Energetics of dehydroepiandrosterone polymorphs I and II from solution and drop-sublimation Calvet microcalorimetry measurements

• Published in: The Journal of chemical thermodynamics 2023-11, Vol.186, p.107137, Article 107137
• Main Authors: Bernardes, Carlos E.S., Feliciano, Inês O., Naese, Christoph, Emmerling, Franziska, da Piedade, Manuel E. Minas
• Format: Article
• Language: English
• Subjects: Calorimetry; Enthalpy of solution; Enthalpy of sublimation; Lattice enthalpy; Polymorphism; Thermochemistry
• ISSN: 0021-9614; 1096-3626
• DOI: 10.1016/j.jct.2023.107137

•Form I of dehydroepiandrosterone has a larger lattice energy than phase II.•Dehydroepiandrosterone forms I and II are monotropically related.•A new calorimetric cell for solution calorimetry is described and tested. The lattice enthalpies and monotropic relationship of two dehydroepiandrosterone (DEHA) polymorphs (forms I and II) were evaluated through a combination of differential scanning calorimetry (DSC), isothermal solution microcalorimetry, and drop-sublimation Calvet microcalorimetry experiments. The standard molar enthalpy of transition between both forms was determined as ΔtrsHmo(II→I, 298.15 K) =  −0.90 ± 0.07 kJ mol−1 and ΔtrsHmo(II→I, 417.8 K) =  −1.7 ± 1.0 kJ mol−1, from measurements of standard molar enthalpies of solution in dimethyl sulfoxide and enthalpies of fusion, respectively. Drop-sublimation Calvet microcalorimetry experiments on form I led to ΔsubHmo(cr I, 298.15 K) = 132.0±3.3 kJ mol−1. This result, when combined with the more precise ΔtrsHmo(II→I) value obtained by solution calorimetry, afforded ΔsubHmo(cr II, 298.15 K) = 131.1±3.3 kJ mol−1. The overall data indicate that on enthalpic grounds form I is more stable than form II from 298.15 K up to fusion. This conclusion, and the fact that DSC experiments indicated that form I has also a considerably higher temperature fusion, namely, Tfus(cr I)= 422.5±0.2 K and Tfus(cr II) = 413.1±0.2 K, suggest that the two polymorphs are monotropically related.