Selective Polymorphism of α-Glycine by Acoustic Levitation

In this work we investigate the crystallisation behaviour of glycine in water and in a binary solvent mixture in an acoustic levitator under controlled environmental conditions. High speed video microscopy was used to monitor the changes in the microdroplet volume upon evaporation of the solvent. The glycine crystals obtained from levitation form an agglomerate, whose exact morphology depends on the solvent system used. The agglomerates have been collected and precisely opened via laser cutting, allowing further investigation of the morphology and structure of the internal crystals. The crystals appear to grow from the external region towards the centre of the sphere, indicating the formation of a solid shell, whose formation depends on the solvent used. The polymorphic outcome was thoroughly investigated by Raman spectroscopy: all of the crystals measured, regardless of the region or the solvent used, were found to be exclusively of the α-form, despite the addition of IPA inducing changes in the induction time and morphology.