Water Dielectric Effects in Planar Confinement

We investigate the dielectric profile of water confined between two planar polar walls using atomistic molecular dynamics simulations. For a water slab thickness below 1 nm the dielectric response is highly asymmetric: while the parallel component slightly increases compared to bulk, the perpendicular one decreases drastically due to anticorrelated polarization of neighboring water molecules. We demonstrate the importance of the dielectric contribution due to flexible polar headgroups and derive an effective dielectric tensorial box model suitable for coarse-grained electrostatic modeling.

Figure 1

Simulation snapshot for a water layer thickness $L_w=1.5\mathrm{nm}$. Selected molecules are shown using van der Waals spheres. Each monolayer consists of 100 decanol molecules. The inset on the right shows the distribution of partial charges on the flexible polar headgroups.

Figure 2

MD results for a water slab thickness $L_w=1.5\mathrm{nm}$. Profiles for (a) mass density, (b) charge density, (c) parallel dielectric response, (d) inverse perpendicular dielectric response, and (e) perpendicular dielectric response are shown, contributions of water and decanol are indicated by blue and red lines, respectively. The horizontal broken lines denote the bulk value of SPC/E water. The inset in (d) zooms into the slab central region where bulk water behavior is obtained. The vertical gray lines indicate the Gibbs dividing surface positions.

Figure 3

Total (solid triangles) as well as self (open circles) and collective (open diamonds) polarization variances as a function of water slab thickness $L_w$. The solid horizontal line represents the expected bulk limit $C_{\alpha }/(A{L}_w{ϵ}_0{k}_{B}T)=70-1=69$, the dashed horizontal line shows the Langevin model prediction for an isolated water molecule $C_{\alpha }^{\text{self}}/(A{L}_w{ϵ}_0{k}_{B}T)=19$.

Figure 4

(a) Parallel and (b) inverse perpendicular dielectric profiles at various water slab thicknesses. The vertical broken lines indicate the corresponding water slab thickness $L_w$.

Figure 5

(a) Shifts of the effective dielectric box widths $L_{\alpha }^{\text{eff}}-L_w$ assuming effective dielectric constants corresponding to the bulk value as defined by Eqs. (10) and (11). The horizontal dashed lines denote the asymptotic shifts ${\delta }_{\alpha }=L_{\alpha }^{\text{eff}}-L_w$ for large separation. (b) Effective dielectric constants ${ϵ}_{\alpha }^{\text{eff}}$ when assuming effective box widths given by $L_{\alpha }^{\text{eff}}=L_w+{\delta }_{\alpha }$.