Surfactant self-assembly and micelle elongation in reline: structural basis and driving forces

Sodium dodecyl sulfate (SDS) forms elongated micelles in urea:choline chloride deep eutectic solvent (DES), unlike its spherical aggregates in water. To understand the driving forces, we employed a multiscale simulation approach combining coarse-grained (CG) (Martini 3) and backmapped atomistic (CHARMM36) models. Simulations confirmed that micelle elongation increases with higher urea:choline chloride ratios and decreases upon hydration. Structural analysis based on radial distribution functions (RDFs), minimum distance distribution functions (MDDFs) and spatial distribution functions (SDFs) revealed that choline cations displace sodium to dominate the Stern layer, providing primary screening. Crucially, we observed differential head group solvation by DES components based on local curvature: urea preferentially accumulates around the sulfate head groups at high-curvature end-caps, while choline favours the lower-curvature cylindrical body. We conclude SDS elongation in reline results from dominant choline screening modulated by specific, curvature-dependent stabilization of end-caps by urea. This highlights the key role of hydrogen bond donor interactions in controlling surfactant self-assembly in DES. This article is part of the discussion meeting issue 'Ionic liquids and the future of soft materials'.