Crystal polymorphic transformation

Due to the difference of the polymer adsorption onto different faces of different crystal forms, polymer adsorption onto the crystal face has played an important role in crystal polymorphic transformationJ Garti and Zour have studied the effects of surfactants on the polymorphic transformation of glutamic acid. Glutamic acid has two crystal forms, a and p, with the p-form being more stable than the ot-form. Those surfactants that preferentially adsorb onto the surface of the ot-growing crystals retard the transformation of the ot-form to the p-form. A Langmuir analysis indicates that the kinetic coefficient of crystal polymorphic transformation is related to the volume of the surfactant adsorbed at the crystal surface. 

Experimental determination of phase diagrams is convenient by using the thermal analysis method at which the temperature of the investigated sample is registered at its cooling by a constant rate of 2-5°C/min. Due to the thermal effects connected with the phase transformations (crystallization, polymorphic transformation), breaks appear on the cooling... 

Polymorphs are different crystal forms of the same compound [55], They are usually prepared by crystallization of the drug from different solvents under diverse conditions. However, exposure to changes in temperature, pressure, relative humidity, and comminution which are encountered in processes such as drying, granulation, milling, and compression may also lead to polymorphic transformations. 

Preferred orientation of the particles must be minimized. One of the most effective ways to achieve this is to reduce the particle size by grinding the sample [1], As already discussed in Section III.A, however, grinding can disorder the crystal lattice. Grinding can also induce other undesirable transitions, such as polymorphic transformations [59]. In order to obtain reproducible intensities, there is an optimum crystallite size. The crystallites have to be sufficiently small so that the diffracted intensities are reproducible. Careful studies have been carried out to determine the desired crystallite size of quartz, but no such studies have been reported for pharmaceutical solids [60]. Care should be taken to ensure that the crystallites are not very small, since decreased particle size can cause a broadening of x-ray lines. This effect, discussed earlier (Eq. 9), usually becomes apparent when the particle size is below 0.1 jum. 

Microscopy is used to identify crystal morphology and size, and to assess physical form issues like agglomeration and solvent occlusion. It can be used to observe polymorphic transformations in real time with the addition of a hot stage fitting. 

Near and Mid infrared spectroscopy can be used to monitor crystal form and polymorphic transformations, as well as solution phase concentration. 

FBRM provides a real time chord length distribution within the crystallizer. The technique is excellent for determining the onset of nucleation and in following general growth rate and rate transitions associated with polymorphic transformation, reaching equilibrium and attrition effects. 

Their normal crystal structure, at ambient conditions, corresponds to the body-centred cubic cI2-W-type structure. At very low temperatures, the close-packed hexagonal hP2-Mg-type structure has been observed for Li and Na, while for Rb and Cs the face-centred cubic close-packed cF4-Cu-type structure is known at high pressure. No polymorphic transformation has been reported for potassium. 

In 2007, in a very exhaustive paper, Paradies and coworkers carried out a comprehensive structural characterization of the colorless and yellow forms of Af-hydroxyphthalimide (NHPI) by means of single-crystal X-ray diffraction, FTIR and Raman spectroscopies and scanning electron microscopy. In the yellow form, the Af-hydroxyl group is significantly out of the plane (1.19°), but the Af-hydroxyl group in the colorless form is only 0.06° out of the plane. The irreversible conversion of the colorless crystalhne form to the yellow crystalhne form is more like a dynamic isomerism than a polymorphic transformation. 

The change of the crystal structure of (l- -3)-a-D-glucan should be compared to the case of (l- -3)-3-D-glucan (curdlan). Although curdlan changes its polymorphic form by annealing (6), neither the fiber patterns (samples B and C) nor the powder patterns (samples A and D) underwent polymorphic transformations due to annealing. 

N. Garti, H. Zour, The effect of surfactants on the crystallization and polymorphic transformation of glutamic acid, J. Cryst. Growth 172 (1997) 486-498. 

The dispersed state has a considerable effect on fat crystal polymorphism. Lopez et al. (2000, 2001c) observed that crystallization in milk fat globules is more disordered than in bulk fat. On slow cooling, milk fat crystallizes in the a form in cream (Lopez et al., 2001a), whereas in anhydrous milk fat, it crystallizes first in the (3 form and then in the a form (Lopez et al., 2001b). Rapid cooling of cream or anhydrous milk fat from 60 to 4°C leads to the formation of a crystalline structures, which transformed into (3 structures... 

The results discussed show that crystal structure transformations are considerably dependent on the thermal history of the samples to be more specific, the crystallite size, particle size and surface area have measurable effects on the transformation. It would, therefore, probably be difficult to reproduce strictly transformation data with different samples. The magnitudes of these effects are, however, not too great to result in the wide variability of temperatures of polymorphic transformations. The wide variations in transformation temperature can only be due to other factors... 

The notion that mechanical stress may cause solid-to-solid phase transitions or other physicochemical transformations is well known in materials sciences. However, examples of molecular crystals undergoing mechanically induced phase transitions or polymorph — pseudo-polymorph transformations between crystalline phases are not very abundant some recent entries in the literature can be found in Ref. 74. 

From the snap, gloss and texture of chocolate to the shelf life of frozen foods, crystalline microstructure plays a very important role in the texture, appearance, shelf life and overall quality of many foods. The total amount of crystaUine phase in a food, as well as the size distribution and shape of the crystals within the food, can affect the physical properties of the product. Furthermore, some mataials in food can crystallize in different polymorphic forms so that control of polymorphic transformations may also be necessary. 

Black and Davey (1988) describe a number of the interrelationships and practical aspects of the control of nucleation, crystal growth, and polymorphic transformation of amino acids. The factors described and demonstrated for primary nucleation of L-glutamic acid include temperature, critical nucleus, relationship of interfacial tension to solubility, thermal history, induction time, agitation, and effect of additive. 

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