Dielectric constant of water in the interface

TL;DR

This paper defines the dielectric constant at microscopic water interfaces, deriving it from linear-response theory and applying it to MD simulations, revealing a lower and size-dependent interfacial dielectric constant.

Contribution

It introduces a precise definition of the dielectric constant at dielectric interfaces based on linear-response theory and applies it to molecular dynamics simulations of water-spherical solute interfaces.

Findings

Interfacial dielectric constant is significantly lower than bulk water.

Interfacial dielectric constant decreases with increasing solute size.

For TIP3P water, the dielectric constant drops from 9 to 4 as solute radius increases.

Abstract

We define the dielectric constant (susceptibility) that should enter the Maxwell boundary value problem when applied to microscopic dielectric interfaces. The dielectric constant (susceptibility) of the interface is defined by exact linear-response equations involving correlations of statistically fluctuating interface polarization and the Coulomb interaction energy of external charges with the dielectric. The theory is applied to the interface between water and spherical solutes of altering size studied by molecular dynamics (MD) simulations. The effective dielectric constant of interfacial water is found to be significantly lower than its bulk value, and it also depends on the solute size. For TIP3P water used in MD simulations the interface dielectric constant changes from 9 to 4 when the effective solute radius is increased from ~5 to 18 A.