Interfacial Ionic Liquids: Connecting Static and Dynamic Structures

TL;DR

This study combines real-time X-ray reflectivity and molecular dynamics simulations to reveal that interfacial ionic liquids exhibit bistable structures with potential-dependent transitions, influenced by temperature and energy barriers.

Contribution

It introduces a combined experimental and computational approach to understand the dynamic interfacial structures of ionic liquids under varying potentials.

Findings

Interfacial structure is bistable and can be described as a mixture of two extreme states.

Transition between structures involves an energy barrier of approximately 0.15 eV.

Temperature affects the width of the transition but not its hysteresis magnitude.

Abstract

It is well-known that room temperature ionic liquids (RTILs) often adopt a charge-separated layered structure, i.e., with alternating cation- and anion-rich layers, at electrified interfaces. However, the dynamic response of the layered structure to temporal variations in applied potential is not well understood. We used in situ, real-time X-ray reflectivity (XR) to study the potential-dependent electric double layer (EDL) structure of an imidazolium-based RTIL on charged epitaxial graphene during potential cycling as a function of temperature. The results suggest that the graphene-RTIL interfacial structure is bistable in which the EDL structure at any intermediate potential can be described by the combination of two extreme-potential structures whose proportions vary depending on the polarity and magnitude of the applied potential. This picture is supported by the EDL structures obtained by fully atomistic molecular dynamics (MD) simulations at various static potentials. The potential-driven transition between the two structures is characterized by an increasing width but with an approximately fixed hysteresis magnitude as a function of temperature. The results are consistent with the coexistence of distinct anion and cation adsorbed structures separated by an energy barrier (~0.15 eV).