Effects of structurally – related impurities on the crystal growth of curcumin spherulites

Abstract

The crystal growth of curcumin in pure propan-2-ol and in this solvent containing two structurally related impurities, demethoxycurcumin (DMC) and bisdemethoxycurcumin (BDMC), has been investigated by seeded isothermal desupersaturation experiments at 283, 293 and 308 K. In situ attenuated total reflectance UV-visible spectroscopy and focused beam reflectance measurement were used as process analytical technologies to monitor the solution concentration over time and to track the particle counts. Both impurities are found to slow down the growth of curcumin spherulites. The product particles collected after growth in the presence of the impurities present a rougher and more porous surface appearance in comparison to curcumin crystalline material grown in pure solutions. A detailed analysis of the powder X-ray diffraction patterns along with compositional analysis by high performance liquid chromatography of grown crystals reveals that impurities are not incorporated into the solid phase except at the highest impurity concentrations explored. By molecular modelling it is shown that the influence of impurities on the diffractograms of the material grown at high impurity concentrations is consistent with the formation of a solid solution. By fitting the Birth and Spread theory to the experimental growth data it is found that the interfacial energy for growth in the presence of the impurities is higher than that for growth of curcumin in the absence of impurities. Two scenarios are envisaged explaining the results. The first is that 3-D nucleation occurs on the impurity encumbered CUR crystallite generating new crystalline material on which crystal growth may continue. The second scenario envisages that a molecular cluster from the solution attaches to the impurity-encumbered surface and condenses into a crystalline surface nucleus without perfect lattice matching.