Crystallisation of glycine and dipicolinic acid (DPA) from microemulsions

Abstract

Crystallisation under confinement conditions can produce nanocrystals with high surface-to-volume ratio, altered intrinsic properties (e.g. melting point depression), preferred orientation, selective polymorphic form and well-controlled morphology. The confinement conditions are normally achieved by nanopores prepared from polymers, which provide physical constraints on the size of the crystal. In this thesis, microemulsions, which have been extensively used for nanoparticle synthesis, are employed to achieve the 3D-nanoconfinement, owing to their spherical morphology to provide volume control and excellent droplet size control (< 50 nm) via the addition of dispersed phase.

Thermodynamic control has been achieved during the crystallisation of glycine under the 3D-nanoconfinement generated by microemulsions. These microemulsions were prepared from heptane, Span 80/Brij 30, glycine solution, and methanol antisolvent. Polymorph selection has been obtained by varying the microemulsion compositions. A majority of y-glycine was observed when the microemulsion droplet size was in the range of 3.3-4.2 nm. This is when the microemulsions just had sufficient glycine for crystallization. Below this, no crystallization was observed. The microemulsions comprised of 0.25 g 4% glycine solution and 0.8-1.5 g methanol per sample, with crystallisation yielding visible crystals in 2-3 weeks. Higher concentration of glycine solution (> 4.5%) yielded mostly the α form and excess amount of methanol (> 1.5 g) produced the β form, due to the loss of confinement within the microemulsion and the participation of solvent templating effects introduced by methanol. The polymorph outcome was not affected by the surfactant-continuous phase ratio.

Following on from the success of glycine and other organic/pharmaceutical compounds studied within the research group, dipicolinic acid (DPA) was chosen to study the thermodynamic control of hydrates within 3D-nanoconfinement. The hydration or solvation control was demonstrated in the microemulsions. In particular, hydration was only observed when the droplet sizes were greater than 4.3 nm with 15 mg/ml of DPA solution in the AOT system. However unexpected formation of metal salts occurred, i.e. Na+ and K+ ions from impurities participated in the crystal structures, regardless of the surfactants or solvents employed. Upon treatment with 2M HCl solution, metal salt participation in DPA crystallisation could be prevented. From the acidified microemulsions employing Trion X-100/1-hexanol as surfactants, no macroscopic sized crystals were seen. Instead, square plate-shaped nanoaggregates (30–100 nm in dimension) of 2–10 nm nanoparticles were obtained for samples with a droplet size of 4.01 nm, prepared from 18 mg/ml DPA in 2M HCl solution. Despite the strong electron density contrast across the nanocrystal surface, single crystal-like diffraction pattern (DP) was seen from these crystals, which was considered to be consistent with these nanocrystals being iso-oriented crystals/mesocrystals. The size and morphology of these nanoparticles can be controlled by varying the microemulsion composition. Lower DPA concentrations of 10-12 mg/ml and smaller droplet sizes of 2.40-3.15 nm produced much less organised nanoaggregates, with 'arcing' and 'rings' appearing in the DP, indicating the polycrystalline nature, whereas the highest DPA concentration of 18 mg/ml and the largest droplet of 4.01 nm could produce organized nanoaggregates that were barely distinguishable from single crystal structure.