Hidden long-range correlations in the ion distribution at the graphite / [bmim][NTf$_2$] electrified interface

TL;DR

This study uses molecular dynamics to reveal hidden long-range correlations in ion arrangements at the graphite/[bmim][NTf2] interface, suggesting subtle effects on the IL's properties related to the capacitor's charge state.

Contribution

It uncovers long-range ionic correlations normal to the interface driven by the capacitor charge, a novel insight into the static and dynamic screening behaviors.

Findings

Long-range ordering of ion pairs normal to the interface.

Correlations manifest as parallel orientation of ion pair dipoles.

Potential impact on spectroscopic properties of ionic liquids.

Abstract

A capacitor consisting of the [bmim][NTf$_2$] ionic liquid (IL) confined in between planar graphite electrodes has been investigated by molecular dynamics based on an all-atom, unpolarizable force field. Despite a few peculiarities due to the size and complexity of the ions, properties such as the density of ions throughout the capacitor, the screening of the surface charge on the electrodes by the IL and exact sum rules for the radial distribution functions of cations and anions generally comply with the results of time honored theories of the electrostatic double layer. This soothing regularity may conceal hidden correlations still compatible with the static screening rules, propagating far inside the IL the information on the state of charge of the capacitor. Evidence in this respect might have been detected by vibrational spectroscopy (see, for instance, Langmuir 2021, vol. 37, 5193-5201) showing changes in optical properties of the IL far from the charged electrodes. We show that grouping the [bmim]$^+$ and [NTf$_2$]$^-$ ions into instantaneous neutral pairs reveals an intriguing long range ordering of ions normal to the interface, driven by the capacitor state of charge. These correlations manifest themselves through the parallel orientation of the dipole moments of the neutral ion pairs. We speculate that this effect changes the average intensity of fluctuating electric fields deep in the IL, while average, static fields vanish in agreement with well established screening laws. This effect, which could change the spectroscopic properties of the IL, is present in the simulated [bmim][NTf$_2$] / graphite capacitor, but too small to be unambiguously confirmed by the present simulations with a safe margin over the error bar. The conceptual interest in these effects, however, will motivate further studies of the same or similar electrode / ionic liquid interfaces.