Simulating retention in gas-liquid chromatography: Benzene, toluene, and xylene solutes

Accurate predictions of retention times, retention indices, and partition c onstants are a long sought-after goal for theoretical studies in chromatogr aphy. Although advances in computational chemistry have improved our unders tanding of molecular interactions, little attention has been focused on chr omatography. let alone calculations of retention properties: Configurationa l-bias Monte Carlo simulations in the isobaric-isothermal Gibbs ensemble we re used to investigate the partitioning of benzene, toluene, and the three xy]ene isomers beta een a squalane liquid phase and a helium vapor phase. T he united-atom representation of the TraPPE (transferable potentials for ph ase equilibria) force field was used for all solutes and squalane. The Gibb s free energies of transfer and Kovats retention indices of the solutes wer e calculated directly from the partition constants (which were averaged ove r several independent simulations). While the calculated Kovats indices of benzene and toluene at T = 403 K are significantly higher than their experi mental counterparts, much better agreement is found for the xylene isomers at T = 365 K.