Polymorphs appearing first

Consequently, the selective production of crystals of a metastable type is desirable. Accordingly, it is necessary to study the crystallization behavior of the polymorph appearing first (primary nuclei) in a supersaturated solution. 

In this paper, correlating equations of solubility of QE in ethanol were proposed using previous data (2). Subsequently, in order to clarify the formation mechanism of polymorphs appearing first in a supersaturated solution, the solutions were cooled to a given temperature rapidly and the time required for... 

The type of polymorph appearing first from a supersaturated solution was considered... 

In crystallization processes involving a material that displays polymorphism, it is quite common for an unstable polymorph to appear first and then transform into a stable form. This observation is summarized by Ostwald s step rule, sometimes referred to as the Law of Successive Reactions, which says that in any process, the state which is initially obtained is not the stablest state but the least stable state that is closest in terms of free energy change, to the original state. What this means, therefore, is that a crystallization process, the initial solid phase, can be the least stable polymorph that will then transform into successively more stable forms until the stable form, at the conditions of the system, is reached. With some systems this can mean the formation of an... 

As myceiium grows out on a nutrient agar, it can display a remarkable diversity of forms. Some mycelia are fairly uniform in appearance others can be polymorphous at first and then suddenly de- velop into a homogeneous looking mycelia. This is the nature of mushroom mycelia—to constantly and evolve. 

Problems caused by polymorphism appear in many fields such as fine chemicals in industries (pharmaceuticals(7,2), foods, etc.), optical electronic materials(i), clathrate compounds(4) and biominerals(5). In crystallizations of these materials the crystallization behavior of the polymorphs is controlled first by basic operational conditions such as temperature, supersaturation degree, stirring rates. In addition to these basic factors, solvents, additives and guest molecules (in clathrate compounds) should be also considered as the important factors((5,7). The crystallization process of the polymorphs is composed of comp)etitive nucleation, competitive growth of polymorphs and transformation from metastable to stable form. Accordingly individual step should be investigated to clarify the crystallization mechanism of polymorphs. 

If the crystal form is observed in the experiment, the discrimination between a and p polymorphs of QE crystals appearing first in the soluticm can be immediately decided. 

The polymorphs of QE appearing first in the supersaturated solution are shown in Figure 2 together with the solubility lines of a and p calculated from equations 6. 

Pressure-induced amorphization of solids has received considerable attention recently in physical and material sciences, although the first reports of the phenomenon appeared in 1963 in the geophysical literature (actually amorphization on reducing the pressure [18]). During isothermal or near isothermal compression, some solids, instead of undergoing an equilibrium transition to a more stable high-pressure polymorph, become amorphous. This is known as pressure-induced amorphization. In some systems the transition is sharp and mimics a first-order phase transition, and a discontinuous drop in the volume of the substance is observed. Occasionally it is strictly not an amorphous phase that is formed, but rather a highly disordered denser nano-crystalline solid. Here we are concerned with the situation where a true amorphous solid is formed. 

The most recent converts are in the health care industry. Pharmaceutical and biological applications have become myriad since the early 1980s. The first widespread application was for the identification/ qualification of incoming raw materials. Since then, applications have appeared for moisture (bound and free), blend uniformity of powders, tablet and capsule assays, counterfeiting, polymorphism, degree of crystallinity, hardness (of tablets), dissolution prediction, isomerism, as... 

Determination of electron density maps for the u-quartz polymorph establishes that the charge transfer between silicon and oxygen is not complete and that a residual charge of +1.0 ( 0.1) electron units (e.u.) remains localized on silicon, whereas a charge of —0.5 ( 0.1) e.u. is localized on each oxygen atom. The interpretation of this fact in terms of the bond ionicity is not as univocal as it may appear at first glance. 

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