Interfacial Layering in the Electric Double Layer of Concentrated Electrolytes and Ionic Liquids

Ionic liquids and concentrated electrolytes form discrete layers of ions at charged surfaces due to strong steric and electrostatic interactions. Here, we present a theory that captures the interplay between overcrowding--the formation of dense layers of counterions at high surface charge densities-- and overscreening-- alternating layers of charge at low surface charge densities. Mathematically, the key to describing the interfacial concentration profile is to represent the electrostatic and excess free energy functional in terms of weighted densities. The concentration and differential capacitance predictions of the theory agree with molecular dynamics simulations of a representative ionic liquid. The theory outputs a simple formula for the length of decaying oscillations which become more long range as the ionic concentration increases. Finally, leveraging the theory, we demonstrate how the structure of an ionic liquid or concentrated electrolyte may change in confinement, and how overscreening and overcrowding phenomena can affect surface force measurements of these confined systems.