Three-Step Mechanism of Antisolvent Crystallization

Synthesis of crystalline materials involves the two most important methods: antisolvent and cooling crystallization. Despite the extensive use of the antisolvent method in the crystallization of various organic and inorganic crystals, the governing mechanism of the antisolvent in activating this process is not fully understood. Thermodynamically, the antisolvent is known to increase the chemical potential, and thereby supersaturation, of solute in the solution leading to crystal nucleation and growth. It is well-known that, before the solute molecules can self-assemble to form crystals, they must leave their solvation shell. Here, we show a previously unrecognized three-step mechanism of antisolvent-driven desolvation, where the antisolvent first enters the solvation shell due to attractive interactions with solute, followed by its reorganization and then expulsion of an antisolvent–solvent pair from the solvation shell due to repulsive forces. To confirm this mechanism, molecular simulations of histidine (solute) in water (solvent) at various concentrations of ethanol (antisolvent) and supersaturation are performed. The simulations reveal competitive binding of ethanol to hydrated histidine followed by its dewetting to allow significant solute–solute interactions for crystal growth. This three-step mechanism is then used to obtain an activation barrier for desolvation of histidine followed by prediction of crystal growth rates using a computationally inexpensive semiclassical approach. Growth rates obtained from the activation barrier reproduce the experimental growth rates reasonably, thereby validating the governing three-step mechanism for antisolvent crystallization.