Giant Dielectric Anisotropy and Enhanced In-Plane Conductivity in Nanoconfined Water

Recent experiments revealed a strong dielectric anisotropy in nanoconfined water, with a giant in-plane dielectric constant exceeding 10^{3} and an ultralow out-of-plane response of order 2-3, far beyond the predictions of conventional continuum electrostatics. A pronounced enhancement of in-plane electrical conductivity was also observed under strong confinement. Here, we develop a unified microscopic theory showing that these anomalies arise from confinement-induced anisotropic polarization correlations in the hydrogen-bond network. Starting from linear-response theory with an exponential polarization-correlation kernel, we show that nanoconfinement selectively enhances longitudinal in-plane dipolar correlations while suppressing perpendicular fluctuations. This leads to universal scaling laws for the dielectric response, κ(h)∝ξ_{c}^{2}f(h/2ξ_{c}), and the conductivity, σ(h)∝κ(h)/h, where ξ_{c} is the confinement-enhanced dipolar correlation length. The resulting growth of in-plane correlations and suppression of out-of-plane fluctuations naturally explain the giant in-plane and ultralow out-of-plane dielectric constants, as well as the enhanced in-plane conductivity. Quantitative agreement with experiment over a broad range of channel thicknesses establishes nanoconfined water as a ferroelectriclike correlated polar fluid governed by collective dipolar fluctuations rather than simple geometric capacitance effects.