Wetting transition of ionic liquids at metal surfaces: A computational molecular approach to electronic screening using a virtual Thomas-Fermi fluid

Abstract

Of particular relevance to energy storage, electrochemistry and catalysis, ionic and dipolar liquids display a wealth of unexpected fundamental behaviors – in particular in confinement. Beyond now well-documented adsorption, overscreening and crowding effects1,2,3, recent experiments have highlighted novel phenomena such as unconventional screening4 and the impact of the electronic nature – metallic versus insulating – of the confining surface on wetting/phase transitions5,6. Such behaviors, which challenge existing theoretical and numerical modeling frameworks, point to the need for new powerful tools to embrace the properties of confined ionic/dipolar liquids. Here, we introduce a novel atom-scale approach which allows for a versatile description of electronic screening while capturing all molecular aspects inherent to molecular fluids in nanoconfined/interfacial environments. While state of the art molecular simulation strategies only consider perfect metal or insulator surfaces, we build on the Thomas-Fermi formalism for electronic screening to develop an effective approach that allows dealing with any imperfect metal between these asymptotes. The core of our approach is to describe electrostatic interactions within the metal through the behavior of a `virtual' Thomas-Fermi fluid of charged particles, whose Debye length sets the Thomas-Fermi screening length λ in the metal. This easy-to-implement molecular method captures the electrostatic interaction decay upon varying λ from insulator to perfect metal conditions, while describing very accurately the capacitance behavior – and hence the electrochemical properties – of the metallic confining medium. By applying this strategy to a nanoconfined ionic liquid, we demonstrate an unprecedented wetting transition upon switching the confining medium from insulating to metallic. This novel approach provides a powerful framework to predict the unsual behavior of ionic liquids, in particular inside nanoporous metallic structures, with direct applications for energy storage and electrochemistry.

Molecular dynamics of ionic polymer-metal composites

“…Further, the proposed representation of the electrodes only seeks to capture physical non-penetration, thereby neglecting their metallic nature that would force a null electric field. More accurate representations of the electrodes will be considered in future studies, following [48–51]. Moreover, the walls we use to mimic the electrodes are assumed to be smooth, in contrast with experimental evidence supporting the formation of rough interfaces that favour charge storage [1].…”

Molecular Simulation of Electrode-Solution Interfaces

“…Electrostatic screening inside the solid can be captured at the continuum level using Thomas-Fermi theory, an approach which can also be exploited for classical molecular simulation. Earlier attempts included this e↵ect of screening on the electrolyte via an external potential [119,120], but new strategies have been proposed very recently, based on fluctuating charges [121] or on mobile charges [122]. This will be particularly useful to address the e↵ect of the metallic character of the electrode on the properties of the interfacial fluids, such as the nanoscale capillary freezing of ionic liquids [123].…”

Microscopic Simulations of Electrochemical Double-Layer Capacitors

“…Another approach has also been proposed, which consists of using a "virtual" Thomas−Fermi fluid of charged particles to account for the screening. 133 It is also possible to go further in that direction by treating the electrode materials using the tight binding model, 134,135 in which the electronic band structure is approximated using atomic wave functions. However, these enhanced electrode models have not yet been used to simulate supercapacitor devices with complex material structures.…”