Note: On the dielectric constant of nanoconfined water

Abstract

Investigations of dielectric properties of liquid water in nanoconfinement are highly relevant for the energy storage in electrochemical systems, mineral-fluid interactions in geochemistry and microfluid based devices in biomedical analysis 1 . It has been reported that polarization shows a strong anisotropy at water interfaces 2,3 and the dielectric constant of nanoconfined water is surprisingly low ( ⊥ ∼ 10) 4 . Here, using a simple capacitor model, we show that the low dielectric constant of nanoconfined water can be largely accounted by the so-called dielectric dead-layer effect known for ferroelectric nanocapacitors 5 .Before talking about the effect of nanoconfinement, one needs to realize that the first effect of having an interface corresponds to a switch in the electric boundary condition. From classical electrodynamics, we know that the electric field E z is discontinuous at a dielectric interface, that is the reason why it is convenient to use the electric displacement D as the fundamental variable instead 6 . In the latter case, the polarization of dielectrics P z is added into the electric field. This makes D continuous in the direction perpendicular to an interface and leads to itsWhen the electric boundary condition is switched from constant electric field E to constant electric displacement D, the dielectric response will be different accordingly 7-10 .The difference in χ due to the electric boundary condition leads to differences in the fluctuation of polarization at zero field and in the corresponding relaxation time. This phenomenon is not limited to water in nanoconfinement where switching of electric boundary condition is enforced by introducing explicit interfaces 2,3 but can also be realized in bulk liquid water by turning on the constant electric displacement simulation 9,10 . For bulk liquid water, ⊥ = even though χ ⊥ is radically different from χ . Now let us go back to the original question: What accounts for the low dielectric constant ⊥ of water slab in nanoconfinement 4 . Is water in nanoconfinement completely different from that in the bulk ?To answer this question, we applied the constant electric displacement simulation with D = 0.6835V/Å to a water slab confined between two hydrophobic walls at a) Electronic mail: chao.zhang@kemi.uu. se FIG. 1. a) A snapshot of MD simulations of water slab confined between rigid walls under constant electric displacement D = 0.6835V/Å . The seperation distance between walls Lw is 30.77Å in this case.; b) The corresponding electrostatic potential profile ϕ(z) generated from the charge density. The slope gives the negative of the deploarization field 4πP ⊥ in the bulk water region. ambient conditions (Fig. 1a). Interactions between water molecules is described by simle point charge/extended (SPC/E) model 11 and the rigid hydrophobic walls are composed of atoms on a dense cubic lattice. All molecular dynamics (MD) simulations were performed with GROMACS 4 package 12 and technical settings are the same as described in the previous work 9 .From the ...

Effect of Hydrogen Bonds on the Dielectric Properties of Interfacial Water

“…Now using this value of δ and eq 11, we calculate the effective dielectric constant at different regions of water, which is given in Table 1. We observe a reduced effective dielectric permittivity for confined water, as reported in previous studies, 9,28 with the least value of permittivity shown by the interface regions. The low dielectric constant at the interface indicates that the interfacial water molecules restrain themselves from aligning along the direction of the external field.…”

Dielectric response with short-ranged electrostatics

“…67), the fact that dielectric properties can be understood within the LMFT framework can be viewed as a demonstration of this result and is perhaps more open to intuitive physical interpretation. This may prove useful as we continue to develop our understanding of dielectrics under confinement (1,4,9,10).…”

Competition between Born solvation, dielectric exclusion, and Coulomb attraction in spherical nanopores

“…Their data were well fitted by a simple model of three capacitors in series made of a bulk layer (with a bulk value ⊥ 80) sandwiched between two thin layers of width 0.74 nm close to the surface. This low value of ⊥ may arise simply from an averaging that includes the thin layers of vacuum close to hydrophobic surfaces (of width equal to twice the van der Waals radius of wall atoms) with = 1, and a bulk value quickly reached in between (even for nanometric water slabs), as has been suggested by all-atom simulations [34].…”