Ways of manipulating the polymorphism of glycine during supercritical fluid crystallisation

The precipitation of glycine from aqueous solution was studied using a mixture of supercritical carbon dioxide and ethanol as drying medium and as anti-solvent. Glycine, which has three polymorphs, was precipitated by a direct spraying process using a coaxial nozzle under high pressure. By simple manipulation of the solute concentration, the process could be tuned to selectively precipitate either pure α or β-glycine, as determined by X-ray powder diffraction. When decreasing the solute concentration or increasing the ethanol fraction in the system, the precipitation of the metastable β-glycine was preferred over the precipitation of α-glycine. Modelling of the mass transfer around a drop showed that the ethanol fraction inside the drop can reach significant values, and that the evaporation occurs in less than a second, leaving little space for recrystallisation. Even though the crystal growth rate was extreme (up to 800 μm/s), the product was crystalline.