‘Hedvall effect’ in cryogrinding of molecular crystals. A case study of a polymorphic transition in chlorpropamide

Abstract

The effect of grinding at ambient temperature and at 77 K (CryoMill Retsch) on the polymorphic transitions in α- and ε- chlorpropamide has been compared. The result of grinding of α-chlorpropamide at room temperature could not be interpreted as a mere formation of traces of the ε-polymorph. The diffraction patterns suggested the presence of small amounts of some unknown polymorph, possibly in a mixture with other polymorph(s). Possibly, mechanical treatment gave a defect nanostructured phase with alternating domains. Cryogrinding of α-chlorpropamide did not result in polymorphic transitions. In contrast, cryogrinding of the ε-polymorph was much more efficient than grinding at ambient temperature: almost no changes could be observed at ambient temperature, whereas cryogrinding gave the α-form. The observed phenomena could be interpreted taking into account that at low temperatures the ε-polymorph undergoes a polymorphic transition into another polymorph— the ε′-form. Without grinding, the ε′-form transforms back to the ε-polymorph when heated back to ambient temperature. If grinding takes place in the temperature range of the ε- to ε′-polymorphic transition, the transformation to the α-form occurs. One phase transition, induced by low temperature, facilitates another one, induced by mechanical treatment. The reason for this interesting phenomenon is to be sought in the similarity of the crystal packing of molecules with different molecular conformations in the ε- and ε′-forms, and of the similarity of the molecular conformations despite different crystal packing in the ε′- and α-forms.