Structural ordering transitions in ionic liquids mixtures

• Structural EDL transitions in ILs mixtures are followed by an abrupt increase in the differential capacitance curves.
• Interfacial structure results from the balance between the short range effects and is also affected by long range forces.
• Ubiquitous properties obtained by mixing ionic liquids arise from the competition between these two kinds of interactions.

Differential capacitance curves for the electrical double layer (EDL) of mixtures of imidazolium-based ionic liquids (ILs) with a common cation (1-ethyl-3-methylimidazolium, [C2MIM]+) and two different anions (bis(trifuoromethylsulfonyl)imide, [Tf2N]−) and tris(pentafluoroethyl)trifluorophosphate [FAP]−) were obtained. Sharp peaks in the differential capacitance curves were observed for a small range of mixtures compositions at positive charge densities. The appearance and position on the potential scale of the peaks were found to be dependent on the mixture composition and temperature. The occurrence of these phenomena is interpreted as corresponding to an abrupt change in the EDL structure arrangement as a result of a complex interplay of electrostatic interactions and steric effects. The use of the non-structured mercury electrode allowed to decouple the eventual potential induced restructuring occurring at the double layer from the well-known surface reconstruction effects often reported for ionic liquids in contact with single crystal face electrodes.

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