Ionic Structure in Highly-Concentrated Confined Electrolytes*

Recent surface force measurements have shown that the effective force between mica surfaces does not decay as sharply as predicted by mean-field models such as the Debye-Huckel theory when the concentration of the confined electrolytes is high (around 2 M for NaCl). In other words, the interaction between charged surfaces is “under-screened” at high electrolyte concentrations. Motivated by these experiments, we use molecular dynamics simulations and strong coupling theory to extract the ionic structure in aqueous NaCl electrolytes confined by two interfaces. Ionic density profiles, effective force (pressure) between surfaces, and pair correlation functions are extracted for electrolytes confined between uncharged, charged, and polarizable interfaces at different concentrations (0.1 M to over 2 M). Effects of varying ion sizes, surface separation, and dielectric properties of the solvent are outlined. Simulation results are correlated with strong coupling theory calculations where “underscreening” behavior is associated with the formation of highly correlated groups of ions that act as individual entities.