Journal of Chromatography A, 853 (1999) 63–69 Short communication Electrophoretic mobility study on ion–ion interactions in an aqueous solution * Shoji Motomizu , Toshio Takayanagi Department of Chemistry, Faculty of Science, Okayama University,3-1-1 Tsushimanaka, Okayama 700-8530, Japan Abstract Ion–ion interactions between anions and their pairing ions in aqueous solutions were studied through the measurements of electrophoretic mobilities of analyte ions in capillary zone electrophoresis, where the electrophoretic method for the analysis of ion association reaction is shown to be more useful than the conductometric method widely used in the analysis of the reactions. The electrophoretic mobility of monovalent inorganic anions was almost identical even when the concentrations of alkali metal ions and quaternary ammonium ions in the migrating solution were varied up to 15 mM. On the other hand, the electrophoretic mobility of organic anions, such as monovalent and divalent anions, decreased with increasing concentrations of quaternary ammonium ions. Changes in the electrophoretic mobilities were analyzed by a non-linear least-squares method giving ion association constants. The results indicate that the proposed method is applicable to the analysis of such reactions to give the mobility change. The ion association constants obtained in an aqueous solution were related to the extraction constants of the ion associates, and the contributions of the association process and the distribution process were clarified.  1999 Elsevier Science B.V. All rights reserved. Keywords: Electrophoretic mobility; Ion–ion interactions; Association constants; Inorganic anions; Phenolic compounds; Organic acids 1. Introduction interaction [1], a protein–sugar interaction [2], a binding of a fluorescence labeled molecule to anti- Recently, capillary zone electrophoresis (CZE) has body [3], a binding of neutral compounds to sodium been widely used for the separation and determi- dodecyl sulfate micelles [4], and a binding of 2- nation of ionic substances. In CZE, many kinds of phenoxypropionic acid enantiomers to b-cyclodex- modifying reagents in the migrating solutions have trin [4]. been effectively used to improve the separation of Ion–ion interactions, such as an ion association the analytes. In addition to the improvement in reactions, have been analyzed mainly by a conduc- separation, CZE is a powerful tool for various kinds tometric method, which is based on the Debye– ¨ of interaction analyses in aqueous solutions. The Huckel activity theory and Onsager conductance interactions so far studied are a ligand–protein equation [5]. Although the conductometric method contains some theoretical assumptions, many ion association constants have been determined by this *Corresponding author. Tel.: 181-86-251-7846; fax: 181-86- method [6,7]. Recently, a novel method using elec- 251-7846. E-mail address: motomizu@cc.okayama-u.ac.jp (S. Motomizu) trophoretic mobility, which is easily measured by 0021-9673 / 99 / $ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0021-9673(99)00453-7