Charge Oscillations in Ionic Liquids: A Microscopic Cluster Model

TL;DR

This paper presents a microscopic cluster model for ionic liquids that explains charge oscillations near charged interfaces, aligning with experimental observations and connecting to the BSK continuum theory.

Contribution

It introduces a detailed ionic cluster-based theory that captures charge oscillations and links microscopic details to macroscopic models in ionic liquids.

Findings

Charge density oscillates near charged boundaries with alternating ion layers.

Short-range oscillations are due to small ion pairs, long-range oscillations involve larger clusters.

The model recovers the BSK equation in the linear limit, explaining its parameters.

Abstract

In spite of their enormous applications as alternative energy storage devices and lubricants, room temperature ionic liquids (ILs) still pose many challenges from a pure scientific view point. We develop an IL microscopic theory in terms of ionic clusters, which describes the IL behavior close to charged interfaces. The full structure factor of finite-size clusters is considered and allows us to retain fine and essential details of the system as a whole. Beside the reduction in the screening, it is shown that ionic clusters cause the charge density to oscillate near charged boundaries, with alternating ion-size thick layers, in agreement with experiments. We distinguish between short-range oscillations that persist for a few ionic layers close to the boundary, as opposed to long-range damped oscillations that hold throughout the bulk. The former can be captured by finite-size ion pairs, while the latter is associated with larger clusters with pronounced quadrupole (or higher) moment. The long-wavelength limit of our theory recovers the well-known Bazant-Storey-Kornyshev (BSK) equation in the linear regime, and elucidates the microscopic origin of the BSK phenomenological parameters.