Molar enthalpy and molar volume of methylene and benzene homologues in reversed-phase liquid chromatography

In this study, thermodynamic properties are measured for methylene and benzene homologues in reversed-phase liquid chromatography using octadecylsilica stationary phases and methanol mobile phase. The change in molar enthalpy (Delta H degrees) is determined from graphs of the logarithm of the capacity factor versus the inverse temperature (15 to 60 degrees C), whereas the change in molar volume (Delta V degrees) is determined from graphs of the logarithm of the capacity factor versus pressure (830 to 5000 p.s.i.). For octadecylsilica phases with low bonding density (2.7 mu mol m(-2)): Delta H degrees and Delta V degrees are small and are relatively unaffected by temperature and pressure. These thermodynamic parameters are linearly related to the homologue number for the methylene homologues, but not for the benzene homologues. For the ethylene group, Delta Delta H degrees and Delta Delta V degrees are in the order of -0.41 kcal mol(-1) and -1.0 cm(3) mol(-1), respectively, at 30 degrees C. As the bonding density increases (5.4 mu mol m(-2)), the molar volume and molar enthalpy decrease in a significant and nonlinear manner with the homologue number. Moreover, these thermodynamic parameters are markedly affected by temperature and pressure. For the ethylene group, Delta Delta H degrees and Delta Delta V degrees are in the order of -3.65 kcal mol(-1) and -14.1 cm(3) mol(-1), respectively, at 30 degrees C. The theoretical and practical implications of these measurements are discussed with respect to the retention mechanism in reversed-phase liquid chromatography.