Interfacial Effect of Water/Oleic Acid Emulsion on Polymorphic Selection in the Cooling Crystallization of Glycine

Abstract

In the crystallization of glycine from the bulk aqueous phase without additives such as pH control agents or surfactants, nucleation of α-glycine is commonly induced. However, in the present work, selective crystallization of γ-glycine was successfully demonstrated from water/oleic acid emulsions without any additives. The carboxyl head groups of oleic acid at the water/oleic acid interface were found to strongly influence the direct nucleation of γ-glycine by interaction with the NH3+-rich (001) face of γ-glycine. Direct nucleation of γ-glycine was preferentially induced when the γ-form was used in solution with a relatively high oleic acid content and slow cooling rate. For example, when an aqueous γ-glycine solution with a saturation temperature of 60 °C was used, γ-glycine crystals were produced with an oleic acid content of 40 wt % at moderate cooling rates of 10 to 45 °C/h. Furthermore, when the oleic acid content was 40 wt % in aqueous α-glycine solution/oleic acid emulsions, γ-glycine crystals were obtained at cooling rates slower than 20 °C/h. However, α-glycine crystals were obtained at a cooling rate of 10 °C/h with an oleic acid content lower than 30 wt %.