Sustainable deep eutectic solvents induced the polymorph selectivity and high purification efficiency

Deep eutectic solvents (DESs), as modern green solvents, have been examined as sustainable crystallization media in recent years. Nevertheless, their roles in affecting nucleation and polymorph control of crystal materials are still poorly understood. Herein, we report the use of DES for controlling polymorph crystallization and investigate systematically the mechanism of DES solvent characteristics on polymorph selectivity. Concomitant crystallization of sulfathiazole (STZ) polymorphs has been widely reported in organic solvents, while we found DES can improve its polymorph selectivity significantly. Pure forms II and IV were selectively and rapidly obtained from solvents containing [thymol][fatty acid] and [thymol][coumarin] DESs, respectively. The microstructural changes of DES exhibited dynamic regulatory effect on STZ polymorph selectivity. Analysis of 1H NMR, viscosity and computational simulations revealed that the DES hydrogen-bonding network structures were preserved for DES contents above 50 wt% in binary solvents, which affected the nucleation kinetics of STZ and its molecular assembly during crystallization process. Further mechanistic investigations suggested that DES hydrogen-bonding network structures exerted confinement effect and nucleation-templating effect through DESsolute interactions (hydrogen-bonding and 7C-7C stacking interactions for [thymol][fatty acid] and [thymol] [coumarin] systems, respectively), which limited the motion of solute molecules and delayed nucleation rate as well as induced the STZ nucleation toward specific orientations, thus achieving polymorph selectivity. In addition, DESs could be recycled and exhibited good reproducibility in achieving STZ polymorph purification. This study offers a sustainably effective strategy for polymorph purification of crystalline materials and promotes advance the fundamental understanding of DES solvent characteristics on polymorph control.