The nanostructure of ionic liquid-solid interfaces revealed with amplitude modulated-atomic force microscopy

Research Doctorate - Doctor of Philosophy (PhD)In this thesis, amplitude modulated – atomic force microscopy (AM – AFM) is used to characterise the <i>lateral</i> arrangement of ionic liquid (IL) ions adsorbed to, and near a solid substrate. The sensitivity afforded by AM-AFM allows the position of surface adsorbed ions to be elucidated with molecular-resolution <i>in situ</i> at a true <i>bulk liquid</i> interface. Additionally, the nanostructure of near-surface IL layers is shown to be far more complex than previously reported. A selection of protic ILs, aprotic ILs, IL-solute, and IL-solvent mixtures are studied at the muscovite (colloquially referred to as mica) and highly ordered pyrolytic graphite (HOPG) interface. The pure IL systems reveal how the interfacial nanostructure varies as a function of the ion species, cation alkyl chain length, hydrogen bonding capacity, solvophobicity and steric hindrance of the ion’s charge centres at both the mica and HOPG interface. In general, the adsorbed layer structure strongly depends on the registry between the cation and the surface, while near-surface nanostructure is dictated by intermolecular forces of both the cation and anion. The adsorbed surface structures dramatically change when an electric potential is applied to the substrate. This is because the IL ions must re-arrange both <i>laterally</i>, and <i>normally</i> to the surface to satiate the applied charge. In the presence of dissolved solute, even in small concentrations, the adsorbed and near-surface IL nanostructures change markedly. This occurs because the dissolved species are capable of competing with the IL ions for surface adsorption. In these systems the resultant interfacial structure reflects a balance of IL-surface and solute-surface surface interactions. Together the structures elucidated in this work reveal how interfacial nanostructure can be tailored via variations in IL ion structure, dissolved solutes, and applied stimuli (electric potential) which will enable the optimisation of IL-solid interfaces for a range of applications including heterogeneous catalysis, lubrication, electrochemical processes and nanofluids