Pure Curcumin Spherulites from Impure Solution Via Non-Classical Crystallization

Abstract

Crystallization experiments performed with highly supercooled solutions produced highly pure (> 99 wt%) and highly crystalline mesocrystals of curcumin from impure solution (~22% of two structurally similar impurities) in one step. These mesocrystals exhibited a crystallographic hierarchy and were composed of perfectly or imperfectly aligned nanometer thick crystallites. X-ray diffraction and spectroscopic analysis confirmed that the spherulites are a new solid form of curcumin. A theoretical hypothesis based on particle aggregation, double nucleation and a repeated secondary nucleation is proposed to explain the spherulite formation mechanism. The experimental results provide for the first time evidence for an organic molecule to naturally form spherulites without the presence of any stabilizing agent. Control experiments performed with highly supercooled pure solutions produced spherulites confirming the formation of spherulites is attributed to the high degree of supercooling and not due to the presence of impurities. Likewise control experiments performed with a lower degree of supercooling produced impure crystals of curcumin via the classical molecular addition mechanisms. These experimental observations all together provide first time evidence for particle mediated crystallization as an alternate and efficient method to purify organic compounds.