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Open AccessFeature PaperArticle

Thermodynamic vs. Kinetic Basis for Polymorph Selection

Synthesis and Solid State Pharmaceutical Centre, Department of Chemical Sciences, Bernal Institute, University of Limerick, Limerick, V94 T9PX, Ireland
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Processes 2019, 7(5), 272; https://doi.org/10.3390/pr7050272
Received: 28 March 2019 / Revised: 24 April 2019 / Accepted: 2 May 2019 / Published: 9 May 2019
(This article belongs to the Special Issue Modeling and Control of Crystallization)
Ratios of equilibrium solubilities rarely exceed two-fold for polymorph pairs. A model has been developed based on two intrinsic properties of polymorph pairs, namely the ratio of equilibrium solubilities of the individual pairs (C*me/C*st) and the ratio of interfacial energies (γst/γme) and one applied experimental condition, namely the supersaturation identifies which one of a pair of polymorphs nucleates first. A domain diagram has been developed, which identifies the point where the critical free energy of nucleation for the polymorph pair are identical. Essentially, for a system supersaturated with respect to both polymorphs, the model identifies that low supersaturation with respect to the stable polymorph (Sst) leads to an extremely small supersaturation with respect to the metastable polymorph (Sme), radically driving up the critical free energy with respect to the metastable polymorph. Generally, high supersaturations sometimes much higher than the upper limit of the metastable zone, are required to kinetically favour the metastable polymorph. View Full-Text
Keywords: classical nucleation theory; polymorphs; interfacial energy; solubility; supersaturation; metastable zone width classical nucleation theory; polymorphs; interfacial energy; solubility; supersaturation; metastable zone width
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Hodnett, B.K.; Verma, V. Thermodynamic vs. Kinetic Basis for Polymorph Selection. Processes 2019, 7, 272.

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