Spectroscopic studies on the transport properties of ionic liquids

Abstract

Ionic liquids are liquids composed entirely of ions. These ions are in constant motion. Ionic liquids have found use in a wide range of electrochemical applications, such as batteries, fuel cells and solar cells. In these applications they are exposed to various external forces including magnetic and electric fields. Gaining a thorough understanding of how these external forces affect the physical and chemical properties of ionic liquids is important; and it remains a fundamental challenge. In this study, fluorescence lifetime spectroscopy was utilised as a technique to gain an insight into the behaviour of the ions when experiencing electric fields. To enable the fluorescence measurements, synthetic processes were explored and developed to produce good quality ionic liquids with low fluorescence backgrounds. The physical and chemical properties of the ionic liquids were characterised by various established techniques such as nuclear magnetic resonance spectroscopy, mass spectrometry, impedance spectroscopy, cyclic voltammetry and elemental analysis. Upon the successful production of low fluorescence background ionic liquids, fluorescence lifetime experiments were conducted to monitor their local viscosities. In this study, molecular rotors were used as probes to estimate local viscosity of the ionic liquids. The fluorescence lifetimes were derived from a molecular rotor, 3,3'-diethylthiacarbocyanine (Cy3) which was dissolved in the ionic liquids (ca. 100 nM). The local viscosities were then estimated from a calibration curve developed from various ionic liquids and these values were also compared to those estimated from a calibration curve usually used for molecular solvent systems. Further to this, attempts to use the obtained correlation between fluorescence lifetime of Cy3 and viscosity to estimate the local viscosities of the ionic liquid under application of +2V potential were conducted. Even though the change of viscosity of ionic liquids was observed upon application of the +2V potential, the explanation to the observation is rather complicated due to many factors that can contribute to the viscosity change. Based on analysis and support data from the literature, the observed change in viscosity could be either attributed to change in molecular dynamics of ionic liquids as the potential is applied or it may be affected by instability of the indium-tin oxide (ITO) electrode over the applied potential which has the potential to cause dissolution of ITO components into ionic liquid system.