Modeling the Competition between Polymorphic Phases: Highlights on the Effect of Ostwald Ripening

This work aims to evaluate the effect of Ostwald ripening on the crystallization of polymorphic phases by means of the kinetic equation model, which was adapted to describe the competition between the nucleation, the growth, and the Ostwald ripening of the different phases. The kinetic equation model is a very convenient way to simultaneously implement these three mechanisms and quantify their respective roles on the formation and the dissolution of the clusters. The polymorphic system studied is l-glutamic acid (LGlu), which exhibits two monotropic polymorphs: α LGlu and β LGlu. The model assumptions consider the main differences between both polymorphs (shape, solubility, interfacial energy, and growth rate) as well as the supersaturation decrease. The simulation results at various temperatures show that the crystallization process of small nuclei (<40 nm) can be greatly affected by the Ostwald ripening mechanism. In particular, Ostwald ripening can induce the total dissolution of the stable phase clusters, leading to a result in agreement with the Ostwald rule of stages. Thus, the numerical results suggest that the Ostwald ripening mechanism could explain the Ostwald rule of stages.