Effect of solvent composition on the van't Hoff enthalpic curve using amylose 3,5-dichlorophenylcarbamate-based...

Van't Hoff plots have been widely used for investigating the thermodynamic properties of adsorption processes in various chromatography systems. By measuring the retention factor k over a certain temperature range, the plot of ln k versus 1/T often yields a straight line with a slope of -ΔH_vH⁰/R. Although this method provides information on adsorption enthalpy changes, its theoretical basis does not account for the effect of the solvent. In this paper, the relationship between apparent enthalpy changes determined directly from van't Hoff plots and solvent modifier concentrations is systematically investigated using three simple solutes-tetrahydrofuran, acetone, and tert-butanol-in an n-hexane-methyl tert-butyl ether (MTBE) mobile phase with an amylose 3,5-dichlorophenylcarbamate-based sorbent. The apparent enthalpy changes of solutes are strongly dependent on MTBE concentration, increasing rapidly with MTBE content at low concentrations but leveling off after ΔH⁰ reaches approximately -15kJ/mol. These data cannot be explained by the thermodynamic model currently used in the literature. A new three-equilibrium-constant thermodynamic model is developed herein to account for solute-sorbent, solvent-sorbent, and solute-solvent interactions. The thermodynamic parameters of the model are estimated from the apparent enthalpy changes at different MTBE concentrations. The results reveal that two key dimensionless groups control the van't Hoff enthalpic curves: the fractions of solute molecules bound to modifier molecules and adsorption sites occupied by modifier molecules. As a result, the shapes of van't Hoff enthalpic curves reflect the adsorption isotherm of MTBE without complexation or information regarding solute-MTBE complexation without MTBE competitive adsorption. The new model is thus demonstrated to be more reliable than the current model for examining the thermodynamic properties of retention mechanisms.