Ion Association Study in Aqueous Solution Using Capillary Electrophoresis.

Abstract

Ion association reactions between monovalent- and divalent- aromatic anions and their pairing ions, quaternary ammonium ions, in an aqueous solution were analyzed by using electrophoretic mobility of aromatic anions (analyte anions). The electrophoretic mobility of analyte anions decreased with increasing the concentrations of quaternary ammonium ions added in the migrating buffer solutions. The electrophoretic mobility of the analyte anions was obtained by using capillary zone electrophoresis. The change in electrophoretic mobility was analyzed to obtain ion association constants (K ass ) by a non-linear least-squares analysis method, as well as some other linear least-squares analysis methods. The larger the decreases in the mobility of the analyte anions with increasing the amounts of pairing ions, the larger the K ass values were. Ion association constants obtained were larger in divalent anions than in monovalent anions, which suggested the contribution of electrostatic interactions to ion associability in the aqueous solution. The K ass values became larger in the anions possessing naphthalene ring than in those possessing benzene ring, and also became larger in the bulkier pairing cations than in less bulky cations. These results indicate that the hydrophobicity of the ions will contribute to the ion associability positively. In comparison of ion associability of position isomers with each other, the ion associablity of β-substituted naphthalene isomers were larger than those of α-substituted ones, which agreed with the acid dissociation property of the anions; that is, the more basic the position isomer is, the more associable the isomer is. Differences in K ass values among functional groups, such as -O - , -CO 2 - , and -SO 3 - , were small, compared with the differences in liquid-liquid extraction constants (K ex ) of ion associates.