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

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.