Thermodynamic Interpretation of Retention Equilibrium in Reversed-Phase Liquid Chromatography

A new model was developed for explaining the retention behavior in reversed-phase liquid chromatography (RPLC) on the basis of two extrathermodynamic relationships, i.e., enthalpy-entropy compensation (EEC) and linear free energy relationship (LFER). Krug’s four tests were used for analyzing the temperature dependence of the retention equilibrium constant normalized by alkyl ligand density. It was demonstrated that a real EEC takes place in the retention equilibrium. A new model based on the real EEC was developed to explain the LFER concerning the retention equilibrium. The model formulates the slope and intercept of the LFER with the compensation temperatures and molecular thermodynamic parameters relating to the changes in enthalpy and entropy. The values of the retention equilibrium constant under different RPLC conditions were estimated with a relative error of 6.9%. The new model provides an alternative approach for interpreting the retention equilibrium in RPLC from the viewpoints of thermodynamics and extrathermodynamics.