Thermodynamics of anion binding to zwitterionic sulfobetaine micelles

One of the most intriguing aspects of zwitterionic surfactant micelles is their propensity to exhibit selectivity in the binding of the anions of added salts. In this work we examine the thermodynamics of the interaction of the strongly bound perchlorate ion and the more weakly bound bromide ion with micelles of N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SB3-14) in aqueous solution employing enthalpies derived from isothermal titration calorimetry combined with Gibbs free energies derived from literature data for the binding equilibria. In both cases, the binding is exothermic and enthalpy driven, but entropically unfavorable, with only modest changes in the Gibbs free energy as a function of the extent of anion binding. Likewise, perchlorate ion binding was found to have little or no effect on the aggregation numbers of SB3-14 micelles determined by time-resolved fluorescence quenching of pyrene by the N-hexadecylpyridinium cation. The results are interpreted within the context of the factors involved in the ion-pairing between the anions and the positive charge center of the zwitterion headgroup and the interplay between electrostatics, solvent reorganization and a net loss of translational degrees of freedom that accompany anion binding. (C) 2021 Elsevier Inc. All rights reserved.

Automated summary

One intriguing characteristic of zwitterionic surfactant micelles is their ability to selectively bind anions of added salts. This study investigates the thermodynamics of the interaction between strongly bound perchlorate ions and weakly bound bromide ions with micelles of N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SB3-14) in aqueous solution. The results indicate that both binding processes are exothermic and driven by enthalpy, but unfavorable in terms of entropy. The Gibbs free energy shows only modest changes as the extent of anion binding increases. Additionally, the binding of perchlorate ions has minimal effect on the aggregation numbers of SB3-14 micelles. These findings are explained by considering factors such as the ion-pairing mechanism between anions and the positive charge center of the zwitterion headgroup, as well as the interplay between electrostatics, solvent reorganization, and the loss of translational degrees of freedom during anion binding.