Incorporation and differential reactivity of anions in mitterionic sulfobetaine micelles

Zwitterionic surfactants, such as sulfobetaines and carboxybetaines, are formally neutral, but their micelles incorporate anions owing to the higher charge density of the cationic ammonium inner surface, compared with that of the anionic sulfonate, or carboxylate, surface and to anion-specific interactions. In this work, we used the following sulfobetaines: N-decyl-, N-tetradecyl- and N-hexadecyl-N, N-dimethylammonio-1-propanesulfonate, N-tetradecyl-N,N-diethyl-, N-tetradecyl-N,N-dipropyl- and N-tetradecyl-N,N-dibutylammonio-1-propanesulfonate and N-tetradecyl-N,N-dimethyl-, N-tetradecyl-N,N-diethyl- and N-tetradecyl-N, N-dipropylammonio-1-butanesulfonate. We have determined the partitioning of bromide and perchlorate ions between water and micelles in terms of anions incorporated in the interfacial region of the micelles as a function of concentrations of added ions in the bulk solution. The concentrations of free and micellar-bound ions were determined by using ion-selective electrodes, in the presence and absence of sulfobetaines, at pH 9.0 (borate buffer). The incorporation of anions into zwitterionic micelles fits Langmuir adsorption isotherms, and the results are related to changes in the micellar surface potential. The incorporation of perchlorate is in the range 15-30% of the concentration of micellized surfactant for the different sulfobetaines, and is greater than that for bromide ion (about 10%). These results fit differences in anionic reactivities and in the NMR line widths of micellar incorporated anions.