Redefining the dielectric response of nanoconfined liquids: insights from water (2412.01347v2)

Abstract: Recent experiments show that the relative dielectric constant $\epsilon$ of water confined to a film of nanometric thickness reaches a strikingly low value of 2.1, barely above the bulk's 1.8 value for the purely electronic response. We argue that $\epsilon$ is not a well-defined measure for dielectric properties at sub-nanometer scales due to the ambiguous definition of confinement width. Instead we propose the 2D polarisability $\alpha_{\perp}$ as the appropriate, well-defined response function whose magnitude can be directly obtained from both measurements and computations. Once the appropriate description is used a very significant reduction, $\sim 30$\%, of the electronic response with respect to the bulk's is observed contrary to what widely assumed, highlighting the importance of electronic degrees of freedom to interpret the dielectric response of polar fluids under nanoconfinement conditions. This is obtained from molecular dynamics simulations.