Origin of the anisotropic dielectric response of nanoconfined water
In plain words
Water squeezed into a gap about a nanometer wide barely responds to an electric field across the gap, as if it were oil, yet it responds enormously to a field along the gap. A single microscopic explanation of both facts is missing.
Precise statement
For water in slits of height h between atomically flat graphite or hBN walls, the perpendicular dielectric constant falls to epsilon_perp ~ 2 for $h \sim 1\ \mathrm{nm}$ (2018 measurement), while a 2025 measurement found the in-plane constant $\epsilon_{\mathrm{par}}$ of about 1000 at few-layer thickness with superionic-level conductivity. Determine whether a model of interfacial layers with an anisotropic local dielectric tensor reproduces both epsilon_perp(h) and epsilon_par(h), or whether nonlocal correlated polarization and proton transport are required. An answer is a first-principles or simulation prediction of epsilon_perp(h), epsilon_par(h) and conductivity that matches both data sets.
What would settle it
Ab initio or validated force-field simulations of the measured geometries reproducing both dielectric components and the conductivity versus h, with the controlling molecular feature identified.
Status in the literature
Unverified note
The 2025 Nature in-plane measurement attributed the enhancement to disordered hydrogen bonding and fast proton exchange; the low out-of-plane value is broadly reproduced by simulations of interfacial ordering.