Capillary zone electrophoretic separation of aromatic anions utilizing enhanced ion associability with viologen cations

Abstract

Ion associability and the separability of positional isomers of aromatic anions with divalent pairing ions were studied in aqueous solution by capillary zone electrophoresis (CZE). As the pairing ions, hydrophobic ion association reagents possessing aromatic moieties, such as methylviologen, ethylviologen, octylviologen and benzylviologen, were examined. Four naphthalenedisulfonate ions and three naphthalenedicarboxylate ions were well resolved by CZE, using the ion association reagents in the migrating solutions. Ion association constants between the divalent cations and the divalent anions, K_ass, were determined by analyzing the electrophoretic mobility changes of analyte anions using a non-linear least-squares method. The K_ass values in methylviologen and ethylviologen were larger than those in pentane-1,5-bis(triethylammonium) [PeB(TEA)²⁺]. The differences in logK_ass between the two viologens and PeB(TEA)²⁺ were about 0.3, which is likely to be introduced by the viologen structure. The logK_ass values further increased with octylviologen and benzylviologen. The increase from PeB(TEA)²⁺, about 0.5, is attributed to the increased hydrophobicity of pairing cations in addition to the effect of viologen structure. The differences in ion associability between PeB(TEA)²⁺ and the viologens are larger in naphthalene than in benzene anions, which suggests a contribution of aromatic–aromatic interactions.