First-Principles Study of the Optical Dielectric Constant in Nanoconfined Water

The out-of-plane dielectric constant of water confined in graphene capacitors was measured as low as 2.1 for thicknesses below 1 nm (Fumagalli et al., Science 360, 2018). It was unclear whether this suppression arose purely from ionic contributions or also involved electronic effects. In prior work on nanoconfined ice, we developed a charge separation protocol that uniquely defines the dielectric constant by separating electrode and dielectric contributions, showing that ice’s electronic response is largely insensitive to nanoconfinement. Building on this framework, we apply the same protocol to liquid water using ensemble-averaged molecular dynamics simulations. We find that a high-density interfacial water layer forms near each capacitor plate, while adjacent layers form compensating low-density regions. Each water molecule has an intrinsic optical dielectric constant of ~1.8, but the reduced density of intermediate layers reduces the local electronic response and reduces the effective optical dielectric constant of the confined film. These results show that the suppression of the optical dielectric constant in nanoconfined water arises not from changes in individual molecular polarizability, but from density variations within the confined film.