Unambiguous characterization of in-plane dielectric response in nanoconfined liquids: water as a case study

TL;DR

This paper introduces the in-plane 2D polarizability as a clear measure of dielectric response in nanoconfined water, overcoming previous ambiguities related to water width dependence.

Contribution

It proposes a new unambiguous characterization method for in-plane dielectric response in nanoconfined liquids, validated through molecular dynamics simulations.

Findings

The in-plane 2D polarizability provides a consistent measure of dielectric response.

Two independent methods yield compatible results for $eta_{ ext{parallel}}$.

Framework established for quantifying dielectric response in experiments and simulations.

Abstract

The in-plane dielectric constant of nanoconfined water has attracted growing interest over the last years. Nevertheless, this magnitude is not well-defined at the nanoscale due to its dependence on the arbitrary choice of water width. We propose the in-plane 2D polarizability, $\alpha_{\parallel}$, as an unambiguous characterization of the in-plane dielectric response under 2D confinement, in analogy to what has been recently done for the perpendicular response. Using classical molecular dynamics simulations, we compute $\alpha_{\parallel}$ via two independent and consistent methods: based on fluctuation--dissipation theory, and from the induced dipole moment when water is placed in a capacitor. Our results provide the framework to quantify the in-plane dielectric response of polar liquids across simulations and experiments.