Exothermic contributions in the DSC signal of endothermic effects

Abstract The understanding, qualitative interpretation and quantitative evaluation of differential scanning calorimetry (DSC) curves is not always an easy task, e.g., due to asymmetric shape of the peak, overlapping effects, etc. Recently, the asymmetry of some endothermic DSC peaks was attributed to the existence of exothermic contributions related to decomposition and the corresponding formation of vapor phase and mass loss. Scope of this theoretical work is to provide unequivocal evidence that the production of vapor phase in a primarily endothermic effect results in exothermic contributions of the DSC signal. The thermodynamic equations describing the DSC signal are derived for the general case of variable specific heat capacity as well as variable mass sample. A simple case of mass loss (desorption of impurities from a solid) is studied under different conditions. It is shown that the exothermic contributions due to mass loss are always present and influence the overall DSC endothermic peak (its area and baseline). The severity of this influence depends on the relative contributions of heat capacities of the various phases and the heat of desorption as well as on the pan (closed or open) that is used for the measurement. The results of this work may be useful for the interpretation of DSC curves regarding complex thermal phenomena which overlap with mass loss.