Amorphous quininium aspirinate from neat mechanochemistry: diffracting nanocrystalline domains and quick recrystallization upon exposure to solvent vapours

Abstract

Quininium aspirinate is mechanochemically prepared as a crystalline solid by liquid-assisted grinding, or as an amorphous phase (as determined by X-ray powder diffraction), by neat grinding or neat ball milling. Our previous work demonstrated using FT-IR spectroscopy that a mechanochemical reaction had occurred in the mechanically treated neat mixtures. Herein is reported that microcrystal electron diffraction (microED) afforded the discovery of two diffracting micron-size particles in the amorphous powder synthesized by manual grinding, among a majority of non-diffracting particles. Remarkably, microED data of one of them led to the known lattice parameters of quininium aspirinate. Furthermore, this so-called ‘X-ray amorphous’ phase quickly recrystallizes upon exposure to vapors of N,N-dimethylformamide, or hexane vapours (at a lower rate); but it remains amorphous for longer than 20 months when stored at ambient conditions in a closed container. The lattice parameters and the degrees of crystallinity of both recrystallized materials are identical within the experimental error. However, slightly more intense and better-resolved X-ray powder diffraction peaks are observed in the material recrystallized from N,N-dimethylformamide vapours than in the analogous phase recovered from hexane. As expected, Williamson–Hall graphs lead to a larger average crystalline domain size for the former solid. These results illustrate the use of microED for the investigation of structural features in amorphous phases, and the generic role of the solvent vapours in promoting their recrystallization.