Thermal properties of drug polymorphs: A case study with felodipine form I and form IV

Abstract

Thermal decomposition kinetics and solution thermodynamics of two polymorphs, felodipine form I and form IV, were investigated. The thermal decomposition kinetics of the polymorphs, containing thermal decomposing mechanism and the kinetic parameters were studied under non-isothermal conditions using Popescu method, and the shelf life was simply calculated. The results showed that nucleation and growth (n = 3/4) of Avremi–Erofeev equation is the most probable mechanism function for form I, and the integral form is G(α) = [−ln(1 − α)]3/4; while the Mample Powel law (n = 1) is the most probable mechanism function for form IV, corresponding to G(α) = α. Notably, the individual amorphous phases of crystal felodipine form I and IV were obtained after the heating–cooling cycle of DSC tests, which were identified by TMDSC and FT-IR measurements. As the interim products before the collapse, it can be inferred that different amorphous intermediates may be the determinant for different thermal decomposition mechanisms of crystal forms I and IV. The solubility data and solution thermodynamic parameters, including enthalpy, entropy and Gibbs free energy have also been calculated by Van’t Hoff equation in ethanol aqueous. The results illustrated the polymorphic pair is enantiotropic with the transition temperature of 322.23 K and the conversion is driven by entropy.