Dielectric Constant and Conductivity of Planar-Confined Water

 

Water is essential for life, and its properties have been extensively studied, yet little is known about its electrical behavior under interfacial or extreme confinement conditions.

In such states, water loses its bulk structure and adopts a laminar

arrangement, modifying its electrical conductivity, polarizability, and

the intermolecular forces that govern numerous physical and chemical processes.

In this work, the in-plane dielectric properties of water confined between two hBN layers—one flat and one patterned with channels—separated by up to 1 nm, were measured.

Scanning Dielectric Microscopy (SDM), based on an Atomic Force Microscope (AFM), was employed to detect capacitance variations of the water confined within the channels formed on the hexagonal boron nitride (hBN) surface.

When the confinement exceeds several nanometers, the dielectric constant and conductivity of water approach their bulk values, and proton conduction increases. As the water layer thickness decreases, conductivity rises sharply, and when the thickness reaches only a few molecular layers, the in-plane dielectric constant attains values on the order of 1000—similar to those of ferroelectric materials—while conductivity reaches several S m⁻¹, comparable to that of superionic liquids.

Bulk water exhibits a high dielectric constant (ε_bulk ≈ 80) and a conductivity of approximately σ_bulk ≈ 10⁻⁵ S m⁻¹, values similar to those of a wide-bandgap semiconductor. This explains its ability to form hydrogen bonds and to dissolve more substances than any other liquid, producing strong dielectric screening essential for the biochemical processes of life.

Recent studies have shown that water confined between monolayers is

non-polarizable in the perpendicular direction (ε_⊥ ≈ 2), in agreement with theoretical predictions, although the parallel dielectric constant (ε_∥) remains experimentally unknown and theoretically unexplained.

This work demonstrates that under extreme molecular confinement, the electrical properties of 2D-confined water change dramatically, providing new insights into the electrical double layer and strongconfinement phenomena, and opening avenues for explaining interfacial and nanoconfined aqueousprocesses in other materials.

For further information see: Nature