Enthalpy−Entropy Compensation in Hydrophobic Interaction Chromatography

Abstract

Exothermodynamic relationships between thermodynamic quantities and molecular structure are employed to facilitate a molecular interpretation of enthalpy−entropy compensation (EEC). For hydrophobic interactions the compensation temperature T C is expressed in terms of the enthalpy and entropy change, both per unit nonpolar surface area of the molecules, and it is concluded that the utility of T C as a diagnostic tool for the mechanistic identity of processes rests on this simple dependence of T C on molecular parameters. Whereas classical EEC is observed only with processes involving no heat capacity change and T C is evaluated from the slopes of linear enthalpy versus entropy plots of data measured at any temperature, this investigation shows that even when the 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. In turn, the compensation temperature changes with the experimental temperature, reflecting mechanistic changes as expected with processes such as hydrophobic interaction chromatography that are governed by hydrophobic interactions and driven by entropy or enthalpy change at low or high temperatures. These compensating processes exhibit at least one isoenergetic temperature , which marks the intersection point of curved van't Hoff plots, where all species have the same free energy change in the same way as at T C in the case of linear van't Hoff plots. In turn, the isoenthalpic and isoentropic temperatures mark the intersection points of the respective plots of enthalpy and entropy versus temperature as described in the literature. The triad of isothermodynamic temperatures is characteristic for processes which can be represented by constant heat capacity change and evince compensation behavior.

Solvent effect on lipase enantioselectivity. Evidence for the presence of two thermodynamic states

“…EEC effects have been observed for such diverse processes as (1) solution of small molecules in water-alcohol mixtures and denaturation of proteins (Lumry and Rajender, 1970); (2) drug-receptor binding (Gilli et al, 1994); (3) hydrophobic interaction chromatography (Vailaya and Horváth, 1996); and (4) enzyme-catalyzed hydrolysis in organic solvents (Tewari et al, 1995). Just as many explanations for the compensation phenomenon have been proposed.…”

Thermodynamic Interpretation of Retention Equilibrium in Reversed-Phase Liquid Chromatography

“…7a for all the alkyl ligand-bonded silica gels. It is assumed that DH f and DS f are similarly correlated with Nc: 14,23 Fig . 5 Temperature dependence of (a) kC1 and kC18 and (b) kC4 and kC8.…”

“…A number of publications have demonstrated the existence of an EEC on the basis of experimental data [1][2][3][4][5][6][7][8][9] and have supported the possibility of an EEC on theoretical bases. [10][11][12][13][14] Compensation temperatures (Tc) have been reported between ca. 500 and 1000 K for the retention equilibrium under different RPLC conditions of the mobile phase solvents, sample compounds, and temperature ranges.…”

Extrathermodynamic Study of Retention Equilibrium in RP-LC Using a C18-Silica Monolithic Stationary Phase

“…The similarity of the retention behavior in RP-LC was discussed from the viewpoints of the EEC between the enthalpy change and the entropy change. There were some publications reporting the possibility of the EEC on theoretical bases [14][15][16][17][18] and on the experimental observations about the establishment of EEC [19][20][21][22]. The presence of EEC suggests that the retention equilibrium is governed by a single mechanism.…”