Composition-induced micelle-vesicle-micelle transitions: A simple and effective strategy to tune the electrocatalytic performance of aqueous mixtures of imidazolium-based surface-active ionic liquids

Aqueous micellar solutions of imidazolium-based surface-active ionic liquids (SAILs) are considered as promising green-electrocatalytic solvent systems for a variety of electrochemical transformations especially the electroreduction of CO2 (ERCO2) and electro-carboxylation with CO2 (ECCO2). However, the limited solubility of CO2 in these solvent systems continues to hinder their use for bulk-scale ERCO2 and ECCO2 reactions. We theorize that the formation of mixed aggregates of cationic and anionic SAILs is the easiest and most reliable approach to tune the size, composition and zeta potential and, hence, the solubility and electrocatalytic performance of the SAIL micellar aggregates. In anticipation of this presumption, the current work was designed to explore the composition dependence of the structural and physicochemical aspects of self-assembled aggregates in micellar mixtures of imidazolium-based SAILs, viz., 1-butyl-3-methyl-imidazolium dodecyl sulfate ([BMIM]DS) and 1dodecyl-3-methylimidazolium chloride ([DDMIM]Cl). The conductometric, UV-Vis spectroscopic, rheological, dynamic light scattering (DLS) and cyclic voltammetric (CV) investigations suggest that besides variation in the structural and physicochemical aspects of mixed micellar aggregates, composition variation of the [DDMIM]Cl plus [BMIM]DS SAIL mixtures results in a spontaneous micelle to vesicle transition. Besides their various physicochemical characteristics, the potential utility of the variedly composed [DDMIM]Cl plus [BMIM]DS SAIL micellar mixtures as electrolyte systems for electrochemical investigations was also investigated. The investigations carried out in this regard suggest that the SAIL mixtures exhibit composition-dependent solubilization, transport and electrocatalytic performance. The mass transport ability of the mixed aggregates and the concentration of the redox probe solubilized within them, investigated via electrochemical studies using a gold ultramicroelectrode (UME), were found to be the highest for the mixture with equal mole fractions of [BMIM]DS and [DDMIM]Cl (chi[DDMIM]Cl = 0.5). The differently composed SAIL micelle mixtures were also tested for their potential use as electrolytes for ERCO2. The findings indicate that the ability of these mixed micellar solutions to solubilize and transport non-polar redox-active probes determines their solubilization potential and electrocatalytic performance towards ERCO2. The study suggests that [BMIM]DS plus [DDMIM]Cl SAIL mixture with chi[DDMIM]Cl = 0.5 exhibits the highest electrocatalytic performance towards ERCO2. Overall, this study provides a comprehensive understanding of the physicochemical properties of aqueous micellar mixtures of imidazoliumbased SAILs and highlights the composition-specific variations in their properties. These findings may have important implications for the development of new materials with enhanced solubilization potential and electrocatalytic performance toward ERCO2.