  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.
