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.
CITATION STYLE
Bernardes, C. E. S., Feliciano, I. O., Naese, C., Emmerling, F., & da Piedade, M. E. M. (2023). Energetics of dehydroepiandrosterone polymorphs I and II from solution and drop-sublimation Calvet microcalorimetry measurements. Journal of Chemical Thermodynamics, 186. https://doi.org/10.1016/j.jct.2023.107137
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