recovery of alumina. The mineral is leached with dilute sulfuric acid. In the hydrated form, the mineral is insoluble in dilute mineral acids, and on drying at 400 to 800 °C, kaolinite is converted to the amorphous form, metakaolin, A1203 2 Si02 ... 

Another physical property that can affect the appearance, bioavailability, and chemical stability of pharmaceuticals is degree of crystallinity. Amorphous materials tend to be more hygroscopic than their crystalline counterparts. Also, there is a substantial body of evidence that indicates that the amorphous forms of drugs are less stable than their crystalline counterparts [62]. It has been reported, for example,... 

Often the stability of a drug in the solid state depends on its physical state (i.e., crystalline or amorphous [8]). If freeze-drying produces an amorphous solid and the amorphous form is not stable, then freeze-drying will not provide an acceptable product. 

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