Redefining the dielectric response of nanoconfined liquids: insights from water

Abstract

Recent experiments show that the relative dielectric constant $ε$ of water confined to a film of nanometric thickness reaches a strikingly low value of 2.1, barely above the bulk's 1.8 value for the purely electronic response. We argue that $ε$ is not a well-defined measure for dielectric properties at sub-nanometer scales due to the ambiguous definition of confinement width. Instead we propose the 2D polarizability $α_{\perp}$ as the appropriate, well-defined response function whose magnitude can be directly obtained from both measurements and computations. Once the appropriate description is used, understanding the interplay between electronic and ionic contributions becomes critical, contrary to what is widely assumed. This highlights the importance of electronic degrees of freedom in interpreting the dielectric response of polar fluids under nanoconfinement conditions, as revealed by molecular dynamics simulations.

Figures (3)

• Figure 1: (a) Schematic view of the simulation box, perpendicular to the confining plane. The gray-shaded regions represent the location of the confining soft potentials. Oxygen and hydrogen atoms in the water molecules are represented as red and white spheres, respectively. (b) Oxygen density profiles along the confining direction for 2D densities $\sigma=$ 0.177 Å$^{-2}$ (black) and 0.195 Å$^{-2}$ (blue).
• Figure 2: 2D polarisation $\mathcal{P}_{\mathrm{2D}}$ vs external traverse electric field $\mathcal{E}^{\mathrm{ext}}_{\perp}$ for TIP4P-2005 water classical trajectories (black) and for DFT calculations using the classical TIP4P-2005 under the same external field (red). In blue, we show the pure electronic response obtained from DFT as explained in the text. Inset: 2D density dependence of $\alpha_{\perp}$ obtained with the TIP4P-2005 classical simulations.
• Figure 3: Dielectric constant across a water thin film as a function of the defined width $w$ of the film for a given value of the 2D polarisability, $\alpha_{\perp} = 6.06$ Å, as obtained with DFT simulations described in the text. Values of $\epsilon_{\perp}$ are highlighted for three values of $w$ for the same film: ($i$) the distance between the origins of the confining potentials $w$ = 8 Å, ($ii$) $w = 5.17$ Å, the accessible perpendicular space as defined by Kumar et al.Stanley, for this bilayer film, and ($iii$) $w=5.31$ Å, obtained from normalising the 2D density of the film with the 3D density of bulk water. Negative values for $\epsilon_{\perp}$ indicate overscreening, a larger polarisation than what is needed to screen the field completely.