Voltage-dependent interfacial structure and properties of room temperature ionic liquids: operando X-ray studies

Overview Room-temperature ionic liquids (RTILs) are salts with molecular anions and/or molecular cations, which are in the liquid phase at or near room temperature. The objective of this project is to study experimentally how the static and dynamic arrangements of RTIL ions near electrode interfaces respond to electric fields; to correlate this behavior with the molecular structures of the ions present in the RTIL; and to seek to understand how the interfacial liquid structure determines the observed electrochemical and electrowetting properties. The primary method to be employed is in situ X-ray reflectivity and grazing incidence X-ray diffraction from solid-RTIL interfaces, using national facilities such as the Advanced Photon Source and the National Synchrotron Light Source II. The dependence of the interfacial structure and its dynamics on the ions present, the nature of the electrode surface, and on applied direct and alternating voltages, will be studied and compared to existing theoretical predictions. Intellectual Merit Unlike aqueous solutions, room-temperature ionic liquids are known to be composed largely of dissociated ions, and these are large molecular ions. Thus their properties and potential applications are very different from those of aqueous solutions, and they can be controlled using relatively small applied voltages. The electrochemical properties of RTILs (e.g. the interfacial capacitance as a function of voltage) are quite inconsistent with theoretical predictions applicable to dilute aqueous solutions. Such experimental evidence indicates that electric fields must modify the interfacial liquid structure, but it does not tell us how it is modified. Although very detailed predictions can be obtained from theory and computer simulations, we do not know what the structure-property relationships are at real RTIL interfaces. This proposed project seeks to see, with subnanoscale resolution, what happens at real RTIL interfaces. The anticipa