Surface Confinement Induces the Formation of Solid-Like Insulating Ionic Liquid Nanostructures

We report on the modification of the electric properties of the imidazolium-based [BMIM] [NTf2] ionic liquid upon surface confinement in the sub-monolayer regime. Solid like insulating nanostructures of [BMIM][NTf2] spontaneously form on a variety of insulating substrates, at odds with the liquid and conductive nature of the same substances in the bulk phase. A systematic spatially resolved investigation by atomic force microscopy of the morphological, mechanical, and electrical properties of [BMIM] [NTf2] nanostructures showed that this liquid substance rearranges into lamellar nanostructures with a high degree of vertical order and enhanced resistance to mechanical compressive stresses and very intense electric fields, denoting a solid-like character. The morphological and structural reorganization has a profound impact on the electric properties of supported [BMIM] [NTf2] islands, which behave like insulator layers with a relative dielectric constant between 3 and 5, comparable to those of conventional ionic solids and significantly smaller than those measured in the bulk ionic liquid. These results suggest that, in the solid-like ordered domains confined either at surfaces or inside the pores of the nanoporous electrodes of photoelectrochemical devices, the ionic mobility and the overall electrical properties can be significantly perturbed with respect to the bulk liquid phase, which would likely influence the performance of the devices.