Nancy C. Forero-Martinez

arXiv2026

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

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.

physics.chem-ph↗

arXiv2025

A kinetic model to simulate charge flow through an electrochemical half cell

A kinetic model of the electron transfer at the electrode / electrolyte solution interface is developed, implemented in a Monte Carlo framework, and applied to simulate this process in idealised systems consisting of the primitive model of electrolyte solutions limited by an impenetrable conducting surface. In the present implementation, a charged, spherical interface surrounding an equally spherical sample of electrolyte solution is introduced to model a single-electrode system, providing the computational analog to the conceptual half-cell picture that is widely used in electrochemistry. The electron transfer itself is described as a simple surface hopping process underlying a first order reaction corresponding to one of the coupled M/M$^+$ and X$^-$/X half reactions. Then, the electron transfer at the interface is combined with the self-diffusion of ions in the electrolyte solutions whose role is to supply reagents and disperse products, allowing the system to settle in a stationary non-equilibrium state. Simulations for the primitive model of electrolyte in contact with a charged impenetrable surface show that, after a brief transient, the samples sustain a steady current through the electrolyte solution. The results quantify the dependence of the current on: the overall charge of the electrode, the electrolyte concentration, the solvent viscosity and the kinetic parameter $k_e$ that represents the rate of the electron transfer for each ion in contact with the electrode. Since the simulated interface is very idealised, strategies to overcome the limitations of the present model are outlined and briefly discussed.

physics.chem-ph↗