Water confined between two plates just a few nanometers apart ceases to behave like an ordinary liquid. Its dielectric permittivity along the plane of the layer soars above a thousand, while perpendicular to the walls it drops to two or three units. At the same time, proton conductivity rises sharply.
The usual notions of water as a simple medium with a constant permittivity do not work here. When the gap thickness is less than five nanometers, almost all the water finds itself within the zone of influence of the surfaces. The molecules cannot orient freely in three dimensions — their dipoles are forced to lie in the plane.
This constraint lengthens the correlations between neighboring dipoles. Instead of local fluctuations, domains arise consisting of hundreds of molecules linked by hydrogen bonds and oscillating in concert. Such collective motions enhance the response to an electric field in the plane and suppress it in the perpendicular direction.
A theoretical model built on the linear response with an exponential kernel of polarization correlations yields universal scaling laws. The dielectric permittivity grows in proportion to the square of the correlation length, while the conductivity is inversely proportional to the layer thickness. The calculations agree quantitatively with experiments over a range of thicknesses from one to five nanometers.
Nanoconfined water behaves as a correlated polar liquid with ferroelectric traits. Its properties are determined not by the geometry of the capacitor but by collective fluctuations of the dipoles. This opens a path to controlling ion transport and chemical activity in nanochannels and nanopores.
Understanding the mechanism makes it possible to predict the behavior of water in biological membranes, graphene channels and nanomaterials, where the layer thickness is measured in nanometers.
