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

TL;DR

This work investigates the graphite/[bmim][NTf$_2$] electrified interface using all-atom MD with an unpolarizable force field, uncovering long-range correlations by partitioning ions into neutral ion-pair dipoles and optimally pairing them via simulated annealing. The simulations show that dipoles retain a net alignment along the electrode normal even where the average electric field vanishes, indicating medium-to-long-range dipole correlations that extend into bulk-like regions. The findings connect to experimentally observed Stark-like (Pockels) effects and offer new insight into interfacial physics relevant to energy storage and electrochemical applications, while proposing future directions such as grand-canonical single-electrode schemes and Berry-phase–style analogies. Overall, the paper provides a mechanistic view of how dipolar correlations can survive strong screening and influence vibrational properties in complex ionic liquids at electrified interfaces.

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. Structural and dynamical properties such as: (i) the density and orientation of the [bmim]$^+$ and [NTf$_2$]$^-$ ions throughout the capacitor; (ii) the electrostatic double layer at the electrode / electrolyte interface; (iii) the ions' mobility perpendicular and parallel to the graphite plates are determined as a function of the electrostatic charge of the capacitor, the concentration of absorbed water, the temperature and pressure. Grouping the [bmim]$^+$ and [NTf$_2$]$^-$ ions into neutral ion pairs reveals an intriguing ordering normal to the interface that is related to correlations among the dipole moments of the neutral ion pairs. These correlations might explain the observation of an anomalous Stark effect (Pockels effect) reported a few years ago in Langmuir, vol. 37, 5193-5201, (2021), and provides useful insight for the multitude of electro-chemical applications that involve electrode / ionic liquid interfaces.

Figures (13)

• Figure 1: The dry graphite/[bmim][NTf$_2$]/graphite capacitor. Gray, blue, red, green and yellow dots represent C, N, O, F and S atoms, respectively. Hydrogen atoms not shown. The regular vertical layers of C atoms are part of the graphite electrodes. The right-most graphite layer in the figure is the periodic replica of the left-most layer.
• Figure 2: (a) Honeycomb structure of the single graphite layer (graphene). (b) stacking of A (red) and B (black) layers whose repetition is makes the finite graphite slab representing the planar plates of the capacitor. Slight irregularities in the atomic positions are due to the fact that the configuration has been taken from an MD simulation at $T=300$ K.
• Figure 3: [bmim][NTf$_2$] neutral ion pair. The color coding is the same as in Fig. \ref{['capac']} The picture shows a generic configuration from a MD trajectory at $T=300$ K.
• Figure 4: Snapshot of the ionic liquid part of Fig. \ref{['capac']} coarse grained through Eq. \ref{['cg']}. Red dots: cations; blue dots: anions. Temperature in measured in scaled energy units, see text.
• Figure 5: Number ($\rho_N(x)$, blue line) and charge ($\rho_Q(x)$, red line) density profiles across the uncharged ($\sigma=0$) capacitor. $L_x$ is the average separation of the outermost graphite layers measured across the [bmim][NTf$_2$] slab.