Colloidal Interactions in Ionic Liquids—The Electrical Double Layer Inferred from Ion Layering and Aggregation

Ionic liquids (ILs) are organic salts that remain liquid in absence of a solvent over a wide range of temperatures, often at room temperature. This chapter summarizes the progress in understanding colloidal interactions mediated by ILs and their electrical double layer (EDL) based on experimental observations and theory. It is well known that short-range oscillatory forces in ILs originate from the overscreening provided by ion layers that accumulate close to the charged surface. In contrast, the origin of the more surprising long-range decaying force is not well understood yet. There is experimental and theoretical evidence opposing the originally proposed dilute behavior of ILs, arising from either ion pair formation or solvent/voids/alkyl tails being the effective charge carrier. Here, we overview experiments and theory that supports an alternative explanation of this long-range force based on ion aggregation.