Amorphous instabilities

When microscopically examined, this group of precipitates lack defined shape and generally assumes a color reflective of the wine. Precipitates in this category include protein and phenolics (and complexes of the two), polysaccharides (glucans, pectin, and starch), and metal casses (copper and iron). 

Proteinaceous materials in wines will stain blue-black using Amido Black lOB and pink to red using Eosine Y as described in method A (Section 17.5.1.1). Method B (Section 17.5.1.2) rehes on the interaction of tannin and protein, which forms a visually apparent haze or precipitate. In method B, formation of pronounced haze in the treated sample as compared with a control is indicative of unstable protein (Fig. 17.4). 

Filter sample containing sediment through a polycarbonate membrane. Cellulose acetate membranes are unstable in presence of the protein stains. 

Leaving the membrane on the filter housing, wet with stain, and hold for 10 min. 

Apply vacuum to the membrane. Carefully rinse with extra methanol-acetic acid solvent (90 mL methanol -I- 10 mL concentrated acetic acid) until the hlter is white to remove excess stain prior to examination under a microscope. 

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It may occasion surprise that an amorphous material has well-defined energy bands when it has no lattice planes, but as Street s book points out, the silicon atoms have the same tetrahedral local order as crystalline silicon, with a bond angle variation of (only) about 10% and a much smaller bond length disorder . Recent research indicates that if enough hydrogen is incorporated in a-silicon, it transforms from amorphous to microcrystalline, and that the best properties are achieved just as the material teeters on the edge of this transition. It quite often happens in MSE that materials are at their best when they are close to a state of instability. 

Polyvinyl chloride (p.v.c.) P.V.C. is one of the two most important plastics in terms of tonnage and shows many properties typical of rigid amorphous thermoplastics. More individually, it softens at about 70°C, burns only with difficulty and is thermally unstable. To reduce this instability, stabilisers are invariably compounded into the polymer. 

These two examples show that regular patterns can evolve but, by definition, dissipative structures disappear once the thermodynamic equilibrium has been reached. When one wants to use dissipative structures for patterning of materials, the dissipative structure has to be fixed. Then, even though the thermodynamic instability that led to and supported the pattern has ceased, the structure would remain. Here, polymers play an important role. Since many polymers are amorphous, there is the possibility to freeze temporal patterns. Furthermore, polymer solutions are nonlinear with respect to viscosity and thus strong effects are expected to be seen in evaporating polymer solutions. Since a macromolecule is a nanoscale object, conformational entropy will also play a role in nanoscale ordered structures of polymers. 

Phase transitions, whether first-order or second-order, are potent sources of instability of solid drugs and can usually be detected and studied by thermal methods of analysis (e.g., DSC, TGA, TMA, ODSC, DMA, DEA). In crystalline solids, typical first-order transitions are polymorphic or desolvation transitions. In amorphous solids, second-order transitions, such as glass transitions, are common. 

Na-cefazolin is instable in its amorphous state. Takeda [1.32J described a method to ensure complete crystallization in which micro crystalline Na-cefazolin were added to at 0 °C supersaturated Na-cefazolin solution, frozen and freeze dried. The product did not contain amorphous or quasi-crystalline components. 

MJ Powell, C van Berkel, and JR Hughes, Time and temperature dependence of instability mechanisms in amorphous silicon thin-film transistors, Appl. Phys. Lett., 54 1323-1325, 1989. 

The results of the polymorph screening step in combination with bioavailability studies, provide the information required by the clinical research team to nominate the desired crystal form of the API for long term manufacture and formulation. This form will usually be the most stable polymorph, where a number of forms have been identified, or a salt form if bioavailability is low or when there are formulation concerns regarding polymorph stability. In some cases it may be necessary to select an amorphous form or metastable polymorph because of crystallization difficulties, time constraints or bioavailability requirement. The nomination of a hydrate or solvate is generally avoided because of their relative instability and compositional variability such constraints are less of a concern for the earlier synthetic intermediates. 

AIH3 has several crystalline and amorphous forms it appears as a non-volatile solid instable above 150°C. In a-A H3, aluminium is octahedrally surrounded by 6 hydrogen atoms. 

When dealing with partially crystalline materials, such as those produced by milling, the effect of water uptake is intensified. The amorphous component likely absorbs greater quantities of water than its crystalline counterpart, leading to reduced Tg, increased molecular mobility, and both physical and chemical instability. 

The chemistry and structure of the hydrogen form of zeolite Y have been thoroughly investigated 82) and are not considered further. The structure of the dehydroxylated zeolite proposed by Uytterhoeven, Christ-ner, and Hall 15) remains unchanged. Recently Ward, on the basis of infrared studies, suggested that this form may be amorphous 27). The extreme instability of dehydroxylated zeolite Y to moisture complicates detailed study 19). The elucidation of the detailed nature of this material lies in the future. At present, completely dehydroxylated Y is little understood and presents a challenging void in our knowledge of the nature of ammonium zeolite Y thermal decomposition products. 

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