Structural Self-Destruction of Deep Eutectic Solvents Induced the Formation of Unusual Diamond-Shaped Crystals of Honokiol and Its Mechanistic Study

Deep eutectic solvents (DESs), as one type of modern green solvent, have gained increasing interest in regulating the crystallization process. Crystal habit is one of the crucial physical properties affecting product processing and the quality of pharmaceutical preparations. Here, a highly unusual diamond-shaped crystal of honokiol (HON, a natural bioactive compound with multiple pharmacological activities) was discovered from volatile HON-based DES. In non-DES solvents, HON grows as hexagonal crystals with the same growth mode along the opposite directions of the nonpolar b-axis. In contrast, the (010) face along the b-axis disappeared in volatile DESs, breaking the original crystal symmetry. Surface tension analysis revealed that the volatilization of co-formers (i.e., menthol and camphor in this work) in the DES destroyed its hydrogen bond network, generating two crystallization environments for HON. Primary trapezoidal HON crystals formed in the hole environment after evaporation of co-formers at an early stage. The (010) face of trapezoidal crystal continuously grows in the molecular cluster environment due to its highest roughness and preferential adsorption with DESs and finally disappears to form diamond-shaped crystals. Furthermore, the diamond-shaped crystals show improved dissolution behavior and powder flowability compared to the hexagonal crystals. This study clarifies the DES supramolecular structure changes caused by its component volatilization and its regulating mechanism on the HON crystallization process, which might provide a new route and guidance for the crystal morphology modification of organic small molecules.