Solid-state phase transformations and storage stability of curcumin polymorphs

Curcumin crystallizes in three polymorphic forms. Form 1 exists as a monoclinic structure, whereas Forms 2 and 3 exist as orthorhombic structures. The aim of this work was to understand the polymorphic behavior of curcumin and the stability of the forms of curcumin. Liquid antisolvent precipitation of curcumin in the presence of ultrasound and additives results in the precipitation of curcumin in orthorhombic forms (Form 2 or Form 3), whereas raw curcumin and curcumin particles precipitated without ultrasound and without additives exist in monoclinic form. Differential scanning calorimetery (DSC) experiments reveal that both of the orthorhombic forms (Forms 2 and 3), which precipitated in the presence of ultrasound and additives, transform to a monoclinic form (Form 1) upon heating. The orthorhombic form (Form 3) in particles, which precipitated in the presence of ultrasound only (no additives), transforms directly to a monoclinic form (Form 1) upon heating. On the other hand, the orthorhombic form (Form 3) in particles, which precipitated in the presence of additives such as HPMC and BSA along with ultrasound, first transforms to Form 2 before finally converting to Form 1 upon heating. All of these transformations occur below the melting points of all three forms. These polymorphic transformations also result in a visible change in particle morphology from rice seed-like or spherical to acicular. While Form 2 and Form 3 were found to be unstable at higher temperatures, these forms, when stored in aqueous suspensions at room temperature (25 C), were found to be stable even after 3 years, with no significant change in particle size or morphology.