| Summary: | <p>The ubiquity of electrolytes is exemplified by the variety of fields in which they are encountered including technology, biology, and geology. Our understanding of dilute electrolytes is well established by Debye-Hückel theory, but in concentrated systems where continuum theories fail, the behaviour is not yet fully understood.</p>
<p>Deep eutectic solvents (DESs) are concentrated electrolytes commonly formed from eutectic mixtures of a quaternary ammonium salt and some molecular component. Recognised as an ionic liquid sub-category, they find use in synthesis, metal processing, and as lubricants. The immense range of potential components means ‘green’ DESs can be manufactured that are biodegradable, non-toxic, and inexpensive.</p>
<p>This thesis explores the interfacial structure and frictional response across electrolyte films. Using the surface force balance and white light interferometry, the film thickness can be controlled with sub-nanometre precision and the surface forces measured with sub-micronewton resolution. Bulk phase behaviour is also studied using differential scanning calorimetry.</p>
<p>Measurements across two DESs, choline chloride:ethylene glycol (ChCl:EG) and ChCl:glycerol, and the corresponding pure molecular fluids show time- and water-dependent interfacial structures. In ChCl:EG, a long-ranged exponentially decaying force is observed that decays faster with added water, suggesting an underscreening-like behaviour previously observed in ionic liquids. The frictional response across the DESs highlights the molecular fluid-controlled nature of the lubrication mechanism. Varying the composition of ChCl:EG reveals that optimal lubrication properties do not coincide with the eutectic composition. The bulk phase behaviour of ChCl:EG mixtures shows a drastically different eutectic point compared with the literature, emphasising the importance of distinguishing thermodynamic and kinetic thermal events.</p>
<p>Studies across concentrated aqueous monovalent nitrate solutions, and ionic liquids under different confinement geometries demonstrate the prevalence of underscreening. It is also shown that the proposed underscreening scaling exponent does not depend on the mean ion diameter definition, but the transition point to underscreening does.</p>
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