Self-Assembly of 2-Hydroxyethyl-1H-imidazolium-Based Surface Active Ionic Liquids and Utilization of Their Aqueous...

Imidazolium-based surface active ionic liquids (SAILs) appended with a hydroxyethyl moiety at a cationic headgroup at a position opposite to an alkyl chain ([C₁₂ImOH][Br] and [C₁₆ImOH][Br]) and alkyl chain functionalized with amide ([C₁₂AImOH][Br]) and ester ([C₁₂EImOH][Br]) groups have been synthesized. Different techniques, i.e., surface tension, ionic conductance, fluorescence, dynamic light scattering, small angle neutron scattering, and isothermal titration calorimetry, have been exploited for establishing their micellization behavior, followed by the utilization of aqueous SAILs to offer enhanced enzymatic activity to cytochrome-c (Cyt-c). The hydrophobic hydration of the hydroxyethyl group retards the micellization in bulk but disturbs the water structure at and beneath the air-solution interface, resulting in better surface-active behavior as compared to their nonhydroxyethyl functionalized counterparts. The variation of characteristic micellar properties, i.e., cmc, counterion binding (β), aggregation number (N_agg), size, compactness, and thermodynamic parameters of micellization, is supported by the increasing hydrophobicity of the cationic SAIL along with contrastingly different H-bonding and stiffness of ester or amide moiety present near the cationic headgroup. The aqueous solutions of SAILs (below cmc) favored the enzyme activity governed predominantly by hydrophobic as well as polar interactions between SAILs and Cyt-c corroborated by molecular docking and circular dichroism (CD) spectroscopy investigations. Cyt-c shows a retarded activity in aqueous micelles of SAILs (above cmc) with the exception of [C₁₂AImOH][Br], where enzyme activity increased further to ∼2.1-fold compared to that observed in buffer. It is anticipated that the current results would pave a new way to synthesize SAILs not only to be used for enzyme activation and prolonged storage in aqueous medium but also for other biological applications.