Anomalous Dielectric Behavior of Nanoconfined Electrolytic Solutions

We report an anomalous dielectric effect of electrolytes under cylindrical nanoconfinement. In bulk phase, the decrease in the water dielectric constant ($ϵ$) with increasing salt concentration is well known, and is due to dielectric saturation. From molecular dynamic simulations of confined water and NaCl solutions, we show a dielectric anisotropy and an unexpected increase in ${ϵ}_{\perp }$ of NaCl solutions with respect to the confined pure liquid until a critical concentration is reached. We infer that this striking dielectric behavior results from the interplay between the effect of confinement and that of ions on the water hydrogen bonding network.

Figure 1

Ratio of the dielectric constant of NaCl solutions in bulk phase (${ϵ}^{\mathrm{bs}}$) and the confined phase of various concentrations to the dielectric constant of water in bulk phase (${ϵ}^{\mathrm{bw}}$). In the confined phase, the dielectric constant is calculated from the average of the local component between $r=0\AA$ and $r=12\AA$ from the accounting of the dipolar relaxation of ions pairs (a) and considering water dipole only (b).

Figure 2

Profiles of ratio of ${ϵ}_{\parallel }$ and ${ϵ}_{\perp }$ of NaCl solutions in confined phase of various concentrations to the dielectric constant of water in bulk phase (${ϵ}^{\mathrm{bw}}$). (a) and (b): The water molecules and ion pair dipoles were considered in the calculation of $ϵ$. (c) and (d): Profiles were obtained by considering the water molecules dipoles only.

Figure 3

(a) Radial profile of hydrogen bond density of confined water (solid line). The dashed line corresponds to the HB density of water in bulk phase. (b) Radial profile of the ratio between ${ϵ}_{\parallel }$ of confined water and that of water in bulk phase (right axis and full circle symbols). Radial profile of the total number of hydrogen bonds per water molecule of nanoconfined water (dotted line and left axis).

Figure 4

Illustration of two hydrogen bonding networks ($A=[<8\AA ]$; $B=[8–10\AA ]$; $C=[>10\AA ]$). The dashed and dotted bonds show the possibility of water molecules to form a hydrogen bond between two networks (water-water and water-SiOH).