Crystal growth of α and γ glycine polymorphs and their polymorphic phase transformations

Abstract

The single crystals of α and γ polymorphs of glycine were grown from aqueous solutions at ambient conditions. The metastable α-form was crystallized from pure aqueous solution and the stable γ-form from aqueous solution in the presence of a critical concentration of a selective additive sodium chloride. Attempt to grow sizable β-form single crystals were not successful because of their high instability. In stability terms at ambient conditions these three glycine polymorphs are like this: β<α<γ. The form of crystallization was confirmed by X-ray powder diffraction (XRD). The α- and β-forms crystallize in the monoclinic system with centrosymmetric space groups P2 1/n and P2 1, respectively, and the γ-form crystallizes in trigonal–hexagonal system with non-centrosymmetric space group P3 2. At high-humidity conditions, the room temperature polymorphic phase transformations from α to γ and solution-mediated phase transformation from β to α were confirmed by XRD. The transformation from γ to α while heating well above the room temperature observed at around 179 °C was recorded by thermogravimetry (TG) and differential thermal analysis (DTA) and confirmed by differential scanning calorimetry (DSC). The morphology of the grown polymorphs were analysed by goniometry. CHN analysis shows the non-existance of sodium chloride species in the grown γ-glycine single crystals. The optical transparancy of the γ-form was recorded by UV–vis–Near IR spectroscopy.