Effects of Common Inorganic Salts on Glycine Polymorphic Transformation: An Insight into Salt-Dependent Polymorphic Selectivity

The effects of typical inorganic salts on solution-mediated polymorphic transformation from metastable α-glycine to stable γ-glycine were investigated, with the measurement of induction times of γ-glycine nucleation being the key objective. Interestingly, it was observed that all the inorganic salts examined in this study considerably shorten induction time, showing that they accelerate γ-glycine secondary nucleation to a great extent. Surprisingly, it was found that the divalent cation salts (Ca­(NO3)2 and MgSO4) exert a peculiar effect in that they greatly promote γ-glycine secondary nucleation despite their significant inhibition to γ-glycine growth. Furthermore, it was revealed that the monovalent cation salts (NaCl, KNO3, and (NH4)2SO4) enhance γ-glycine nucleation far more than the divalent cation salts. These obtained results provide an insight into the general observation that the polymorphic selectivity of glycine from unseeded solution crystallization is salt-dependent. All the experimental observations were discussed and explained at the molecular level.