Electrical double layer in ionic liquids: Structural transitions from multilayer to monolayer structure at the interface

•Structural transition in the EDL of a model RTIL is revealed by MD simulation study.•Structure of the EDL in RTIL can vary between multiple layers and a single monolayer.•An ordered monolayer of counter-ions is formed at certain surface charge densities.•The monolayer coexists with apparently non-structured RTIL further in the electrode. We have studied structural transitions in the electrical double layer of ionic liquids by molecular dynamics simulations. A model coarse grained room temperature ionic liquid (RTIL) with asymmetric sized ions confined between two oppositely charged walls has been used. The simulations have been performed at different temperatures and electrode charge density values. We found that for the studied charge densities the electrical double layer has a multilayered structure with multiple alternating layers of counter- and co-ions at the electrode–RTIL interface; however, at certain charge densities the alternating multilayer structure of the electrical double layer undergoes a structural transition to a surface-frozen monolayer of densely packed counter-ions (Moiré-like structure). At this point the dense ordered monolayer of counter-ions close to the electrode surface coexists with apparently non-structured RTIL further from the electrode. These findings might bring possible explanations to experimental observations of formation of Moiré-like structures in ionic liquids at electrified interfaces. Moreover, we report the formation of herring-bone interfacial structures at high surface charge densities, that appear as a result of superposition of two ordered monolayers of RTIL ions at the electrode–RTIL interface. Similar structures were observed experimentally; however, to the best of our knowledge they have not been modelled by simulations. We discuss the dependence of the electrical double layer structure in RTILs on the ion size and the surface charge density at the electrodes.