Polymorphism preparative methods

Various polymorphs have been reported for SnS with band gap widths in the range 1.0-1.5 eV, depending on the preparation method. The a-SnS (herzenbergite) is the most frequently occurring phase and is a p-type semiconductor with a direct optical transition at 1.3 eV and a high absorption coefficient (> 10" cm ). The orthorhombic S-SnS phase possesses a direct gap between 1.05 and 1.09 eV. 

The existence of two polymorphs was reported for a NO-releasing derivative of acetyl-salicylic acid [28]. Selection crystallization of one form or the other was achieved from a number of solvent systems (14 solvents and 3 preparative methods), but several systems were identified that yielded mixtures of the two forms. The single-crystal structure of Form I was reported, but the habit of the Form II crystals precluded their characterization. The transition point of the two forms was calculated from intrinsic dissolution data to be higher than the melting points of both polymorphs and thus the two forms bear a monotropic relationship. 

Preformulation should include rigorous studies to determine the number of polymorphs that exist, relative degree of stability of the various polymorphs, solubilities, method of preparation of each form, effect of micronization and tableting, and interaction with formulation ingredients. 

Zircon, complete solid-solution behavior is observed, and a plot of the unit cell volume against x shows that Vdgard s Law is followed. When the end members are not is structural, a systematic change in the solubility range in both structures is found as A is varied, and the data have been systematized in terms of a simple, potentially predictive, structure-field map. The pervasive polymorphism of these ABO4 compounds, involving both reconstructive and displacive transformations and metastable structures produced by different sample preparation methods, indicates that the crystal structural stability of substituted compounds needs to be carefully evaluated as a function of temperature to assess the structural integrity of waste-form materials. 

Tozuka Y, Kawada D, Oguchi T, Yamamoto K. Supercritical carbon dioxide treatment as a new method for polymorph preparation of deoxycholic acid. AAPS Pharm Sci 2002 4(4). 

Solids may exhibit polymorphism, which means that they can exist in different crystal modifications which differ in their physical properties [3, 20, 31, 32]. No less than eleven different crystal modifications of phenobarbital are known. It depends on the physical conditions such as temperature which crystal modification is stable. The other modifications are metastable. For substances that exhibit polymorphism, the solubility of the metastable form is higher than that of the stable form. This means that if the metastable modification is in equilibrium with the solution (i.e. saturated for the metastable form), the solution is supersaturated for the stable modification. In other words, the stable form can grow over time at the expense of the metastable form. Some fats also exhibit polymorphism [33]. The preparation method and the storage temperature will influence for example the melting behaviour of suppositories, and thereby probably also the release rate. 

Weissbuch I Leisorowitz L, Lahav M (1994) Tailor-made and chtiige-transfer auxiliaries for the control of the crystal polymorphism of glycine. Adv Mater 6 952—956 Weissbuch I, Zbaida D, Addadi L, Leiserowitz L, Lahav M (1987) Design of polymeric inhibitors for the control of crystal polymorphism— induced enantiomeric resolution of racemic histidine by crystallization at 25-Degrees-C. 1 Am Chem Soc 109 1869—1871 Yamashita K, Nakate T, Okimoto K, Ohike A, Tokunaga Y, Ibuki R, Higaki K, Kimura T (2003) Estabhshment of new preparation method for soUd dispersion formulation of tacrolimus. Int J Pharm 267 79-91... 

Ferroelectric polymer materials like PVDF or its derivatives are mentioned, since they behave as ferroelectric materials (see Fig. 2.2) - They have crystallinity and the crystals show polymorphism by controlling the preparation method. Much detailed work has been carried out on piezoelectric and/or pyroelectric properties, together with their characteristics as electroactive actuators. These materials have long been mentioned as typical electroactive polymers. Through these materials, it is considered that the strain induced in the polymer materials is not large. The electrostrictive coefficient is known to be small for polymers. These are non-ionic polymers and the induced strain originates from the reorientation or the deformation of polarized crystallites in the solid materials. 

Sol-gel techniques are usually employed to produce Ti02 catalysts. The resulting materials are generally amorphous or not well ciystallised and, consequently, they must be subjected to calcination to obtain active samples. In this work we report on the preparation of polymorphic Ti02 nanoparticles obtained by thermolydrofysis of TiCLi in various aqueous solutions at 100° C. The preparation method is very simple and does not require the use of expensive thermal or hydrothermal treatments. The content of anatase, brookite and mtile is easily tailored by varying the composition of the hydrolysis solution. The degradation of 4-nitrophenol was chosen as model reaction to evaluate the photoactivity of the various samples. 

Which polymorphic form of a compound is formed depends on the preparation and crystallization conditions method of synthesis, temperature, pressure, kind of solvent, cooling or heating rate, crystallization from solution, fusion or gas phase, and presence of seed crystals are some of the factors of influence. 

US patent 6,720,453, Formoterol tartrate polymorph [105], A method of preparation of a highly pure salt of R,R-formoterol L-tartrate is disclosed. The process provides the most thermodynamically stable polymorph by recrystallization of a novel polymorph. 

US patent 6,759,521, Polarization switching to control crystal form [113]. This patent describes a method to select and prepare polymorphs of materials by switching the polarization state of light and employing non-photochemical laser-induced nucleation. 

US patent 6,767,921, Polymorphic forms of fasidotril, their methods of preparation and pharmaceutical compositions comprising them [115]. The subject of this invention are polymorphic Forms I, II, III, IV of benzyl (S,S)-2-(2-acetylsul-phanylmethyl-3-benzo[l,3]dioxol-5-ylpropionyl-amino)propionate (i.e., fasidotril), their methods of preparation, and novel pharmaceutical compositions containing them. 

Using zinc oxide as the internal standard, the relative amounts of two polymorphic forms of fenretinide were quantified [55], Mixtures containing 25, 50, and 75% w/w form I were prepared by mixing authentic standards of form I and form II fenretinide. After the addition of the internal standard, the maximum intensities (peak heights) of the 4.6 A line (peak at 19.1° 20) of fenretinide form I (/19. i) and the 2.8 Aline (peak at 31.8° 20) of zinc oxide (/318) were determined. A plot of the intensity ratio (/19.//31.8) as a function of the weight percent form I was linear. The method was reported to be precise and accurate to within 6%. 

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