Microcrystalline structure

Solid cellulose forms a microcrystalline structure with regions of high order, i.e., crystalline regions, and regions of low order that are amorphous. Naturally occurring cellulose (cellulose I) crystallizes monoclinic sphenodic. The molecular chains lay in the fiber direction ... 

Amorphous NiP alloys with > 10% P (generally obtained by deposition from acidic electrolytes) are non-magnetic (see [66] and references therein), as required of the underlayer for thin-film media. Although the structure of these alloys is generally assumed to be a solid solution of P in Ni, a recent report [67] has suggested that NiP with 7.4-10% P deposited from acid sulfate electrolytes is better represented by a microcrystalline structure composed of 4-5 nm fee NiP solid-solution grains. 

The waxes consist of both cyclic and paraffin hydrocarbons. At similar melting points, the waxes have a much higher molecular weight than paraffin waxes and are less stable. Ceresine waxes have a very delicate crystalline structure, with fine needle or short plate crystals. Properly constituted blends of ceresine and paraffin waxes have a dense microcrystalline structure. 

Adsorbents are natural or synthetic materials of amorphous or microcrystalline structure. Those used on a large scale, in order of sales volume, are activated carbon, molecular sieves, silica gel, and activated alumina [Keller et al., gen. refs.]. 

The Michael addition of nucleophiles to coumarins catalyzed by solid bases provides an interesting approach to the synthesis of 4-substituted 3,4-dihydrocumarins, because with the conventional Michael catalysts the alkaline hydrolysis of the 8-lactone predominates (Scheme 44). Results were obtained when the Michael addition of diethyl malonate to coumarin was catalyzed by the activated Ba(OH)2 292). An unusual 1,2-addition-elimination process at the C = 0 bond was observed. The mechanism of this reaction was explained on the basis of the microcrystalline structure of the catalyst. It was suggested that the rigid coumarin molecule interacts with the Ba ions through the lone-pair electrons of both oxygen atoms of the... 

In recent years, it has been realized that techniques based on X-ray absorption provide important additional possibilities for catalyst characterization. Techniques such as X-ray absorption fine structure (XAFS) spectroscopy have had a significant impact on catalyst research. For example, the application of these techniques has for the first time allowed structural descriptions of many catalysts which, because of the presence of microcrystalline structures (nanophase particles) or amorphous phases, cannot be elucidated by XRD. 

The constitutional formula and molecular weight of cellulose determined on the basis of chemical and physico-chemical experiments has been confirmed by X-ray analysis, which has also led to the discovery of the microcrystalline structure of cellulose. Today the structural model proposed by Meyer and Mark [21] and Mark and Misch [22] based on the X-ray measurements of Polanyi [23] and Sponsler and Dore [24] and taking into consideration Haworth s conclusions about the existence... 

The differences in the microcrystalline structure of the different forms of cellulose are compared in Table 36. 

At the present moment it is difficult to decide which of the two hypotheses concerning the structure of cellulose is correct the idea of an amorpho-crystalline structure, or that postulating solely an amorphous texture. Nikitin assumes that the first hypothesis is the more probable, more especially as it is well in line with the most recent work of Zaydes and Sinitskaya [45] who conclude on the basis of electron diffraction investigations that in the natural cellulose fibre of Chinese nettle, there exist phases having a distinct microcrystalline structure. This suggests that structures shown in Figs. 78, 79 and 80 are the most probable. 

Herzog and Londberg [20] have examined the X-ray diagrams of cellulose produced by denitration of a carefully nitrated product. The picture of the regenerated cellulose was in principle identical with that of cellulose before nitration. This observation suggests that a cautious nitration of cellulose that proceeds without oxidation or hydrolysis does not change the microcrystalline structure of the cellulose, i.e. this is a topochemical reaction, in which the product preserves the same... 

Like cellulose, nitrocellulose demonstrates optical anisotropy (double refraction) which is regarded as one of the facts confirming the microcrystalline structure of the substance. A relationship between this property of nitrocellulose and the nitrogen content has been discovered by Ambronn [127] who studied the double refraction of cellulose. Differences in the refractive index for rays of different wavelengths in two directions at right angle are of the order —0.50 x 10-3 to +2.8 x 10"3 (Table 60). 

Microcrystalline petroleum waxes are characterized not only by microcrystalline structure but by very high average molecular weight, manifested by a milch higher viscosity than that of paraffin wax. The chlorophyll present in plants is considered to be a microcrystalline wax. 

F2-10-Ac-docetaxel (74) was first studied in a polycrystalline form by the RFDR technique. Based on the standard simulation curves derived from molecules with known F-F distances (distance markers), the F-F distance of two fluorine atoms in 74 was estimated to be 5.0 0.5 A (Figure 6). This value corresponds quite closely to the estimated F-F distances for the conformers B and C (F-F distance is ca. 4.5 A for both conformers) based on our RMD studies for F2-paclitaxel (65) (vide supra). This means that the microcrystalline structure of 74 is consistent with the hydrophobic clustering conformer B or C, but not with the conformer A in which the F-F distance is ca. 9.0 A. 

Moisture is to be removed, in a drier, from compacted material with a microcrystalline structure. The drier, fitted with a heating jacket and a regulator to maintain a constant temperature, is charged with 100 kg product containing 30% water. The final product must have a residual moisture of 0.1% or less. Because of the nature of the product, its temperature should not exceed 50 °C and, to achieve the required dryness, a pressure of 2 mbar is required. It is proposed to carry out the process with a single-stage oil-sealed rotary pump (Seff = 100 m3 hr1, water vapour tolerance = 50 mbar at 40 °C and pult (with gas ballast) = 1 mbar). 

Heyn, A. N. J. (1966). The microcrystalline structure of cellulose in cell walls of cotton, ramie, and jute fibers as revealed by negative staining of sections./. Cell. Biol. 29 181-197. 

Examination by means of X-rays does show lines corresponding to a finely divided crystalline structure on a polished metal (Boas and Schmid, Naturwiss., 20, 416 (1932) but this is almost certainly because the X-rays penetrate below the amorphous layer and reveal the microcrystalline structure below. Electrons do not penetrate to more than a few atoms depth. 

BDD electrodes are semiconductor electrodes with microcrystalline structure and relatively rough surfaces on the micrometric scale. Diamond-coated electrodes used for disinfection are chemically, mechanically, and thermally very resistant and show very low corrosion even under high electric charge. Diamond electrodes present no surface redox processes as known from other carbon electrodes (for example glassy carbon). 

Tlie term crystallite is used in polymer science to imply a component of an interconnected microcrystalline structure. Metals also belong to the class of microcrystalline solids, since they consist of tiny ordered grains connected by strong boundaries. 

It is beyond the scope of this chapter to indulge in more details about this technique, but it might be of interest to note that it allows the evaluation of the influence, on the product structure, of different factors such as the initial concentration of the starting fluid, the rate of freezing (macro or microcrystalline structures), the pertinence of the drying cycle and of the final conditioning. 

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