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Title: Ion pairing in ionic liquids
Author: Bakis, Eduards
ISNI:       0000 0004 9356 8788
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2019
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In liquids, a plurality of forces exist that add together and lead to physical properties that can be determined by a single measurement. It is therefore necessary to understand these forces and evaluate their contribution in dictating one or another physical property of the liquid. Only then a meaningful material design will be possible. Ionic liquids are low-melting salts with a phenomenal long-range structuring and complex interaction landscape. Among other sustainable applications, these conductive materials are primarily considered for energy storage and conversion. Many ionic liquid properties are explained by their strong Coulombic interactions. Ion pairing is a process that often is associated with the limited electrical conductivity of ionic liquids, though evidence for existence of long-lived ion pairs in these materials is scarce. In this study, conditions that could force the ion pairing in ionic liquids have been challenged by designing and studying strongly-interacting functionalized and dicationic ionic liquids. A polarity and ion pairing study using Kosower's salt, a [1+][1–] electrolye, was performed and revealed a lack of preferential ion association via ion pairing in these highly polar ionic liquids. Novel ion pairing dyes with increased ion charges [2+][2–] were then designed and applied. The spectroscopic and physical properties were fine-tuned via multiple syntheses, and bis-(pivaloyloxymethyl)-4,4'-bipyridinium tetrathiosquarate found as a suitable [2+][2–] ion pairing probe. Study of a conventional ionic liquid with this dye revealed no fundamentally different behaviour than with Kosower's salt. It was concluded that [2+][2–] and [2+][1–] solutes form ideal solutions with ionic liquids and no preferential association via ion pairing in these solutions can be observed. This contradicts the point of view that ion pairing accounts for the reduced charge carrier mobility in IL electrolytes. In fact, forcing ion pairing in these charge-screened materials appears to be very difficult, if not impossible.
Supervisor: Welton, Tom Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral