(335i) Ionic Liquids and Dilute Electrolytes: The Surprising Connection

Solid-electrolyte interfaces are critically important elements of electrochemical devices. Nevertheless, key questions surround the nature of electrode-electrolyte interfaces in highly concentrated electrolytes, particularly in nanoconfined environments. I will discuss using molecular force measurements with in situ electrochemical control to observe electric double layer formation in nanoconfined ionic liquids. As single component liquids composed entirely of ions, it was initially thought that ionic liquids should contain extremely high free ion densities. Yet, the conductivities of many ionic liquids are orders of magnitude lower than aqueous electrolytes. I will present our discovery that electric double layers formed by ionic liquids can extend to more than 20 ion diameters away from charged surfaces, indicating that greater than 99.99% of the ions actually behave as neutral pairs that are in equilibrium with a very small population of thermally-dissociated charged species. This picture provides insight into the low conductivities of typical ionic liquids, and I will also highlight a molecular framework for designing high performance ionic liquids. More broadly, our work tests fundamental ideas concerning electrostatic screening and ion pairing in concentrated electrolytes, and our findings suggest new ways of envisioning concentrated electrolytes with implications for diverse areas, ranging from engineering electrochemical interfaces to understanding biomolecular assembly.