Thermodynamics and kinetics of solute transfer in reversed-phase liquid chromatography: Effect of annelation in polycyclic aromatic hydrocarbons

Abstract

A series of four-ring polycyclic aromatic hydrocarbons (PAHs) with varying annelation structure was studied by reversed-phase liquid chromatography. Using a polymeric octadecylsilica stationary phase over a temperature range from 273 to 303K and an average pressure range from 585 to 3585psi (1psi=6894.76Pa), the thermodynamic and kinetic aspects of the retention mechanism were examined. Thermodynamic behavior was characterized by the retention factor, together with the associated changes in molar enthalpy and molar volume, whereas kinetic behavior was characterized by the rate constants, together with the associated activation enthalpies and activation volumes. The data indicate that pyrene, with a more condensed annelation structure, exhibits smaller changes in molar enthalpy and molar volume (ΔHsm=−4.4kcal/mol, ΔVsm=−1.9ml/mol; 1cal=4.184J) than PAHs with a more linear structure such as chrysene (ΔHsm=−8.2kcal/mol, ΔVsm=−11.7ml/mol). The kinetic data indicate that pyrene undergoes faster rates of transport than chrysene (kms=313 and 14s−1, respectively), but the non-planar benzo[c]phenanthrene undergoes the fastest transport (kms=330s−1). The activation enthalpies and activation volumes are similarly affected by the annelation structure. It is noteworthy that deviations from the exponentially modified Gaussian (EMG) model are observed for some PAH zone profiles at the lowest temperature, which suggests a possible change in retention mechanism. In order to characterize these deviations, the non-linear chromatography (NLC) model and a new bi-exponentially modified Gaussian (E2MG) model were examined. The regression results indicate that neither the NLC nor E2MG model offer significant improvements in the statistical quality of fit or provide a better description of the observed retention behavior.