Dielectric Susceptibility of Water in the Interface

It has long been anticipated that dielectric constants of polar liquids are reduced in the interfacial layer. Recent experiments and computer simulations support these expectations. A strong reduction of the dielectric constant is found in the direction perpendicular to a planar substrate, while the parallel response is bulk-like. This Perspective highlights recent theoretical calculations and simulations with an eye on relating them to properties observable in the laboratory. The average interface dielectric constant computed from simulations connects to thin films experiments, but this cannot be extended to screening of charges. In contrast to dielectric theories where a single dielectric constant gauges both the polarization energy and screening, these two signatures of dielectric polarization diverge on the molecular scale. The reduction of the dielectric constant of water in thin films is currently viewed as a combined effect of geometric confinement imposed by the substrate and the reconstruction of water hydrogen bonds in the surface layer.