Entropy-enthalpy compensation: Perturbation and relaxation in thermodynamic systems

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Abstract
The response of an equilibrium molecular system to perturbations depends on its environmental constraints. For example, the response of an equilibrium P, V, T system to a small temperature perturbation (specific heat) depends on whether the environmental constraint on the system is constant pressure or constant volume. In general, there are two classes of thermodynamic quantities of a system; one is completely determined by its equilibrium distribution, and the other also depends on how the distribution responds to macroscopic changes. The former class is independent of the environment of the thermodynamic system, while the latter class is a function of environmental constraints. In response to a small perturbation, the free energy change of an equilibrium system belongs to the first class but the entropy and enthalpy changes belong to the second. Therefore the thermodynamics of perturbation exhibit compensation between entropy and enthalpy of systems with different environments. The thermodynamic analysis presented here provides a framework for the interpretation of experimental observations of this phenomenon, and is illustrated by a real experimental example.