Thermodynamic Study of Ion-Association Reactions between Aromatic Anions and Tetrabutylammonium Ion in an Aqueous Solution

A temperature effect on ion-association reactions in an aqueous solution has been studied between nine kinds of aromatic anions and tetrabutylammonium ion (TBA(+)). The ion-association constants (K(ass)) were determined by analyzing the change in the electrophoretic mobility of anions, which was obtained by capillary zone electrophoresis, by a non-linear least-squares method. The mobility of the analyte anions decreased with increasing the amount of TBA(+) added in migrating solutions. The thermodynamic parameters in the reaction were obtained from the change in the K(ass) values by the change in temperature. The values of the entropy change (ΔS°) were positive in the ion-association reactions of all of the anions examined, being in the ranges from 5.8 to 46.1 J K(-1) mol(-1); the contribution of TΔS° in the examined ion associates to the Gibbs free-energy changes is larger than that of enthalpy changes (-ΔT°), except for phthalate ion. The results suggest that some hydrated water molecules were released in the formation of such hydrophobic ion associates, as well as in the cases of previously reported inorganic ion associates.