Amorphous materials, chemically

Iron(III) hydroxide [1309-33-7], FeH02, is a red-brown amorphous material that forms when a strong base is added to a solution of an iron(III) salt. It is also known as hydrated iron(III) oxide. The fully hydrated Fe(OH)3 has not been isolated. The density of the material varies between 3.4-3.9 g/cm, depending on its extent of hydration. It is insoluble in water and alcohol, but redissolves in acid. Iron(III) hydroxide loses water to form Fe203. Iron(III) hydroxide is used as an absorbent in chemical processes, as a pigment, and in abrasives. Salt-free iron(III) hydroxide can be obtained by hydrolysis of iron(III) alkoxides. 

EXAFS is a nondestructive, element-specific spectroscopic technique with application to all elements from lithium to uranium. It is employed as a direct probe of the atomic environment of an X-ray absorbing element and provides chemical bonding information. Although EXAFS is primarily used to determine the local structure of bulk solids (e.g., crystalline and amorphous materials), solid surfaces, and interfaces, its use is not limited to the solid state. As a structural tool, EXAFS complements the familiar X-ray diffraction technique, which is applicable only to crystalline solids. EXAFS provides an atomic-scale perspective about the X-ray absorbing element in terms of the numbers, types, and interatomic distances of neighboring atoms. 

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,... 

Direct mass spectrometry based techniques such as DI-MS, DE-MS and DTMS are unique in their ability to yield complete chemical structure assignments and to identify a great variety of resinous amorphous materials. 

An alternative scheme for incorporating chemically reactive anthracene monomers made use of anthracene mono-carboxylic acid, therefore resulting in chain capping of the PET chains with anthracene units. Reaction (either in solution or via reactive extrusion) with bismaleimides resulted in chain extension, increasing polymer molecular weights from 6000-10000 to 20000-25 000 in as little as 3 min reaction time (Figure 6.8) [71, 72], While such an approach could hold great promise for the rapid manufacture of polyesters, it should be pointed out that these chain-extended materials all were amorphous materials. 

The solubility of the drug is affected by several physiological and physicochemical factors [26], Drug properties are defined not only by the chemical structure but also by the solid material, and a drug can potentially exist in many different solid state forms which may differ in solubility. Amorphous materials tend to show much higher aqueous solubility than crystalline forms of the same compound and different crystal modifications of the same compound may also have different solubility (e.g., [25]). 

The chemical stability of an amorphous formulation is usually also a function of its storage temperatme relative to Tg. The enhanced molecular mobility achieved near the glass transition translates into an increase in translational diffusion-dependent degradation pathways, such as aggregation in proteins. It should be noted that the reaction kinetics near the Tg do not obey Arrhenius kinetics, and that extrapolation of the accelerated stability data generated near the Tg to stability at the storage temperature should be viewed with extreme caution. Amorphous materials must be stored well below the glass transition (at least 10°C, and typically 40 to 50°C below Tg) to maintain their physical and chemical stability. 

Effect of 6- Caprolactone and Adipic Acid Molar Ratio for Copolyester III on the Hydrolysis by R. delemar Lipase. The hydrolysis of various copolymers by R. delemar lipase was exam ed to see whether there was an optimum chemical structure or not. Mn of those copolyesters was selected from 17 0 to 2220, to diminish the effect of molecular weight. Optimum molar ratio of e- caprolactone and adipic acid was about from 90 10 to 70 30 (Figure 5). The Tm at the optimum molar ratio was the lowest of all. So it seemed that the existence of optimum molar ratio came from the lowest Tm, which would show the most amorphous material, rather than the optimum chemical structure. 

Various a-methylenemacrolides were enzymatically polymerized to polyesters having polymerizable methacrylic methylene groups in the main chain (Fig. 3, left). The free-radical polymerization of these materials produced crosslinked polymer gels [10, 12]. A different chemoenzymatic approach to crosslinked polymers was recently introduced by van der Meulen et al. for novel biomedical materials [11]. Unsaturated macrolactones like globalide and ambrettolide were polymerized by enzymatic ROP. The clear advantage of the enzymatic process is that polymerizations of macrolactones occur very fast as compared to the chemically catalyzed reactions [13]. Thermal crosslinking of the unsaturated polymers in the melt yielded insoluble and fully amorphous materials (Fig. 3, right). 

The solid state stability of indinavir sulfate has been evaluated under a variety of storage conditions and containers. For materials stored in open dishes or in double polyethylene liners within fiber containers, changes in crystallinity i.e., conversion of the crystalline etlranolate to amorphous material or to a hydrate crystal form) have been detected using XRPD or KF methods [7]. Changes in chemical purity i.e., formation of degradation products) have been detected using GC and HPLC methods... 

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