Thermodynamics of the Micellization of a Simple Zwitterionic Surfactant in the Presence of Imidazolium Salts

Although formally neutral, in aqueous media, zwitterionic micelles selectively concentrate the anions of added inorganic salts at the micelle-water interface. The resultant negative electrostatic potential of the micelles can be additionally modulated by the metal cations of the added salt. In the present work, isothermal titration calorimetry (ITC) has been employed to determine the critical micelle concentrations (CMC) and standard thermodynamic parameters of micellization (Δ<i>G</i> _mic ⁰, Δ<i>H</i> _mic ⁰, and Δ<i>S</i> _mic ⁰) of the zwitterionic surfactant <i>N</i>-tetradecyl-dimethylammoniumpropanesulfonate (SB3-14) in aqueous solution at 298.15 K as a function of the concentration (100-1000 mmol L^-1) of six salts with organic cations: the chloride salts of 1-methylimidazolium (C_omimCl) and 1-ethyl-3-methylimidazolium (C₂mimCl) and the chloride, bromide, iodide, and perchlorate salts of 1-butyl-3-methylimidazolium (C₄mimCl, C₄mimBr, C₄mimI, and C₄mimClO₄, respectively). C_omimCl, C₂mimCl, C₄mimCl, and C₄mimBr increase the CMC of SB3-14 in this concentration range, while C₄mimI and C₄mimClO₄ initially decrease the CMC, followed by an increase in the CMC at higher salt concentrations. Although the Δ<i>H</i> _mic ⁰ values of SB3-14 are consistently negative and the Δ<i>S</i> _mic ⁰ values decrease relative to those in the absence of added salt, the overall behavior is distinct from that in the presence of the corresponding sodium salts, especially for C₄mimI and C₄mimClO₄ with the most strongly interacting anions. We demonstrate that, by explicitly incorporating specific cation-dependent interactions into the formalism developed earlier for inorganic salts [Patiño-AgudeloÁ. J., Colloids Surf., A2025, 725, 137728], it is possible to separate the contributions of the organic cation to the micellization of SB3-14 from those of the anion, providing a coherent picture of the complex interplay between anion, cation, and the zwitterionic interface.