Modeling electrolytes in nanopores by Monte Carlo simulations and the Bazant--Storey--Kornyshev model

Abstract

We study electrolyte-filled cylindrical nanopores through Monte Carlo (MC) simulations and the Bazant--Storey--Kornyshev model, for different ionic valencies, sizes, and bulk concentrations, pore radii and surface charge densities. Our model accounts for finite ion size through a Stern layer and we use de~Souza and Bazant's mechanical equilibrium principle to derive a boundary condition for the outer Helmholtz plane. For 1:1 electrolytes and moderate surface charge densities, we find that both the classical Poisson--Boltzmann--Stern (PB--Stern) and our Bazant--Storey--Kornyshev--Boltzmann--Stern (BSKB--Stern) model fit MC data well---for 2:1 and 3:1 electrolytes, the BSKB--Stern outperforms the PB-Stern model. Conversely, the BSKB--Stern model poorly fits MC data in other scenarios. First, BSKB--Stern does not capture drying and apparent like-charge attraction in 3:1 electrolytes and small surface charge densities. Second, BSKB--Stern only fits MC data for finely-tuned ionic diameters; for other ionic diameters, the ionic charge densities show oscillations or extended near-surface regions caused by ionic packing and strong Coulomb interactions, not captured by the BSKB--Stern model.