Current Status of Enthalpy–Entropy Compensation Phenomenon

For similar physical–chemical processes in chemistry and biology, the phenomenon of linear enthalpy– entropy compensation (EEC) is a thermodynamic puzzle remaining unexplained for a long time. The basic thermodynamic rules do not rigorously support the EEC phenomenon. In some restricted conditions EEC may appear linear with nonrealistic (i.e. hypothetical) values of the slope (the compensation temperature), and the intercept (the compensation free energy). The compensation temperature (Tcomp) is normally higher than the experimental temperature. Compensation temperature may even become negative, and the related phenomenon is called anti-enthalpy–entropy compensation (AEEC). Negative Tcomp is unrealistic. Both EEC and AEEC are not explainable; the derived Tcomp and ΔGcomp (free energy of compensation) of the EEC plot are impractical. The neglect of the Gibbs free energy changes (of similar processes in the EEC plot) makes the phenomenon arbitrary. In a restricted condition (i.e. narrow free energy window range) linear compensation is an assumed solution. In overall consideration, the reported correlations are physicochemically uncertain. The said compensation may arise for both kinetic and equilibrium processes. The manifestations are nearly same. Our demonstration and discussion in this paper pertain to equilibrium processes.