Dielectric response of thin water films: A thermodynamic perspective

TL;DR

This study uses a thermodynamic approach and molecular simulations to show that thin water films behave like bulk water in terms of dielectric response, challenging previous assumptions of reduced interfacial dielectric constants.

Contribution

It demonstrates that a simple dielectric continuum model accurately describes the dielectric response of thin water films, even at nanometer scales, by carefully considering boundary placement.

Findings

Good agreement between simulations and dielectric continuum model

Bulk dielectric permittivity applies up to the water-vapor boundary

Proper boundary placement resolves experimental discrepancies

Abstract

The surface of a polar liquid presents a special environment for the solvation and organization of charged solutes, which differ from bulk behaviors in important ways. These differences have motivated many attempts to understand electrostatic response at aqueous interfaces in terms of a spatially varying dielectric permittivity, typically concluding that the dielectric constant of interfacial water is significantly lower than in the bulk liquid. Such analyses, however, are complicated by the potentially nonlocal nature of dielectric response over the short length scales of interfacial heterogeneity. Here we circumvent this problem for thin water films by adopting a thermodynamic approach. Using molecular simulations, we calculate the solvent's contribution to the reversible work of charging a parallel plate capacitor. We find good agreement with a simple dielectric continuum model that assumes bulk dielectric permittivity all the way up to the liquid's boundary, even for very thin ($\sim 1$ nm) films. This comparison requires careful attention to the placement of dielectric boundaries between liquid and vapor, which also resolves apparent discrepancies with dielectric imaging experiments.