How is an ionic liquid structured when it comes into contact with a wall?



Publication of the Physics Laboratory and Chemistry Laboratory in the journal Langmuir on November 16, 2023. News by CNRS Chemistry on December 19, 2023.

Water flowing over soluble rocks can create patterns of multiple troughs bordered by sharp ridges. By combining field measurements, a numerical model and laboratory experiments, a team led by the MSC laboratory (CNRS/Université Paris Cité), in collaboration with the LPG (CNRS/Nantes Université/Université d’Angers) and the RDP (CNRS/ENS de Lyon/Inrae) has shown that the appearance of these shapes results from a geometric mechanism. The results are published in the journal PNAS.

Room temperature ionic liquids (ILs) can create a strong accumulation of charges at solid interfaces by forming a very thin and dense electrical double layer (EDL). The structure of this EDL has important consequences in numerous applications involving ILs, for example, in supercapacitors, sensors, and lubricants, by impacting the interfacial capacitance, the charge carrier density of semiconductors, as well as the frictional properties of the interfaces. We have studied the interfacial structure of a long chain imidazolium-based IL (1-octyl-3-methylimidazolium dicyanamide) on several substrates: mica, silica, silicon, and molybdenum disulfide (MoS2), using atomic force microscopy (AFM) experiments and molecular dynamics (MD) simulations. We have observed 3 types of interfacial structures for the same IL, depending on the chemistry of the substrate and the water content, showing that the EDL structure is not an intrinsic property of the IL. We evidenced that at a low water content, neutral and apolar (thus hydrophobic) substrates promote a thin layer structure, where the ions are oriented parallel to the substrate and cations and anions are mixed in each layer. In contrast, a strongly charged (thus hydrophilic) substrate yields an extended structuration into several bilayers, while a heterogeneous layering with loose bilayer regions was observed on an intermediate polar and weakly charged substrate and on an apolar one at a high bulk water content. In the latter case, water contamination favors the formation of bilayer patches by promoting the segregation of the long chain IL into polar and apolar domains.

Reference

Effect of Surface Chemistry on the Electrical Double Layer in a Long-Chain Ionic Liquid. Layla Bou Tannous, Mirella Simoes Santos, Zheng Gong, Paul-Henri Haumesser, Anass Benayad, Agilio A. H. Padua, and Audrey Steinberger. DOI : 10.1021/acs.langmuir.3c02123