The effect of anion and cation in N-alkylimidazolium 4-alkoxybenzoates on thermotropic and aqueous behavior as well as conductivity in acetonitrile

“Salt-free” catanionic surface active ionic liquids (CASAILs) are promising for applications such as halide-free electrolytes or surfactants. Recently, we found that 1-alkyl-3-methylimidazolium alkylcarboxylates in bulk revealed a temperature-wise broad lamellar (SmA) mesophase; while in water, we observed low minimum surface tensions of the order of 20 mNm⁻¹. Surprisingly, for both thermotropic behavior and micellization, the chain length of the carboxylate anion had a stronger impact than that of the imidazolium cation. Here, to investigate whether this behavior is related to the linear structure of the anion, we used 4-alkoxybenzoate instead of alkyl carboxylate anions. For this new type of CASAILs, the SmA mesophase was also found in bulk. While in the CASAILs with large cations, which exhibited SmA regions more than 100 K wide, the chain length of the 4-alkoxybenzoate anion again had only a minor influence on the phase behavior, its influence becomes increasingly pronounced for small cations. Surface tension measurements and UV/Vis spectroscopy revealed an analogous linear decrease of log(cmc) with increasing anion and cation chain length due to the increasing hydrophobic effect. As expected for catanionic surfactants, the presence of vesicles was demonstrated for [C₁₀mim][C₇OBCOO] at 10 times the cmc and 25 °C using dynamic light scattering and freeze fracture electron microscopy. Potentiostatic electrochemical impedance spectroscopy of these CASAILs in acetonitrile showed that the conductivity decreased with increasing chain length of either the anion or the cation due to slower diffusion.