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

Abstract

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.

Dehydration Determines Hydrotropic Ion Affinity for Zwitterionic Micelles

“…32 This result is also consistent with the dipolar moment determined experimentally. 21 Addition of Cl − , Br − , and acetate did not change the distribution, with average angles of 101 and 102 degrees, respectively. Addition of ClO 4 − displaced the peak position of the θ angle distribution to a more stretched surfactant, with an average angle of 113 degrees (Figure 2a).…”

“…29 We note that salt addition has minimal effects on the aggregation number of micelles of this surfactant. 21 Maintaining the same aggregation number allowed direct comparison of the specific ion effects (SIEs) on micelles.…”

“…The GAFF force field 27 was used for all salts and DPS, with the dihedral parameters obtained as already described. 21 The partial charges of the atoms were calculated with Gaussian 09 28 (HF/6-31G*). During the micelle simulation, at most one monomer exited the micelle, and only micellized DPS molecules were considered in the calculations.…”

“…Thus, perchlorate anions adsorb in the interface to a larger degree than bromide, as seen by zeta potential measurements, but still does not produces changes in aggregation number or micellar shape. 20,21 Our studies with polyatomic anions in zwitterionic micelles show that, in contrast to what is observed in cationic micelles, there are no specific changes in micelle interfacial hydration induced by greater ion association, 21 mostly because anion association to zwitterionic micelles does not produce changes in shape or aggregation number. 5,7,8 Specific ion effects for polyatomic anions are observed, for example, in the critical micelle concentration for sulfobetaine micelles and also in the aggregation behavior of zwitterionic imidazolium micelles (such as 3-(1-alkyl-3-imidazolium) propanesulfonate).…”

Spectroscopically Quantifying the Influence of Salts on Nonionic Surfactant Chemical Potentials and Micelle Formation

“…Interactions between ions and aqueous molecular interfaces are important for numerous chemical and biological processes, including the solubility of nonelectrolytes, enzyme catalysis, , molecular self-assembly, − membrane permeability, , and cellular exocytosis . Although the associated ion-specific (Hofmeister) effects − are in principle traceable to the influence of salts on molecular chemical potentials, open questions remain regarding the corresponding ion–molecule interactions. − Here we obtain such information by spectroscopically and theoretically analyzing the influence of two salts (NaSCN and Na 2 SO 4 ) on the critical aggregation (micelle) concentration C A of the nonionic surfactant 1,2-hexanediol (12HD).…”