Ionic Structure and Decay Length in Highly Concentrated Confined Electrolytes*

Under dilute electrolyte conditions, Debye-Huckel theory predicts that the characteristic decay length scales inversely with the square root of the electrolyte concentration. Recent experiments have reported an increase in the decay length with increasing electrolyte concentration for highly concentrated electrolytes (e.g., for >1 M in aqueous NaCl). We investigate the ionic structure in electrolytes confined by homogeneously-charged planar interfaces using coarse-grained molecular dynamics simulations. Our results reveal two distinct regimes of screening behavior as the concentration is changed from 0.1 M to 2.5 M for a wide range of electrolyte systems generated by tuning the interfacial separation, surface charge density, and ionic size. For low concentrations, the integrated charge exhibits a monotonic decay with a characteristic decay length decreasing sharply with increasing concentration. For high concentrations (e.g., >1 M for aqueous NaCl), the integrated charge has a non-monotonic behavior due to the formation of structured layers of cations and anions near the interfaces, driven by enhanced steric ion-ion correlations. The associated decay length rises with increasing concentration, albeit much more slowly than the rise observed in experiments.