Understanding the anomalously low dielectric constant of confined water: an ab initio study

TL;DR

This study uses ab initio molecular dynamics to reveal that the low dielectric constant of confined water is due to interfacial ferroelectric ordering and dipole alignment, independent of slit width and substrate electronic structure.

Contribution

It provides the first ab initio evidence that interfacial water polarization causes the low dielectric constant in nanoconfined water, challenging classical force-field assumptions.

Findings

Interfacial water exhibits ferroelectric ordering near surfaces.

The dielectric constant reduction is independent of slit width.

Water structure and polarization are similar near graphene and hBN.

Abstract

Recent experiments have shown that the out-of-plane dielectric constant of water confined in nanoslits of graphite and hexagonal boron nitride (hBN) is vanishingly small. Despite extensive effort based mainly on classical force-field molecular dynamics (FFMD) approaches, the origin of this phenomenon is under debate. Here we used ab initio molecular dynamics simulations (AIMD) and AIMD-trained machine learning potentials to explore the structure and electronic properties of water confined inside graphene and hBN slits. We found that the reduced dielectric constant arises mainly from the anti-parallel alignment of the water dipoles in the perpendicular direction to the surface in the first two water layers near the solid interface. Although the water molecules retain liquid-like mobility, the interfacial layers exhibit a net ferroelectric ordering and constrained hydrogen-bonding orientations which lead to much reduced polarization fluctuations in the out-of-plane direction at room temperature. Importantly, we show that this effect is independent of the distance between the two confining surfaces of the slit, and it originates in the spontaneous polarization of interfacial water. Our calculations also show no significant variations in the structure and polarization of water near graphene and hBN, despite their different electronic structures. These results are important as they offer new insight into a property of water that plays a critical role in the long-range interactions between surfaces, the electric double-layer formation, ion solvation and transport, as well as biomolecular functioning.