Determination of the Relative Stability of a Multipolymorph System via a Novel Pure Component Free Energy Calculation

The determination of the relative stability of multipolymorph compounds is very important to chemical processes and product development. Two thermodynamic methods are used in this study: a eutectic method and a pure component method. The eutectic method, requiring suitable additives to form eutectic mixtures,(1) was reported by L. Yu in 2005. The pure component method is newly developed by the authors. This method involves two innovative approaches. First, thermodynamic equations are derived for the Gibbs free energy difference between a pair of polymorphs, based upon a pathway from one polymorph, through a supercooled liquid, to another polymorph. Second, HyperDSC is used for the accurate measurement of heat capacity and melting data for each polymorph at a high heating rate to avoid form conversion/recrystallization between polymorphs and potential product decomposition. A polymorphic system consisting of five polymorphs is used for demonstrating these two methods. The Gibbs free energy difference between polymorphs with respect to the reference polymorph calculated from the novel pure component method is comparable with that from the eutectic method. The locations of the crossover temperatures from these two methods are in very good agreement with results from the experimental competitive slurrying experiments.