Thermodynamics of a complex melting process: the case of spironolactone

An investigation of "melting" in spironolactone under different experimenta l conditions demonstrates that the melting is always accompanied by loss of mass, and by an endothermic and temperature-activated "decomposition" proc ess. However, experiments performed under different atmospheres reveal a mo re complex phenomenology than expected for a simple melting/decomposition p rocess. It is shown that the byproducts of decomposition may undergo an exo thermic process, which we will call "oxidation". The "oxidant" may be the o xygen of the atmosphere or may be provided by decomposition. By combining a number of experiments, we estimate the melting enthalpy, the specific enth alpy of decomposition and that of oxidation. We show that an "adiabatic" ba lance between decomposition and oxidation is eventually attained. The conce pt of "adiabatic balance" provides simple explanations for several experime ntal observations. (C) 2001 Elsevier Science B.V. All rights reserved.