Redefining the dielectric response of nanoconfined liquids: insights from water

TL;DR

This paper challenges the use of dielectric constant as a measure for nanoconfined water's response, proposing 2D polarizability as a more accurate descriptor that captures electronic and ionic effects revealed through simulations.

Contribution

It introduces 2D polarizability as a well-defined response function for nanoconfined liquids, replacing the problematic dielectric constant.

Findings

Dielectric constant is ambiguous at sub-nanometer scales.

2D polarizability can be directly measured and computed.

Electronic degrees of freedom are crucial in nanoconfined dielectric response.

Abstract

Recent experiments show that the relative dielectric constant $\epsilon$ of water confined to a film of nanometric thickness reaches a strikingly low value of 2.1, barely above the bulk's 1.8 value for the purely electronic response. We argue that $\epsilon$ is not a well-defined measure for dielectric properties at sub-nanometer scales due to the ambiguous definition of confinement width. Instead we propose the 2D polarizability $\alpha_{\perp}$ as the appropriate, well-defined response function whose magnitude can be directly obtained from both measurements and computations. Once the appropriate description is used, understanding the interplay between electronic and ionic contributions becomes critical, contrary to what is widely assumed. This highlights the importance of electronic degrees of freedom in interpreting the dielectric response of polar fluids under nanoconfinement conditions, as revealed by molecular dynamics simulations.