Water in Carbon Nanotubes: Pronounced Anisotropy in Dielectric Dispersion and Its Microscopic Origin

We report a remarkable anisotropy in the dielectric properties of water under cylindrical nanoconfinements in carbon nanotubes (CNTs). We employ linear response theory and use a new relation to define and calculate the two distinct eigenvalues of the dielectric tensor, namely, the axial (ε) and perpendicular (ε) components. We discover that not just the dielectric tensor but dielectric relaxation also exhibits strong anisotropy. The present study reveals that the parallel (ε) and the orthogonal (ε) components approach the bulk value in an opposite manner when the diameter of the CNT is increased. Rather unexpectedly, the polarization fluctuations remain quenched in the perpendicular direction even for the largest systems studied. The microscopic origin of the observed anisotropy is attributed to the propagation of surface effects into the confined liquid.