Specific Ion Effects on Zwitterionic Micelles Are Independent of Interfacial Hydration Changes

Zwitterionic micelles adsorb anions and several techniques were used to determine the specificity of this interaction. Although at a lower intensity, this adsorption can be compared to those observed in cationic micelles, which showed that interfacial dehydration is a fundamental property for the geometry and size of micelles. Because there is no information on the interfacial hydration of zwitterionic micelles, we used dielectric relaxation spectroscopy (DRS) together with molecular dynamics (MD) simulations to evaluate the importance of surface dehydration promoted by the binding of anions at the micellar interface (sodium bromide, sodium methanesulfonate, sodium trifluoroacetate, and sodium triflate) in N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (DPS) micelles. Our results, showing good agreement between DRS and MD simulations, strongly suggest that specific ion effects on zwitterionic micelles are unrelated to global changes in the interfacial hydration and depend on specific interactions of the headgroups with selected anions.