Lower dielectric constant in nano-confined water: Interpreting anisotropy in confinement*

Interface intrinsically breaks isotropy. Systems with confinement thus experiences different environment from isotropic systems which results in unusual dynamic properties. With growing interest in confined systems, lower dielectric constant of confined water is considered as one of the significant problems yet to be fully understood. Experiments and simulations verified that as the extent of confinement in water increases, the dielectric constant decreases. It is commonly thought that water molecules are electrostatically affected by surface of the confinement, forming specific structures and exhibiting distinctive dynamics. Examining the surface induced reduction, we conducted molecular dynamics simulation designed to produce different dielectric properties of confining walls. Interestingly, reduction of dielectric constant calculated from totally different dielectric conditions was almost identical. On the other hand, freeing surface fluctuation which was restricted by rigid surface recovers dielectric property to a certain extent. Longer correlation of dipole is also likely to responsible, as we investigated universal reduction in fluctuation of local polarization. From this study, we expect to explain the abnormal properties resulted from the anisotropy in more physical and concrete words.