Isothermal and Non-Isothermal Measurements Can Yield Different Activation Energies From Thermal Analysis

Thermal measurements applying differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) are commonly used in the study chemical reactions and phase transformations involving the solid state. Because these techniques typically apply heating (sometimes cooling), it is necessary to understand how the kinetics are affected compared to isothermal measurements obtained using the same instrumentation. This issue has resurfaced thanks to a recent article [Vyazovkin, S. Correlation between activation energy and reaction temperature as observed in thermal analysis kinetics. Thermochim. Acta 2025, 743, 179911] that revealed strong evidence of unexpected linear correlations between the activation energy, E, and the reaction temperature, T, across different classes of conversions interrogated with non-isothermal kinetic methods. This work rationalizes the physical origin of that finding and proposes why, under isothermal conditions, the corresponding E values should remain temperature-invariant. Unlike the Kissinger method, isoconversional methods should be unaffected by treating non-isothermal kinetic data.