Tuning the Dielectric Response of Nanoconfined Water via Surface–Water Interactions

Understanding and controlling the dielectric properties of nanoconfined water are fundamentally important for nanoscale electrochemical, biological, and energy-storage systems. Here, we systematically investigate how surface-water interactions govern the dielectric response of water confined between two parallel planar walls separated by nanometer-scale gaps, using extensive molecular dynamics simulations. By tuning the van der Waals (vdW) potential well depth and the polar-atom coverage ratio of the confining walls, we demonstrate that the overall out-of-plane dielectric permittivity of the system (<i>ε̅</i>_⊥) can be effectively modulated through interfacial interactions. It is unexpected that the <i>ε̅</i>_⊥ does not vary monotonically with the vdW or Coulomb components of the wall-water interaction. Only moderate levels of vdW and Coulomb wall-water interaction yield relatively large <i>ε̅</i>_⊥ values, whereas excessively strong or weak interactions in either channel suppress <i>ε̅</i>_⊥. Furthermore, the inaccessibility of the wall-water interfacial regions to water is identified as an important factor leading to the low <i>ε̅</i>_⊥ compared with that of bulk water. These results establish general interfacial design principles for tuning the dielectric properties of nanoconfined water and are directly relevant to electrochemical interfaces, energy-storage materials, and biological confinement environments where surface chemistry can be engineered.