Inverse Transformation of Glycine by Crystal Size Control

This study proposed a method for inverse phase transformation of stable γ-glycine into metastable α-glycine by using the characteristic that solubility increases as crystal size decreases according to the Ostwald–Freundlich equation. First, we measured the change in solubility according to the particle size of glycine. The solubility of γ-glycine bulk crystals at 10 °C in water was 173 g/L, and when the crystal size decreased to about 0.9 μm, the solubility increased to about 185 g/L. This concentration was higher than the solubility of α-glycine bulk crystals, 180 g/L. Based on the above results, γ-glycine can be inverse transformed into α-glycine in aqueous solution. To demonstrate this inverse transformation, in a glycine solution, γ-glycine crystals with a size of about 2 μm were ground with glass beads for 24 h to reduce the crystal size to about 0.8 μm. And the concentration of the solution was made higher than the solubility of α-glycine bulk. α-Glycine bulk crystals (about 110 μm) were placed into this solution and grown to 170 μm. Through this, inverse phase transformation was achieved in which γ-glycine crystals were dissolved and α-glycine crystals grew. The above inverse phase transformation process was confirmed using a microscope, XRD, and ATR–FTIR.