Spyridon KOUTSOUKOS1, Jana EISERMANN1, Alberto COLLAUTO1, Maxie Magdalena ROESSLER1, Tom WELTON1
1Imperial College London, London, United Kingdom
The ‘designer solvent’ concept of ionic liquids requires in-depth understanding of how molecular properties lead to macroscopic behaviours. For dynamic properties of solutes, this remains elusive. It is crucial to find a method to measure both structural and dynamic properties of solutes in ionic liquids, as this could have major impact on their potential applications, such as their use as electrolytes. Electron paramagnetic resonance (EPR) spectroscopy could be a powerful tool for this goal, as it can directly detect the environment around a solute with unpaired electrons.
EPR spectroscopy offers two main advantages compared to other spectroscopic methods: a) it is highly versatile in terms of experimental conditions, since it allows measurements to be performed over a wide temperature range, without being limited by the physical state of the sample – something that is a common problem with several other spectroscopic techniques (e.g. NMR, UV-Vis or fluorescence spectroscopy); b) ionic liquids are silent substrates for EPR investigations, since they do not contain any EPR-active groups and, as a result, they do not give rise to a background signal in the measurements.
In this project we have used stable nitroxide radicals as probes to understand the ionic liquids’ nanostructuring and immediate chemical environment around the radical. Using a combination of continuous wave (CW) and pulsed EPR experiments we have investigated a variety of ionic liquid systems, ranging from macroscopically homogeneous ionic liquids to ionic liquid crystals. The CW experiments are mainly focused on the characterisation of the rotational profile of the spin probe, which is indicative of the ‘local viscosity’ that the solute experiences. We are also comparing the ionic liquids with molecular mixtures of the same bulk viscosity, in order to disentangle the viscosity effects from the effects of the ionic liquid themselves. In the case of the ionic liquid crystals, the CW EPR experiment can distinguish and characterise the different microdomains.
Our pulsed EPR studies, mainly performed at cryogenic temperatures, are centred around two aspects. First, we measure the instantaneous diffusion of the added spin probes to compare the calculated local concentrations with their bulk concentration derived from CW EPR measurements. Second, we combine pulsed EPR techniques (such as ESEEM, HYSCORE and ENDOR) to probe the local structure directly. This insight will help us to link the differences we see in the rotational dynamics with the nanostructure inside the ionic liquids.