Separation of Divalent Aromatic Anions by Capillary Zone Electrophoresis Using Multipoint Ion Association With Divalent Quaternary Ammonium Ions

Abstract

Positional isomers of naphthalenedisulfonate (NDS), naphthalenedicarboxylate (NDC) and phthalate were separated using an ion association reaction in aqueous solution followed by capillary zone electrophoresis with a polymer coated capillary. By using divalent quaternary ammonium ions, four NDS isomers and three NDC isomers were completely resolved, although some of the isomers were not resolved with tetrabutylammonium ion. Ion association constants, K_ass, between the anions and the divalent quaternary ammonium cations were also determined by analyzing the change in electrophoretic mobility using a non-linear least-squares method. The K_ass values of the anions obtained with the divalent cations were larger than those with tetrabutylammonium ion because of an increased electrostatic interaction. The ion associability was also explained on the basis of a distance-fitting concept and the hydrophobicity concept of the pairing cations. Ion associability was also improved by using a better distance-fitting pairing ion. From the results obtained, it was suggested that the charge–charge distance in cationic and anionic molecules must be close, and the charge–charge distance of the divalent cations was about 0.2 nm longer than that of divalent anions for good ion associability (a distance-fitting concept).