Reliable and Selective Crystallization of the Metastable <i>α</i>-Form Glycine by Seeding

This paper describes a reliable method to selectively produce metastable α-form glycine crystals with a controlled size in batch cooling crystallization. Seeding had a large effect both on the polymorphism and the size distribution of the product crystals. In the case of sufficient seed loadings of the α-form crystals, the metastable α-form crystals were obtained selectively as a product. Simultaneously, the crystal size distribution became unimodal with no fine crystals. However, at low seed loadings, mixtures of both the metastable α- and stable γ-form crystals were obtained. The measurement of supersaturation transient suggested that the low supersaturation caused by the growth of sufficient seeds played a key role in suppressing nucleation of the metastable α- and γ-form crystals, whereas, with no seeding, only the stable γ-form crystals were obtained. The seeding technique presented here could be a general and practical method to produce metastable polymorphs with controlled crystal size.