Chapter 1
An Introduction to Ionic Liquids

1.1 Prologue

The benefits of using salts in their liquid form as electrolytes or reaction media have long been recognized. For example, Faraday developed his laws of electrolysis in the 1830s using molten metal halide salts. However, most researchers would rather avoid using solvents that require heating to hundreds of degrees. Therefore, while 'high-temperature molten salts' are extremely useful for certain applications, they are not in widespread use in research laboratories or industry. In contrast, 'ionic liquids' - and we will discuss the definition of these below - are proving to be very exciting for a very wide range of applications at more moderate temperatures. Particularly over the last decade, scientists working in many different areas of research have started to realize the special properties of ionic liquids (ILs) and to embrace the promise of these new materials. As more applications are discovered, and more families of ILs are developed, so the field continues to grow. Thousands of papers on ILs are now published every year, and, more importantly, increasing numbers of researchers are experimenting with ILs and discovering firsthand how their unique properties can help their work.

So here we are, just over 100 years since the first 'room-temperature ionic liquid' was discovered, at a point where these materials are now proving so promising and widely applicable that it is important for a broad range of scientists and engineers to have an appreciation of the basics of ILs. Thus, the purpose of this book is to serve as an introduction to the key concepts and applications of ILs for those venturing into this field for the first time. Hopefully, the book will also inspire further curiosity and enthusiasm for exploring these exciting and very unique materials.

Our goal in this book is to provide a thorough introduction to the field appropriate to the level of a finishing undergraduate science student or a beginning postgraduate student. Our emphasis is on illustrative examples and the background chemistry sufficient to understand the fundamentals of ILs and their applications. For further reading, we have referenced more extensive reviews where they exist. To provide background on fundamental concepts and methods that may not be readily accessible in standard textbooks, we have included Concept Toolbox items as breakout text boxes in various places throughout the chapters. This first chapter provides a broad overview of the field, the materials involved, their properties, and their applications, of which more details can be found later in this book.

1.2 The Definition of an Ionic Liquid

The phrase 'ionic liquid' was coined only relatively recently to refer to ambient-temperature liquid salts, and the definition has since been the subject of much discussion and some evolution. The most useful practical definition of an IL is

'A liquid comprised entirely of ions.'

We can delve into this a little deeper. By this definition, is an IL different from a molten salt? The answer is: 'No' - the term 'molten salt' refers to the liquid phase of a crystalline salt, for example, NaCl. 'Ionic Liquid' covers that, but also covers a broader range of possibilities. Imagine a mixture of the two salts Na[fsi] and [C3mpyr][NTf2] (see Table 1.1 for an explanation of abbreviations). This mixture of salts is a liquid at room temperature and is properly called an IL by our definition. In fact an IL could contain a very large number of different ions. Note that, in common usage, the term 'molten salt' has also come to mean mixtures of salts, although the term itself clearly indicates a single compound.

Table 1.1 Glossary of structures and nomenclature abbreviations used in this book

a Also referred to as 'amides'.

In principle (in fact an important thermodynamic principle), the IL obtained by mixing Na[fsi] and [C3mpyr][NTf2] is exactly the same as that obtained by mixing the appropriate quantities of Na[NTf2] and [C3mpyr][fsi]. The points of origin are irrelevant in defining the IL; only the quantities of the individual ions present are important. In fact, such ILs with very high concentrations of Na or Li salts are proving to be highly effective as electrolytes for Na and Li batteries [1].

Some definitions of IL add a temperature range, such as 'below 100 °C', to the definition but this is not necessary. In fact, it is limiting to do so, since it can blinker our perspective on which compounds or mixtures may be useful for certain applications. Indeed, there are many, quite valuable, applications of ILs at temperatures above 100 °C, for example, the preparation of MnOx water oxidation catalysts by electrodeposition at 130 °C [2]. The key requirement for this is that the IL be a liquid at 130 °C. It is convenient if it is also liquid at room temperature, but it is not necessary for this to be the case and one should certainly not exclude from consideration compounds having melting points >100 °C for an application such as this. Similarly, a definition that includes 'a salt having a melting point below 100 °C' (or some other temperature such as room temperature) is also an unnecessary restriction because in some cases the melting point may be practically difficult to find and measure. The supercooling of liquids below their equilibrium melting points is a well-known phenomenon, and in some cases the liquid becomes so viscous that the crystalline phase never forms on a practical timescale. This is particularly true of mixtures of salts, which we have agreed are perfectly good ILs, because the melting points of individual compounds is often sharply depressed in mixtures. We will discuss the melting points of ILs in Chapter 2, and multicomponent phase diagrams and behavior further in Chapter 5. With all of this in mind, referring to a melting point in our definition of an IL becomes unhelpful.

The meaning of the word 'ion' in this definition also needs some discussion. Species such as Cl- are obvious, as are simple molecular ions such as [NO3]-. Things become more subtle when we consider metal coordination complex ionic species such as [AlCl4]-, which were used extensively during the early work on ILs, as reviewed briefly in the following. These are certainly ionic as written, and as long as they continue to stay bound in the real liquid, for long times, then they fit our definition. However, there is always an equilibrium process by which such complex species are formed, and therefore we must always recognize the presence, in equilibrium, of some amount of the component species. In the case of [AlCl4]-, this might be Al3+ and Cl-; as long as these components involved are also ionic, we still have an IL, although with more complex 'speciation' than is at first apparent. This speciation, and how it responds to variables such as temperature, can influence the IL's properties significantly.

In the case of Co(H2O)62+, which sits in equilibrium with H2O and Co2+,...