Beyond Dielectric Formulations: Local Descriptors of Electrolyte Properties in Cylindrical Nanopores

Dielectric behavior is often invoked to explain ion exclusion from, ion adsorption on, and ion–ion interactions within man-made and naturally occurring water-filled nanopores. In slit pores, the predicted dielectric permittivity has successfully explained nanoconfined pure water behavior. However, in cylindrical pores─a key class of nano materials─rigorous dielectric permittivity profile (ϵr(r)) formalisms derived for local radial dielectric behavior of confined water yield unphysical anomalies near the pore center. Adding ions has further led to complexities not found in bulk electrolytes because the separation of time-scales between water and ion contributions to dielectric response should no longer be valid in the nanoconfined direction. In this work, we conduct molecular dynamics simulations of both pure water and electrolytes confined within nonpolarizable carbon nanotubes and cylindrical pores in silica at both hydrothermal and ambient conditions. We show that the anomalous pore-center oscillations in ϵr(r) can be regularized, but local confinement signatures are difficult to extract for water at T = 300 K. In the more concentrated ion regime, ϵr(r) is dramatically modified in ways consistent with ion-dominated screening behavior but inconsistent with strictly local dielectric screening. We propose instead that single-value quantities derived from ion-pairing potentials-of-mean-force and average Madelung potentials are alternative descriptors that can augment dielectric studies and elucidate confinement behavior of ions in cylindrical nanopores.