Polymorphic transformation of chlortetracycline hydrochloride crystals studied by infrared spectrophotometric method

A comprehensive spectroscopic study of the polymorphs of diflunisal and their phase transformations

Understanding phase transitions in pharmaceutical materials is of vital importance for drug manufacturing, processing and storage. In this paper we have carried out comprehensive high-resolution spectroscopic studies on the polymorphs of the non-steroidal anti-inflammatory drug diflunisal that has four known polymorphs, forms I-IV (FI-FIV), three of which have known crystal structures. Phase transformations during milling, heating, melt-quenching and exposure to high relative humidity were investigated using Raman and terahertz spectroscopy in combination with differential scanning calorimetry and X-ray powder diffraction. The observed phase transformations indicate the stability order FIII>FI>FII, FIV. Furthermore, crystallization experiments from the gas phase and from solution by fast evaporation of different solvents were carried out. Fast evaporation of an ethanolic solution below 70°C was identified as a reliable and convenient method to obtain the somewhat elusive FII in bulk quantities.

Characterization of the solid state: quantitative issues

Quantitative analysis of solid state composition is often used to ensure the safety and efficacy of drug substances or to establish and validate the control of the pharmaceutical production process. There are a number of common techniques that can be applied to quantify the phase composition and numerous different methods for each technique. Each quantitative option presents its own issues in ensuring accuracy and precision of the solid state method. The following article describes many of the common techniques that are used for quantitative phase analysis and many of the considerations that are necessary for the development of such methods.

The kinetics of solvent-mediated phase transformations

A number of mineralogical and synthetic precipitates undergo solid to solid phase changes via a solution phase. A review of the literature reveals a lack of both experimental data and a framework for its interpretation. A model is developed, for the case of a polymorphic phase transformation, which involves the dissolution of the metastable phase and growth of nuclei of the stable phase. The concepts of dissolution and growth time scales have been introduced and it is shown that their sum is the time required for the disappearance of the metastable phase. Mechanistic insight is best obtained by measure­ment of the supersaturation profile rather than conversion data. It is shown that such profiles are dominated by the plateau supersaturation, which is the point at which dissolution and growth are balanced. Its value is determined by the relative surface areas of the phases and their kinetic constants. The model has been successfully used to simulate available kinetic data for the α → β polymorphic transformation in copper phthalocyanine.

Kinetic study of the polymorphic transformations of phenylbutazone

The polymorphic transformations of phenylbutazone from metastable forms alpha and beta to stable form delta were studied quantitatively at four temperature and five humidity levels by X-ray powder diffractometry. The transformation of form alpha conformed with the Avrami-Erofe'ev kinetic model and form beta conformed with apparent first-order kinetics. In the two transformation systems, the induction periods depended on the storage conditions and were prolonged with lowering of temperature and humidity. The transformation rate of form alpha was not affected by humidity, whereas that of form beta increased according to a rise in humidity. The temperature dependency of the transformation rate constant was remarkable. The Arrhenius treatment was applicable to the beta----delta transformation at low temperatures. The overall half-life, including induction period, revealed that form alpha was more stable than form beta under any storage condition. A good linear relationship existed between the induction period and the transformation rate constant, irrespective of the storage conditions. The scanning electron photomicrographs of forms alpha and beta demonstrated that acicular crystals of form delta grew as the transformation progressed. This could be confirmed as the change in particle diameter of the samples.