Equilibrium analysis of reactions between aromatic anions and nonionic surfactant micelles by capillary zone electrophoresis.

Separability of some positional isomers of aromatic anions by capillary zone electrophoresis was improved by adding nonionic surfactants to a migrating solution. Eleven kinds of aromatic anions, including positional isomers, were used as analytes, and Brij-35, Brij-58 and Brij-78 were investigated as nonionic surfactants to form micelles, where hydrophobicities are different from each other. Increasing the concentration of the surfactants developed the separability of the anionic isomers. The interaction between the anions and the nonionic surfactant micelles is also investigated through the change in the electrophoretic mobility, and the binding constants are determined. Apparent electrophoretic mobility of the anions decreased with increasing concentrations of the nonionic surfactants. The decrease in the mobility, as well as the binding constant, was larger in the monovalent anions than in the divalent anions, which indicates that the interaction or reactivity of the monovalent analytes is higher than that of the divalent analytes. The reactivity of each anion was almost identical even when the kinds of the surfactants were changed, suggesting that the hydrophobicity of the polyoxyethylene group in the surfactant would have the main role for binding the analytes. The reactivity tendency among the positional isomers was almost similar to that in ion association-capillary zone electrophoresis using tertabutylammonium ion as a pairing ion. The results obtained in this work suggest that the anions are bound to the micelles by the hydrophobic interaction between analyte anions and the polyoxyethylene moiety of the surfactant micelles. Changes in the fluorescence intensity of the anions were also investigated; the results can explain well the mobility changes of the analytes.