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Mixing Ionic Liquids

Final Report Summary - MIL (Mixing Ionic Liquids)

Ionic Liquids are, by definition, liquids composed exclusively of ions. Many of these are only liquid at very high temperatures (e.g. molten NaCl at > 801 °C). Consequently the applications of these were restricted to processes that could withstand high temperatures (e.g. the refining of aluminium from its ore). Towards the end of the last century new ionic liquids were discovered that were liquid at room temperature. Not surprisingly, this led to an explosion of interest in these exciting new liquids, both in terms of their fundamental properties and their potential applications. This has been remarkably productive and there are already three large-scale industrial processes using these new low temperature ionic liquids.

Mixing is one of nature’s most fundamental processes. Sometimes the mixing of chemicals will lead to chemical reactions, but sometimes the chemicals will mix without reaction. Even when no reaction occurs, dramatic changes in the behaviours of the chemicals can be generated. In this project we have been interested in when mixing happens without chemical reaction. Mixtures such as these form some of our most familiar chemical products, such as shampoo. Since mixing is both fundamentally scientifically interesting and potentially very useful, it now seems remarkable that until this project almost nothing was known about the behaviour of ionic liquid mixtures.

In this project, we have made a wide range of different ionic liquid mixtures of varying compositions and studied their behaviours. We have measured important physical properties such as the conductivity, viscosity, density, melting point and decomposition temperature (vital information for safe industrial application). We have related these observed properties to how the ions in the ionic liquids mix with each other on the molecular scale and, most importantly, we have related these to fundamental properties of the ions that can be calculated.

Our measurements properties have shown us that the behaviours of mixtures of ionic liquids are highly regular. Our measurements of the structures of mixtures of ionic liquids have helped us to understand why this occurs. Our calculations of the fundamentals of the ions have given us the ability to predict the properties of ionic liquid mixtures without needing to make them first. Simple pure ionic liquids had already earned the reputation of being ‘designer solvents’ due to the wide range of accessible properties and the degree of fine-tuning afforded by varying the constituent ions. The knowledge gained during this project offers us the opportunity, through forming mixtures, to further fine-tune the properties of ionic liquids. We have shown that it is really possible to design (from the chemical structure up) an ionic liquid that meets our needs.