Evaluation of Electrostatic Potential Induced by Anion-Dominated Partition into Zwitterionic Micelles and Origin of Selectivity in Anion Uptake

Abstract

The surface potentials of n-dodecyltrimethylammoniopropanesulfonic acid (DDAPS) micelles in various electrolytes have been evaluated by capillary electrophoresis. This zwitterionic micelle has an inner cationic surface and an outer anionic surface and accommodates anions better than cations, indicating that a negative surface potential is induced by anion-dominated partition. Selectivity terms, i.e., solvation changes of ions and ion association between ions and charged groups in the DDAPS micelles, are introduced into the Poisson-Boltzmann equation for the spherical geometry. This model allows the interpretation of differences in the ionic partition and surface potential between electrolytes. The selectivity parameters have been determined by assuming agreement between the zeta potential determined by capillary electrophoresis and the calculated outer surface potential of the micelle. The obtained selectivity parameters can also explain the potentiometrically evaluated partition of ClO4and I -. It has been confirmed that capillary electrophoresis has wide applicability in surface potential measurements and can detect surface potentials of less than 1 mV. The selectivity origin in the partition into the DDAPS micelles is also discussed on the basis of evaluated parameters. The hydration changes mainly govern the uptake of well-hydrated anions, whereas poorly hydrated anions are partitioned into the micelle principally by ion-pair formation with the cationic groups in the micelles.

Hydration of Ions in Confined Spaces and Ion Recognition Selectivity

“…[46][47][48][49] The zeta potential of a micelle is one of the most useful physicochemical properties of a micelle. Although there is a debate on whether the shear plane corresponds to the real surface or not, the rigorous definition of the micellar surface is of no physical significance because micelles are dynamic systems that are subject to continuous thermal perturbation.…”

“…However, in an electrolyte, a relatively large charge is induced by the imbalance between the partition of cations and that of anions into the micelle. 46,51 Figure 7 summarizes the zeta potential of DDAPS micelles measured in various electrolytes. Interestingly, negative potential is induced in any electrolyte, and the nature of a cation affects the zeta potential only slightly.…”

Weak Binding of N-Alkylpyridinium Ions to Nonionic Surfactant Micelles as Studied by Capillary Zone Electrophoresis

“…Moore et al also indicated by fluorophotometry that the binding constants of an anionic dye to nonionic surfactant micelles 9 were larger than that of cationic crystal violet. 8 Okada et al have indicated the selective partition of anions to nonionic polyoxylated surfactant micelle 29 and to zwitterionic surfactant micelle 30,31 through the negative ζ potential of the micelle. In this study, the weak binding affinity of nonionic Brij 58 micelle toward cationic N-alkylpyridinium ions has also been investigated with the analysis through the electrophoretic mobility.…”

Ion Exchange and Protonation Equilibria of an Amphoteric Ion-exchange Resin in the Presence of Simple Salt

“…The nature of these anions should play a crucial role in the selectivity for the amphoteric ion-exchange resin, similar to that for amphoteric micelles. 17,18 The affinity of an anion to the amphoteric ionexchange resin should be correlated with both the dehydration energy of the anion and the electrostatic interaction between the anion and the functional group. 19,20 …”