Analysis of ion association reactions in aqueous solutions between alkali metal-crown ether complexes and pairing anions by capillary zone electrophoresis

In an aqueous solution, capillary zone electrophoresis was used to analyze ion association reactions between alkali metal-crown ether complexes and some organic (sulfonate, benzoate, phenolate and their nitro derivatives) and inorganic (ClO4- and SCN-) pairing anions. The method involved measuring the change in electrophoretic mobility of the crown ethers on increasing the pairing anion concentrations in the solutions, the total concentration of the metal ion being kept constant. The apparent electrophoretic mobility of crown ethers was found to decrease with increasing concentration of pairing anion, and the ion association constants (K-ML.X) were determined using such a mobility change. The ion association constants obtained were found to increase with increasing molecular volume of the organic pairing anions. Picrate ion showed the largest ion association constant of the reagents investigated, and pairing anions having two nitro groups as substituents exhibited higher associability than the pairing anions with a single nitro group. In all the ion associates examined, the ion association constants for dibenzo-18-crown-6 complexes were larger than those for benzo-18-crown-6 complexes. Further, it was found that Na+ complexes showed higher ion associability than K+ complexes. Such differences in the ion association constants between Na+ and K+ complexes, and also the pairing anions, were explained not only from the point of view of the hydrophobicity of the pairing anions, but also on the basis of factors such as the number of benzene rings, the charge dispersion of anions by the electron withdrawing groups and the structural orientation of the pairing anions. The results established the utility of this approach for the study of electrophoretic separations of positional isomers in aqueous solutions.