Strongly Anisotropic Dielectric Relaxation of Water at the Nanoscale

We carried out atomistic simulations of water at the nanoscale, and we investigated the dielectric response of the liquid as a function of the distance between hydrophobic confining surfaces. We found that dipolar fluctuations are modified by the presence of surfaces up to strikingly large distances, i.e., tens of nanometers. Fluctuations are suppressed by approximately an order of magnitude in the z direction, perpendicular to the interface, and they are enhanced (up to 50%) in the x-y plane, giving rise to strong anisotropies in the components of the dielectric relaxation. Such anisotropies originate from the very presence of interfaces, and not from the details of the interaction between water and the hydrophobic surfaces. Our findings are consistent with recent terahertz and ultrafast infrared pump-probe spectroscopy experiments and bear important implications for the study of solvation under confinement.