Grinding Method for Phase Transformation of Glycine

Glycine had three polymorphs, two metastable phases (α-glycine, β-glycine) and one stable phase (γ-glycine). However, the phase transformation of glycine from α-phase to γ-phase was well known as the kinetically unfavorable process. In this study, a simple and effective grinding method for phase transformation of glycine from α-phase to γ-phase is proposed. In an aqueous solution, α-glycine and γ-glycine had bulk solubilities of 180 and ∼172 g/L, respectively. According to the Ostwald-Freundlich equation, however, as the crystal size of α-glycine was reduced to ∼0.6 μm by grinding, the saturated concentration of α-glycine increased from 180 to 191 g/L. As long as the solution concentration exceeds a critical point (σ = 0.1), it can be possible to suddenly induce the nucleation of γ-glycine by grinding the α-glycine crystal in the solution. Subsequently, the complete transformation of α-phase to γ-phase was achieved without additives. Similarly, the grinding method was effective for producing the γ-glycine crystal in the cooling crystallization whereas the α-glycine crystal was always produced in the cooling crystallization without grinding. This study showed that physical grinding can effectively facilitate phase transformation by triggering the nucleation of stable polymorph.