In-plane dielectric constant and conductivity of confined water

TL;DR

This study investigates how water's electrical properties change when confined at the nanoscale, revealing giant dielectric constants and high conductivity in ultra-thin layers, which differ significantly from bulk water.

Contribution

It provides direct measurements of in-plane dielectric constant and conductivity of nanoconfined water, showing dramatic property changes at molecular thicknesses.

Findings

Dielectric constant reaches ~1000 in few-molecule-thick water

Proton conductivity peaks at a few S/m in ultra-thin water

Enhanced in-plane polarization due to disordered hydrogen bonds

Abstract

Water is essential for almost every aspect of life on our planet and, unsurprisingly, its properties have been studied in great detail. However, disproportionately little remains known about the electrical properties of interfacial and strongly confined water where its structure deviates from that of bulk water, becoming distinctly layered. The structural change is expected to affect water's conductivity and particularly its polarizability, which in turn modifies intermolecular forces that play a crucial role in many physical and chemical processes. Here we use scanning dielectric microscopy to probe the in-plane electrical properties of water confined between atomically flat surfaces separated by distances down to 1 nm. For confinement exceeding a few nm, water exhibits an in-plane dielectric constant close to that of bulk water and its proton conductivity is notably enhanced, gradually increasing with decreasing water thickness. This trend abruptly changes when the confined water becomes only a few molecules thick. Its in-plane dielectric constant reaches giant, ferroelectric-like values of about 1,000 whereas the conductivity peaks at a few S/m, close to values characteristic of superionic liquids. We attribute the enhancement to strongly disordered hydrogen bonding induced by the few-layer confinement, which facilitates both easier in-plane polarization of molecular dipoles and faster proton exchange. This insight into the electrical properties of nanoconfined water is important for understanding many phenomena that occur at aqueous interfaces and in nanoscale pores.