Anomalously low dielectric constant of confined water

TL;DR

This study measures the dielectric constant of water confined at the nanoscale, revealing an unexpectedly low value near surfaces, which challenges existing theories and offers new insights into interfacial water behavior.

Contribution

First direct measurement of dielectric constant of interfacial water at nanometer scale, showing an extremely low value and providing critical data for theoretical models.

Findings

Interfacial water has a dielectric constant around 2.

The dead layer is 2-3 molecules thick.

Results challenge existing theories of interfacial water behavior.

Abstract

The dielectric constant of interfacial water has been predicted to be smaller than that of bulk water (= 80) because the rotational freedom of water dipoles is expected to decrease near surfaces, yet experimental evidence is lacking. We report local capacitance measurements for water confined between two atomically-flat walls separated by various distances down to 1 nm. Our experiments reveal the presence of an interfacial layer with vanishingly small polarization such that its out-of-plane dielectric constant is only approximately 2. The electrically dead layer is found to be two to three molecules thick. These results provide much needed feedback for theories describing water-mediated surface interactions and behavior of interfacial water, and show a way to investigate the dielectric properties of other fluids and solids under extreme confinement.