Structure in ionic liquids

Ionic Liquids (ILs) are a subset of molten salts distinguished by melting points below 373 K. ILs are unusual among solvents in that they are composed entirely of ions, with no neutral species present. Over the last decade or so, ILs have emerged as an attractive class of solvents for a range of chemical applications, mostly due to their ‘green’ characteristics and remarkable liquid properties. Understanding the ion arrangements in ILs is important as many of these applications and properties are related to their (bulk or interfacial) solvent structure. Historically, ILs were considered structurally homogeneous solutions of freely dissociated ions or ion pairs. Whilst these concepts are adequate for molten salt melts, IL ions can participate in a range of attractive interactions (van der Waals, π-π, hydrogen bonding, or solvophobic) in addition to Coulombic forces. Notably too, ion-ion interactions in ILs are tuneable, because changes in anion/cation size, shape or functional groups alter the balance of inter-ionic forces. These features suggest different solvent structures may be present in ILs compared to molecular solvents or molten salts. Further, many IL ions (usually the cation) are amphiphilic with both charged and uncharged groups. This means that there is potential for self-assembly in a fashion similar to aqueous surfactant dispersions, microemulsions or liquid crystals, but on much smaller length scales. Recent experimental and theoretical research has tested this hypothesis for aprotic ILs. The results show that aprotic ILs are heterogeneous on the nanoscale, forming polar and apolar domains in the bulk liquid due to clustering of charged and uncharged molecular groups. In this Thesis, the nature of protic IL structure in the bulk phase is examined using model fits to neutron diffraction data. It is shown that protic ILs are nanostructured solvents and that the solvent structure can be controllably varied. Secondly, aprotic IL structure at the Au(111) electrode interface are elucidated using atomic force microscopy. This provides fundamental insight to the IL electrical double layer structure that will underpin future IL-based electrochemical technologies.