Thermodynamics of a complex melting process: the case of spironolactone

An investigation of “melting” in spironolactone under different experimental conditions demonstrates that the melting is always accompanied by loss of mass, and by an endothermic and temperature-activated “decomposition” process. However, experiments performed under different atmospheres reveal a more complex phenomenology than expected for a simple melting/decomposition process. It is shown that the byproducts of decomposition may undergo an exothermic process, which we will call “oxidation”. The “oxidant” may be the oxygen of the atmosphere or may be provided by decomposition. By combining a number of experiments, we estimate the melting enthalpy, the specific enthalpy of decomposition and that of oxidation. We show that an “adiabatic” balance between decomposition and oxidation is eventually attained. The concept of “adiabatic balance” provides simple explanations for several experimental observations.