Supramolecular Self‐Assembly of Nanoconfined Ionic Liquids for Fast Anisotropic Ion Transport

Abstract Materials involving nanoconfinement of ionic liquids (ILs) have been pursued for functionalities and ionic devices. However, their complex synthesis, challenges to achieve long‐range order, and laborious tunability limit their practical implementation. Herein, these challenges are addressed by complexing surfactants to ILs, yielding a facile, modular, and scalable approach. Based on structural screening, ionic complexation of di‐n‐nonylamine to the terminal sulfonic acid of 1‐(4‐sulfobutyl)‐3‐methylimidazolium hydrogen sulfate IL is selected as a proof of concept. Spontaneous homeotropic smectic order over micrometers is observed, with alternating ionic and alkyl layers. The 1 nm thick ionic layers involve 2D crystalline internal order up to 150 °C, strongly promoting anisotropic ion transport (σ_||/σ_⊥ > 6500), and curiously, still allowing fluidity. High ionic conductivity of 35 mS cm⁻¹ and mesoscopic diffusion coefficient of ≈10⁻⁵ cm² s⁻¹ at 150 °C along the ionic layers are observed. Fast anisotropic ion transport by simply complexing two components open doors to functional materials and applications.