Micellization Behavior of Morpholinium-Based Amide-Functionalized Ionic Liquids in Aqueous Media

Morpholinium-based amide-functionalized ionic liquids (ILs) [C_nAMorph]­[Br], where n = 8, 12, and 16, have been synthesized and characterized for their micellization behavior in aqueous medium using a variety of state of the art techniques. The adsorption and micellization behavior of [C_nAMorph]­[Br] ILs at the air–solution interface and in the bulk, respectively, has been found to be much better compared to that observed for nonfunctionalized homologous ILs and conventional cationic surfactants, as shown by the comparatively higher adsorption efficiency, lower surface tension at the critical micelle concentraiton (γ_cmc), and much lower critical micelle concentration (cmc) for [C_nAMorph]­[Br] ILs. Conductivity measurements have been performed to obtain the cmc, degree of counterion binding (β), and standard free energy of micellization (ΔG_m°). Isothermal titration calorimetry has provided information specifically about the thermodynamics of micellization, whereas steady-state fluorescence has been used to obtain the cmc, micropolarity of the cybotactic region, and aggregation number (N_agg) of the micelles. Both dynamic light scattering and atomic force microscopy have provided insights into the size and shape of the micelles. 2D ¹H–¹H nuclear Overhauser effect spectroscopy experiments have provided insights into the structure of the micelle, where [C₁₆AMorph]­[Br] has shown distinct micellization behavior as compared to [C₈AMorph]­[Br] and [C₁₂AMorph]­[Br] in corroboration with observations made from other techniques.