Investigation of the phase transitions occurring during and after the dehydration of xylazine hydrochloride monohydrate

Abstract

This paper reports an investigation of a complex solid state phase transition where two inter-converting polymorphs (X and A) of the pharmaceutical molecule xylazine hydrochloride formed and transformed during and after the dehydration of its monohydrate (H). The crystal structures of all three forms were compared. During the investigation of this solid state phase transition it was determined that the dehydration of H produced either a pure X form, or a mixture of the X and A forms. The phase composition depended on the sample preparation procedure and the experimental conditions. It was found that grinding of the hydrate enhanced the formation of polymorph X as a product of dehydration, whereas higher humidity, temperature, or mechanical compression enhanced the formation of polymorph A. The transition mechanism of this complex process was analysed and explained by taking into account the crystal structures of these three forms.

Keywords: Mechanical compression; Molecular packing; Polymorph; Relative humidity; Sample grinding; Solid state phase transition.