Micellization of Zwitterionic Surfactant with Opposite Dipoles is Differently Affected by Anions.

We used isothermal titration calorimetry to investigate the influence of hydrotropic anions on the micellization of the zwitterionic surfactants N-dodecylphosphocholine (DPC) and N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (DPS), as well as the effect of these anions on the micellar interface. Our results showed an increased enthalpic contribution to the Gibbs energy for micellization when the hydrotropic anions contained CF(3) or C(6)H(5) hydrophobic groups. We found that the critical micelle concentration of DPS was more sensitive to the effects of the anions than that of DPC, likely because the positive charge of DPS is located closer to the micellar core. Overall, the magnitudes of the anion effects on the micellar properties of DPC and DPS were relatively minor compared with those observed in cationic micelles. These findings offer valuable insights into the role of dipole orientation and charge distribution in the interactions between zwitterionic surfactants and anions as well as the effects of hydrotropic anions on these systems.