Potentiometric and chromatographic evaluation of ion uptake by zwitterionic micelles

Abstract

A model has been derived from the Poisson–Boltzmann equation to describe the uptake of ions by a zwitterionic micelle. Two mechanisms representing uptake selectivity are included in the model; (1) a difference in the standard chemical potential of an ion between the bulk solution (partition mechanism) and (2) the micelles and ion-pair formation at the charged surfaces (ion-pair mechanism). In order to confirm the applicability of the derived model, the uptake of ions is evaluated by two independent experiments, i.e. potentiometry and chromatography, and compared with the calculation. Both the partition and the ion-pair mechanism similarly explain the ion uptake by the zwitterionic micelles. The standard chemical potential differences are estimated -5.0 kJ mol^-1 for I^- and -8.5 kJ mol^-1 for SCN^- when employing the partition mechanism, while the ion-pair formation constants are 0.12 M^-1 for I^- and 0.7 M^-1 for SCN^- with the ion-pair mechanism. These parameters depend strongly on the micellar sizes, but well reflect the nature of anions irrespective of size parameters involved in the calculation.