Redefining the dielectric response of nanoconfined liquids: Insights from water

Recent experiments show that the relative dielectric constant <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mi>ε</a:mi> </a:math> 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 <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mi>ε</b:mi> </b:math> is not a well-defined measure for dielectric properties at subnanometer scales due to the ambiguous definition of confinement width. Instead, we propose the 2D polarizability <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:msub> <c:mi>α</c:mi> <c:mo>⊥</c:mo> </c:msub> </c:math> as the appropriate, well-defined response function whose magnitude can be directly obtained from both measurements and computations. Once the appropriate description is used, understanding the interplay between electronic and ionic contributions becomes critical, contrary to what is widely assumed. This highlights the importance of electronic degrees of freedom in interpreting the dielectric response of polar fluids under nanoconfinement conditions, as revealed by molecular dynamics simulations.