Crystal growth Crystallinity

Over 50 acidic, basic, and neutral aluminum sulfate hydrates have been reported. Only a few of these are well characterized because the exact compositions depend on conditions of precipitation from solution. Variables such as supersaturation, nucleation and crystal growth rates, occlusion, nonequilihrium conditions, and hydrolysis can each play a role ia the final composition. Commercial dry alum is likely not a single crystalline hydrate, but rather it contains significant amounts of amorphous material. 

In the analysis of crystal growth, one is mainly interested in macroscopic features like crystal morphology and growth rate. Therefore, the time scale in question is rather slower than the time scale of phonon frequencies, and the deviation of atomic positions from the average crystalline lattice position can be neglected. A lattice model gives a sufiicient description for the crystal shapes and growth [3,34,35]. 

We have discussed simulations that were intended to elucidate aspects of crystal growth under diverse conditions. In most cases a direct simulation of growth using realistic conditions is impractical. The growth rate may be many orders of magnitude slower than that required to produce observable crystalline material in the available computer time. We have described several methods to obtain information about the crystallization process in this situation. 

Cement formation requires a continuous structure to be formed in situ from a large number of nuclei. Moreover, this structure must be maintained despite changes in the character of the bonds. These criteria are, obviously, more easily satisfied by a flexible random structure than by one which is highly-ordered and rigid. Crystallinity implies well-satisfied and rigidly-directed chemical bonds, exact stoichiometry and a highly ordered structure. So unless crystal growth is very slow a continuous molecular structure cannot be formed. 

Pharmacists should also take a dim view of changes in the particle size, size distribution, or particulate nature of semisolid suspensions. They are the consequence of crystal growth, changes in crystalline habit, or the reversion of the crystalline materials to a more stable polymorphic form. Any crystalline alteration can lead to a pronounced reduction in the drug-delivery capabilities and therapeutic utility of a formulation. Thus, products exhibiting such changes are seriously physically unstable and unusable. 

Lateral growth occurs in real systems but is not accounted for in the model of Flory. What allows its incorporation into these new calculations is the assignation of the chains to their most probable positions the chains continuously seek positions of equilibrium as crystallization proceeds. This means that all amorphous links have the same propensity for crystallization, which therefore tends to eliminate a distinction between lateral and longitudinal crystal growth (keep in mind that different levels of crystallinity favor one growth pattern over the other -low crystallinity favors fibrils, high crystallinity favors lamellae). 

Oxytetracycline occurs as yellow crystals or crystalline powder, has a bitter taste and is odorless. Oxytetracycline hydrochloride should have potency not less than 835 pg C22H24N2O9 per mg [1], whilst for oxytetracycline not less than 832 pg/g [1] or 95.0-102.0% as anhydrous substance [2] or not less than 88.0% of C22H24N2O9 calculated as anhydrous basis [3,4]. For oxytetracycline calcium, it should contain not less than 90.0% and not more than 100.5% [2], or 865 pg C22H24N2O9 per mg [1]. Oxytetracycline was produced by the growth of certain strain of Streptomyces rimosus or obtained by other means. 

Characterization. It is both important for crystal growth and for checking the purification of materials. It takes place at every stage, from synthesis to the monocrystal. There are two aspects the control of material purity before crystal growth and the control of the crystalline quality of raonocrystals. 

Titania films prepared by the methods described above are, however, just partially crystalline. Although WAXS patterns indicate formation of anatase crystals of ca. 10-12nm in size (Fig. 9.3a), the electron microscopy study demonstrates that the elongated crystals are actually embedded into an amorphous mesoporous matrix (Fig. 9.3c). The degree of crystallinity for such films usually does not exceed 60% attempts to increase it by calcination at higher temperatures cause uncontrolled crystal growth, which leads to collapse of mesoporos-ity and a drastic decrease in the surface area (Fig. 9.3d). 

The thermal stability of mesoporous frameworks substantially increases with an increase in the wall thickness and pore size, which can be varied even for the same template by changing the processing conditions. Ozin et al.55 developed a way to prepare crystalline titania films with a 2D-hexagonal architecture by replacement of ethanol in the Pluronic-containing precursor solution with more hydrophobic butanol-1. The latter promotes phase separation at low surfactant-to-titania ratios, resulting in thicker pore walls, which are more compatible with the crystal growth during subsequent calcination. 

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