Surfactant self-assembly in water-in-salt electrolytes

The surfactant DTAB forms micelles in all WiSEs studied, persisting at salt concentrations far beyond those in conventional aqueous solutions. In LiTFSI WiSEs, salt concentration determines micelle structure. Hydrophobic TFSI^- counterions screen the micelle surface at 5 mol/kg, forming elongated micelles, while at ≥8 mol/kg they intercalate between headgroups, producing globular micelles. At certain high salt/low surfactant compositions, traditional micelles do not form. Instead, small, disordered clusters are formed that are further enhanced at elevated temperatures. In contrast, hydrophilic NO₃^- counterions (LiNO₃ and NaNO₃ systems) maintain near identical morphologies regardless of salt concentration, though co-ion type strongly affects morphology. Na⁺ produces well-defined wormlike micelles while Li⁺ causes instability, resulting in highly polydisperse assemblies. These findings provide a platform for enhancing WiSEs through protective electrode interfaces that further reduce water activity, inhibit lithium dendrite formation, and control ion transport. This work also advances fundamental understanding of surfactant self-assembly in concentrated electrolytes and in nanostructured fluids.