Entropy-enthalpy compensation in chemical reactions and adsorption: an exactly solvable model.

The free energies of reaction or activation for many systems respond in a common fashion to a perturbing parameter, such as the concentration of an "inert" additive. Arrhenius plots as a function of the perturbing parameter display a "'compensation temperature" at which the free energy appears to be independent of the perturber, an entropy-enthalpy compensation process. Thus, as the perturber's concentration varies, Arrhenius plots of the rate constant or equilibrium constant exhibit a rotation about the fixed compensation temperature. While this (isokinetic/isoequilibrium) component of the phenomenon of entropy-enthalpy compensation appears in a huge number of situations of relevance to chemistry, biology, and materials science, statistical mechanical descriptions have been almost completely lacking. We provide the general statistical mechanical basis for solvent induced isokinetic/isoequilibrium entropy-enthalpy compensation in chemical reactions and adsorption, understanding that can be used to control of rate processes and binding constants in diverse applications. The general behavior is illustrated with an analytical solution for the dilute gas limit.