Physicochemical Understanding of Polymorphism and Solid-State Dehydration/Rehydration Processes for the Pharmaceutical Material Acrinol, by Ab Initio Powder X-ray Diffraction Analysis and Other Techniques

The extensively used antibacterial agent acrinol is known to form a monohydrate phase (H) on crystallization from water. We demonstrate here that acrinol also forms two anhydrous polymorphs, denoted form I (AI) and form II (AII). Polymorph AI is obtained directly on heating H, by a solid-state dehydration process, and polymorph AII is obtained subsequently from AI, by a polymorphic transformation. The crystal structures of AI and AII have been determined directly from powder X-ray diffraction data, employing the direct-space genetic algorithm technique for structure solution, followed by Rietveld refinement. From the structural properties of AI and AII, mechanistic aspects of the dehydration process and the polymorphic transformation have been established. From measurements of initial dissolution rates and enthalpies of dissolution, AII is assigned as the thermodynamically stable polymorph, with a monotropic relationship between AI and AII. The hydration properties of AI and AII differ significantly, with hydration of AI occurring at substantially lower relative humidity than AII.