Analysis of Chemical Equilibria in Aqueous Solution Related with Separation Development Using Capillary Zone Electrophoresis

Capillary zone electrophoresis (CZE) has become a popular technique for the analysis of ionic substances. The separation mechanism is based on the differences in electrophoretic mobility of analyte ions, as well as on the electroosmotic flow. Although the separation efficiency in CZE is amazingly high, various types of interacting reagents are often used to control the electrophoretic mobility of analytes, when the resolution among analytes is not sufficient. The interacting reagents used are so−called ”modifiers,” and various types of inter−molecular interactions have been utilized. Investigation of the modifiers has contributed to establish new analytical systems. Since the nature of molecular interactions utilized in the CZE separation is essentially the chemical equilibria in solution, we can analyze the chemical equilibria by measuring the change in electrophoretic mobility of analytes, vice versa . Journal of Chromatography A has issued a special volume entitled ”Estimation of Physicochemical Properties by Chromatographic and Electrophoretic Techniques,” this year (2004) [1]. In that issue, various chromatographic techniques were introduced to analyze the properties including liquid−liquid partition, vapor pressure, diffusion coefficients, acid−base, complex−forming, critical micelle concentration, and binding constants. Capillary zone electrophoresis has its predominant characteristics as mentioned above, and the advantages utilized have been introduced for the equilibrium analysis of acid−base, metal complex, and binding affinity of biomolecules [2]. In the CZE analysis of the chemical equilibria, the general procedure includes that the electrophoretic mobility of analyte is measured with a series of migrating solutions containing different concentrations of the interacting reagent, and that the changes in the apparent electrophoretic mobility is mathematically analyzed to give equilibrium constants. This paper reviews the potential of the CZE techniques for equilibrium analysis in aqueous solution achieved by the present author’s group. The method was validated by the complex formation analysis of crown ethers, and was further utilized for the analysis of weak ion−ion interactions in aqueous solution: several interesting factors were elucidated. The ion association reaction in aqueous solution led to understand the stepwise reactions in liquid− liquid distribution of ion−pair extraction. Binding reactions to nonionic surfactant micelle are also interpreted in this review. Focusing Review