High temperature polymorphic conversion of carbamazepine in supercritical CO2: A way to obtain pure polymorph I

Article abstract of DOI:10.1016/j.molliq.2020.114630

In this work, we studied the high-temperature polymorphic conversion of carbamazepine (CBZ) in a high-density supercritical CO₂ (scCO₂) medium in the temperature range of 110–200°C. In order to understand the mechanism of transformation we performed a detailed IR analysis of the supercritical fluid (SCF) phase being in permanent contact with an excess of CBZ. We studied the conformational equilibrium of CBZ molecules in scCO₂ phase under isochoric heating conditions. Three temperature ranges, where different types of CBZ–scCO₂ equilibria are realized, were considered: i. ?CBZ solid – SCF solution?; ii. phase transition region related to the melting of CBZ in high-density scCO₂; iii. ?CBZ melt – SCF solution?. An analysis of the IR spectroscopy data on CBZ dissolved in the scCO₂ phase obtained for the third temperature range shows that when ?CBZ melt – SCF solution? equilibrium exists, the scCO₂ phase contains only one CBZ conformer. Relying on this finding, we hypothesized that it is possible to obtain pure CBZ polymorph I by the crystallization from CBZ solution in scCO₂. This statement has been proven by the micro-Raman analysis of the crystalline substance being obtained from such fluid solution.