An Adventurous Journey Around the Thermodynamics of Liquid Chromatography

The characterization of the thermodynamics of separations, especially the calculation of the standard molar enthalpy and entropy through the van ’t Hoff plot, is a popular approach among chiral chromatographers, although it is already known that this representation was adopted into chromatography without real critical consideration. In our work, chiral chromatography is modeled with the column-coupling method to obtain the contribution of each adsorption site. Besides the original problems of the van ’t Hoff plot, it was found that the respective change in entropy (ΔS) and change in enthalpy (ΔH) values of each adsorption site differs significantly from the values obtained when both adsorption sites are present. The circumstances of the chromatographic adsorption were also examined to see whether the calculated values are important, or unique values are obtained for each system. Pressure especially affects the determined values, thus the chromatographic system used—high performance liquid chromatography (HPLC) or ultrahigh-pressure liquid chromatography (UHPLC)—has to be distinguished. We also have to ask: Is there any suitable equation that is less problematic to calculate thermodynamic quantities?