QUADRUPOLAR EFFECTS ON THE RETENTION OF AROMATIC-HYDROCARBONS IN REVERSED-PHASE LIQUID-CHROMATOGRAPHY

Because of their large molecular quadrupole moments, aromatic hydrocar bons can give rise to strong electrostatic interactions with polar mol ecules such as acetonitrile and water, Such interactions may have some significant effects on the retention and separation of the aromatic c ompounds in reversed-phase liquid chromatography, We report here an ex perimental investigation of these effects in which we studied the rete ntion behavior of a series of aromatic hydrocarbons in comparison with that of aliphatic hydrocarbons which have no appreciable quadrupole m oments, The elution order of benzene, cyclohexane, and n-hexane was de termined and shown to follow inversely the increasing magnitude of the molecular quadrupole moments, A linear dependence of retention on mol ecular polarizability was observed for both the aliphatics and aromati cs, the latter being consistently less retained than the former under the same chromatographic conditions, The molecular polarizability and quadrupole moment were regressed as independent variables against the logarithmic retention factors of both the aliphatic and aromatic solut es, and an excellent correlation was found with all five different chr omatographic systems studied. These results clearly show that, white t he retention of aromatic hydrocarbons is primarily governed by the dis persion interactions with the nonpolar stationary phase, the electrost atic interactions of the aromatic solutes with the polar mobile phase act to reduce the retention and thus contribute favorably to the contr ol of the relative retention and selectivity in reversed-phase liquid chromatography.