Leapfrogging Ostwald’s Rule of Stages: Crystallization of Stable γ-Glycine Directly from Microemulsions

Abstract

Crystallization of glycine from aqueous Span 80 (sorbitan monooleate) /Brij 30 (polyethylene glycol dodecyl ether) and AOT (sodium dioctyl sulfosuccinate) microemulsions by adding the antisolvent methanol has been studied as a function of increasing initial supersaturation. All three polymorphs of glycine obtainable at ambient pressure could be crystallized using the Span 80/Brij 30 microemulsions. Crystallization in the Span 80/Brij 30 microemulsions only occurred at initial supersaturations that were significantly higher than those required for glycine crystallization in bulk solution. Crystallization of the most stable γ-glycine polymorph was achieved when crystallization was only just possible in these microemulsions, corresponding to mean initial supersaturation ratios, (c/csat)initial, of 2.3 with respect to γ-glycine. The γ-glycine crystallization occurred even though the water, the methanol, and the surfactants, Span 80 and Brij 30, favored crystallization of the metastable α- and β-polymorphs and despite the glycine solutions being highly supersaturated initially with respect to all three polymorphs. The γ-glycine crystallization arises because crystallization is brought under thermodynamic control under conditions where crystallization is only just possible in the microemulsions. With (c/csat)initial ≈ 2.6 and above, the α- and β-polymorphs became the dominant products in the Span 80/Brij 30 microemulsions for methanol/water mass ratios of 2:1 and 3.8:1, respectively. In contrast, microemulsions stabilized with AOT, which is known to aid γ-glycine crystallization at planar interfaces, led to γ-glycine crystallization at a much lower (c/csat)initial of 1.2. The nonionic Span 80/Brij 30 surfactants were less able to induce nucleation of the metastable forms in the microemulsions due to the high interfacial curvature and ultralow interfacial tension that hindered glycine adsorption at the surfactant interface. The longer range electrostatic interactions in the AOT system, however, could still operate effectively to aid the crystallization of polar γ-glycine.