Direct Growth of γ-Glycine from Neutral Aqueous Solutions by Slow, Evaporation-Driven Crystallization

This study reports the selective growth of γ-glycine crystals via concentrating microdroplets of aqueous glycine solutions through slow evaporation of water using an evaporation-based crystallization platform. In prior studies, γ-glycine crystals could only be obtained from non-neutral pH solutions, by applying electromagnetic fields, or in the presence of impurities that suppress the formation of the kinetically favored α-glycine polymorph. Here in our work, pure γ-glycine crystals form below a certain rate of evaporation (i.e. below a certain rate of supersaturation). Below this rate the crystallizing solution stays close to equilibrium throughout the evaporating process, allowing the system to sample the lowest free energy state during the formation of nuclei. These results point to the interplay of kinetic and thermodynamic effects on selective crystallization of different polymorphs. Polymorphic analysis was performed by examining all samples as randomized polycrystalline particles. The resulting multiframe diffraction patterns were combined to generate a single powder X-ray diffraction (PXRD) spectrum of each sample. In comparison to traditional powder diffraction methods, the quantitative polymorphic analysis procedure reported here eliminates the need to mechanically grind crystalline material, thereby avoiding the potential for undesired polymorphic transformations prior to data collection.