Tunable dielectric constant of water at the nanoscale

Using molecular dynamics simulations, the influence of the surface charge density of a nanotube on the static dielectric permittivity $\epsilon$ of confined water was reported. Whereas the dielectric anisotropy between the radial and axial directions of water confined in hydrophilic and hydrophobic membranes and the increase in axial dielectric permittivity with respect to the bulk value have previously been described, we found that an increase in the surface charge density leads to a drastic decrease in $\epsilon$ into the axial direction. The decrease in $\epsilon$ is accompanied by a strong slowdown in the rotational dynamics of water molecules. We show that this effect is due to the strong orientation of water molecules induced by the surface charge. Thus, by controlling the surface charge in nanotubes and nanocavities, it is possible to tune the dielectric permittivity of confined fluids at the nanoscale.

Figure 1

Snapshot of a typical configuration of confined water within a silica nanotube. Yellow, red, and white colors represent the sulfur, oxygen, and hydrogen atoms, respectively.

Figure 2

Cation (octyl isoquinolium) and the anion [bis(trifluoromethyl)sulfonimide] of the ionic liquid.

Figure 3

(a) Normalized normal axial $\left({\epsilon }_{\left|\right|}/{\epsilon }_b\right)$ dielectric permittivity profiles of water as a function of the surface charge density. ${\epsilon }_b$ corresponds to the dielectric permittivity of water in the bulk phase. The horizontal solid line corresponds to the bulk value. The position $r=0$ Å corresponds to the pore center and $r=6$ Å corresponds to the pore wall. The vertical arrow indicates the direction of evolution of the dielectric permittivity. (b) Radial profiles of the water density as a function of the surface charge density.

Figure 4

Axial dipolar relaxation time ${\tau }_{\left|\right|}$ as a function of the surface charge at the center of the pore and close to the solid-liquid interface.

Figure 5

(a) Distribution of the angle between the dipole moment vector of the water molecules and the normal of the silica nanotube as a function of the surface charge density. The vertical arrow indicates the direction of evolution of the angular profile as a function of the surface charge density. The color code is similar to that in Fig. 3. (b) Parallel component of the dielectric permittivity profile as a function of the surface charge density for the CC nanotube pore. (c) Normalized radial $\left({\epsilon }_r/{\epsilon }_b\right)$ dielectric permittivity as function of the surface charge density of the confined water in a spherical nanocavity. In (b) and (c) the vertical arrows indicate the direction of evolution of the dielectric permittivity.

Figure 6

Profiles of ${\epsilon }_{\left|\right|}$ of (a) water confined in two CNTs (10,0) and (16,0), of different radii and (b) an ionic liquid confined within a CNT(16,0). The position $r=0$ Å corresponds to the pore center and $r=6$ Å corresponds to the pore wall. The horizontal lines indicate the bulk values.