Association constants, Kc, were derived from the electrophoretic mobilities of the anionic solutes (seven benzoates with hydroxy or chloro substituents) by capillary zone electrophoresis in different solvent systems, consisting of binary mixtures of water with up to 20% (v/v) methanol or acetonitrile, respectively. The association constants expectedly are found to decrease with increasing organic solvent concentration. The effect of organic solvents on the Kc of the benzoates with -cyclodextrin was analyzed applying the concept of the transfer activity coefficient (or the medium effect). This concept enables the evaluation of the significance of the contributions of the individual species involved in the complexation equilibrium in the different solvents: the benzoate ion, -cyclodextrin, and the anionic benzoate--cyclodextrin complex. The medium effect on benzoate was calculated from the change in acidity constant of benzoic acid in the different mixed solvents and the corresponding transfer activity coefficients of the proton and the molecular acid. The transfer activity coefficients for -cyclodextrin results from its solubility at saturation in the different solvents. In this way, an estimation of the standard free energy of transfer, ¢Gt 0, of each species involved in the complexation equilibrium was possible for the transfer from water into the respective mixed solvent. It was found that the organic solvents do not significantly affect ¢Gt 0 for the benzoate anion. However, the organic solvents play a different role concerning the stabilization of -cyclodextrin and the complex anion: whereas the addition of acetonitrile has nearly no influence on ¢Gt 0 of the anionic complex, the reduction in Kc is caused by the enhanced stabilization of -cyclodextrin (reflected by its better solubility). Addition of methanol, on the other hand, lowers the solubility of -cyclodextrin, thus giving positive values for ¢Gt 0. Thus, the overall effect on Kc in methanolic solutions must be related to the pronounced destabilization of the benzoate-- cyclodextrin complex.
Medium effect (transfer activity coefficient) on beta-cyclodextrin/benzoate complexation in capillary zone electrophoresis.
Fanali S;
2003
Abstract
Association constants, Kc, were derived from the electrophoretic mobilities of the anionic solutes (seven benzoates with hydroxy or chloro substituents) by capillary zone electrophoresis in different solvent systems, consisting of binary mixtures of water with up to 20% (v/v) methanol or acetonitrile, respectively. The association constants expectedly are found to decrease with increasing organic solvent concentration. The effect of organic solvents on the Kc of the benzoates with -cyclodextrin was analyzed applying the concept of the transfer activity coefficient (or the medium effect). This concept enables the evaluation of the significance of the contributions of the individual species involved in the complexation equilibrium in the different solvents: the benzoate ion, -cyclodextrin, and the anionic benzoate--cyclodextrin complex. The medium effect on benzoate was calculated from the change in acidity constant of benzoic acid in the different mixed solvents and the corresponding transfer activity coefficients of the proton and the molecular acid. The transfer activity coefficients for -cyclodextrin results from its solubility at saturation in the different solvents. In this way, an estimation of the standard free energy of transfer, ¢Gt 0, of each species involved in the complexation equilibrium was possible for the transfer from water into the respective mixed solvent. It was found that the organic solvents do not significantly affect ¢Gt 0 for the benzoate anion. However, the organic solvents play a different role concerning the stabilization of -cyclodextrin and the complex anion: whereas the addition of acetonitrile has nearly no influence on ¢Gt 0 of the anionic complex, the reduction in Kc is caused by the enhanced stabilization of -cyclodextrin (reflected by its better solubility). Addition of methanol, on the other hand, lowers the solubility of -cyclodextrin, thus giving positive values for ¢Gt 0. Thus, the overall effect on Kc in methanolic solutions must be related to the pronounced destabilization of the benzoate-- cyclodextrin complex.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
