Electrolyte effect on the aggregation behavior of 1-butyl-3-methylimidazolium dodecylsulfate in aqueous solution

Effect of three inorganic electrolytes (LiCl, NaCl, and MgCl2) and four organic electrolytes, viz. tetraalkylammonium bromides ((CH3)4NBr, (C2H5)4NBr, (C3H7)4NBr, and (C4H9)4NBr) on the aggregation behavior of the anionic halogen-free surface active ionic liquid, 1-butyl-3-methylimidazolium dodecylsulfate ([C4mim][C12SO4]), in aqueous solution was studied by surface tension, steady-state fluorescence quenching, and dynamic light scattering measurements. The results show that all the electrolytes investigated have a salting-out effect, which promotes aggregate formation of [C4mim][C12SO4]. The stronger hydrophobicity of organic electrolytes is crucial for the superior influence on the surface activity of [C4mim][C12SO4]. However, the stabilization energy results obtained by quantum chemical calculations prove that although the promoting effect of organic cations (tetraalkylammonium cations) on the micellization process of [C4mim][C12SO4] is powerful, they mainly act as counterions. For a given electrolyte (i.e., NaCl), critical micelle concentration of [C4mim][C12SO4] decreases with increasing electrolyte concentration. The average aggregation number and aggregate size of [C4mim][C12SO4] were shown to change slightly in the presence of various electrolytes, except for MgCl2. Anyway, hydrophobicity together with bulkiness and hydration ability of cations of the added electrolytes are suggested to play important roles in modifying the aggregation behavior of [C4mim][C12SO4] in aqueous solution.