Mechanism of Surfactant-Mediated Morphology and Growth Kinetics Regulation of Adipic Acid Crystals

The precise control of crystal morphology plays a significant role in ensuring product quality in the pharmaceutical and chemical industries. However, the underlying regulating mechanisms and their impact on crystallization kinetics of crystals still require further elucidation. Herein, adipic acid, which typically exhibits a flaky morphology, was selected as the model compound to investigate the regulating mechanism of surfactants on the morphology evolution process and the crystallization kinetics. Combined with visualization experiments and growth kinetics analysis, the relative growth rate of different crystal faces was determined and a mathematical model correlating crystal growth rate and supersaturation was established. Furthermore, density functional theory (DFT) calculations combined with molecular dynamics simulations were adopted to investigate the regulating mechanisms. The results demonstrate that the polar head groups of surfactants interact with the carboxyl groups exposed on the adipic acid crystal faces via hydrogen bonding and van der Waals forces, which hinders the incorporation of solute molecules into the crystal lattice and thereby suppresses the growth of the crystal faces. Overall, the specific binding of surfactants to different crystal faces allows for the control of their growth rates, enabling the directed regulation of adipic acid crystal morphology from a flaky habit to a bulk-like habit.