Abnormal in-plane permittivity and ferroelectricity of confined water: From sub-nanometer channels to bulk.

Dielectric properties of nano-confined water are important in several areas of science, i.e., it is relevant in the dielectric double layer that exists in practically all heterogeneous fluid-based systems. Molecular dynamics simulations are used to predict the in-plane dielectric properties of confined water in planar channels of width ranging from sub-nanometer to bulk. Because of suppressed rotational degrees of freedom near the confining walls, the dipole of the water molecules tends to be aligned parallel to the walls, which results in a strongly enhanced in-plane dielectric constant (ε∥) reaching values of about 120 for channels with height 8 Å < h < 10 Å. With the increase in the width of the channel, we predict that ε∥ decreases nonlinearly and reaches the bulk value for h > 70 Å. A stratified continuum model is proposed that reproduces the h > 10 Å dependence of ε∥. For sub-nanometer height channels, abnormal behavior of ε∥ is found with two orders of magnitude reduction of ε∥ around h ∼ 7.5 Å, which is attributed to the formation of a particular ice phase that exhibits long-time (∼µs) stable ferroelectricity. This is of particular importance for the understanding of the influence of confined water on the functioning of biological systems.

Out-of-plane permittivity of confined water.

The dielectric properties of confined water is of fundamental interest and is still controversial. For water confined in channels with height smaller than h=8Å, we found a commensurability effect and an extraordinary decrease in the out-of-plane dielectric constant down to the limit of the dielectric constant of optical water. Spatial resolved polarization density data obtained from molecular dynamics simulations are found to be antisymmetric across the channel and are used as input in a mean-field model for the dielectric constant as a function of the height of the channel for h>15Å. Our results are in excellent agreement with a recent experiment [L. Fumagalli et al., Science 360, 1339 (2018)SCIEAS0036-807510.1126/science.aat4191].