Experimental Evidence of Long-Lived Electric Fields of Ionic Liquid Bilayers

Herein we demonstrate that ionic
liquids can form long-lived double
layers, generating electric fields detectable by straightforward open
circuit potential (OCP) measurements. In imidazolium-based ionic liquids
an external negative voltage pulse leads to an exceedingly stable
near-surface dipolar layer, whose field manifests as long-lived (∼1–100
h) discrete plateaus in OCP versus time traces. These plateaus occur
within an ionic liquid-specific and sharp potential window, defining
a simple experimental method to probe the onset of interfacial ordering
phenomena, such as overscreening and crowding. Molecular dynamics
modeling reveals that the OCP arises from the alignment of the individual
ion dipoles to the external electric field pulse, with the magnitude
of the resulting OCP correlating with the product of the projected
dipole moment of the cation and the ratio between the cation diffusion
coefficient and its volume. Our findings also reveal that a stable
overscreened structure is more likely to form if the interface is
first forced through crowding, possibly accounting for the scattered
literature data on relaxation kinetics of near-surface structures
in ionic liquids.