ION ASSOCIATION STUDY IN AQUEOUS-SOLUTION USING CAPILLARY ELECTROPHORESIS

Ion association reactions between monovalent-and divalent-aromatic ani ons and their pairing ions, quaternary ammonium ions, in an aqueous so lution were analyzed by using electrophoretic mobility of aromatic ani ons (analyte anions). The electrophoretic mobility of analyte anions d ecreased with increasing the concentrations of quaternary ammonium ion s added in the migrating buffer solutions. The electrophoretic mobilit y of the analyte anions was obtained by using capillary zone electroph oresis. The change in electrophoretic mobility was analyzed to obtain ion association constants (K-ass) by a non-linear least-squares analys is 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 we re. Ion association constants obtained were larger in divalent anions than in monovalent anions, which suggested the contribution of electro static interactions to ion associability in the aqueous solution. The K-ass values became larger in the anions possessing naphthalene ring t han in those possessing benzene ring, and also became larger in the bu lkier pairing cations than in less bulky cations. These results indica te that the hydrophobicity of the ions will contribute to the ion asso ciability positively. In comparison of ion associability of position i somers with each other, the ion associability of beta-substituted naph thalene isomers were larger than those of alpha-substituted ones, whic h agreed with the acid dissociation property of the anions; that is, t he more basic the position isomer is, the more associable the isomer i s. Differences in K-ass values among functional groups, such as -O-, - CO2-, and -SO3-, were small, compared with the differences in liquid-l iquid extraction constants (K-ex) of ion associates.