ENTHALPY-ENTROPY COMPENSATION IN HYDROPHOBIC INTERACTION CHROMATOGRAPHY

Exothermodynamic relationships between thermodynamic quantities and mo lecular structure are employed to facilitate a molecular interpretatio n of enthalpy-entropy compensation (EEC). For hydrophobic interactions the compensation temperature T-C is expressed in terms of the enthalp y and entropy change, both per unit nonpolar surface area of the molec ules, and it is concluded that the utility of T-C as a diagnostic tool for the mechanistic identity of processes rests on this simple depend ence of T-C on molecular parameters. Whereas classical EEC is observed only with processes involving no heat capacity change and T-C is eval uated from the slopes of linear enthalpy versus entropy plots of data measured at any temperature, this investigation shows that even when t he heat capacity change is finite and constant or varies linearly with the temperature, EEC can occur with processes if they are subject to the same mechanism at a fixed temperature. Tn turn, the compensation t emperature changes with the experimental temperature, reflecting mecha nistic changes as expected with processes such as hydrophobic interact ion chromatography that are governed by hydrophobic interactions and d riven by entropy or enthalpy change at low or high temperatures. These compensating processes exhibit at least one isoenergetic temperature T-G, which marks the intersection point of curved van't Hoff plots, w here all species have the same free energy change in the same way as a t T-C in the case of linear van't Hoff plots. In turn, the isoenthalpi c T-H and isoentropic T-S* temperatures mark the intersection points of the respective plots of enthalpy and entropy versus temperature as described in the literature. The triad of isothermodynamic temperature s is characteristic for processes which can be represented by constant heat capacity change and evince compensation behavior.