MOLAR ENTHALPY AND MOLAR VOLUME OF METHYLENE AND BENZENE HOMOLOGS IN REVERSED-PHASE LIQUID-CHROMATOGRAPHY

In this study, thermodynamic properties are measured for methylene and benzene homologues in reversed-phase liquid chromatography using octa decylsilica stationary phases and methanol mobile phase. The change in molar enthalpy (Delta H degrees) is determined from graphs of the log arithm of the capacity factor versus the inverse temperature (15 to 60 degrees C), whereas the change in molar volume (Delta V degrees) is d etermined from graphs of the logarithm of the capacity factor versus p ressure (830 to 5000 p.s.i.). For octadecylsilica phases with low bond ing density (2.7 mu mol m(-2)): Delta H degrees and Delta V degrees ar e small and are relatively unaffected by temperature and pressure. The se thermodynamic parameters are linearly related to the homologue numb er for the methylene homologues, but not for the benzene homologues. F or 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), respec tively, 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 thermo dynamic parameters are markedly affected by temperature and pressure. For the ethylene group, Delta Delta H degrees and Delta Delta V degree s are in the order of -3.65 kcal mol(-1) and -14.1 cm(3) mol(-1), resp ectively, at 30 degrees C. The theoretical and practical implications of these measurements are discussed with respect to the retention mech anism in reversed-phase liquid chromatography.