Ionic Structure in Dense Electrolytes Confined by Interfaces*

Recent surface force measurements have shown that the effective force between mica surfaces does not decay as sharply as predicted by mean-field models when the concentration of the confined electrolytes is high (around 2 M for NaCl). Motivated by these experiments, we use molecular dynamics simulations to extract the ionic structure in aqueous electrolytes confined by two interfaces. Ionic density profiles and contact-region densities are extracted for electrolytes confined between uncharged, charged, and polarizable interfaces at different concentrations with implicit-solvent models. Simulation results show that the net charge density in the contact region near charged surfaces exhibits a distinct trend with increase in interface separation for highly concentrated electrolyte systems (around 2 M for model NaCl) compared to electrolytes at low salt concentration. This behavior is further probed by varying ion sizes, interface charge density, ion valency, and interface polarizability, as well as by computing ion-ion pair correlation functions near the interface. Effects of explicit solvent molecules on the observed ionic structure are also discussed.