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

Abstract

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.