A Look At Ionic Liquids

If ever there was a case of a reporter becoming part of the story, it would have to be Michael Freemantle’s pivotal role in the growth of the field now known as ionic liquids. Ionic liquids are quite literally defined as salts that melt below 100 °C, an admittedly arbitrary definition. The macroscopic property of being a liquid combined with properties inherent in many salts fueled an expansion of research interest into the use of ionic liquids as solvents, with green chemistry often a purported goal. For most researchers engaged in the study of ionic liquids today, it is the range and tunability of the physical, chemical, and biological property sets possible in salts that are liquid at or near ambient temperatures that have led to an explosion of interest in a wide range of diverse applications—lunar mirrors, paint compatibilizers, heat pump fluids, cellulose solvents, propellants, and pharmaceuticals.

Freemantle’s early reports in Chemical & Engineering News continue to be highly cited as pivotal in bringing new people into the field, as a result of both the timing of the stories and the style of writing. His new book, “An Introduction to Ionic Liquids,” provides a good overview and introduction, starting in the mid-1990s, through the eyes of a trained reporter.

I remember my first meeting with Freemantle in Greece in 2000 when he was a guest at the NATO Advanced Research Workshop “Green Industrial Applications of Ionic Liquids” that Ken Seddon, director of the Queen’s University Ionic Liquid Laboratories in Belfast, Northern Ireland; Sergei Volkov of the V. I. Vernadsky Institute of General & Inorganic Chemistry of the National Academy of Sciences, Ukraine; and I had organized. Freemantle, camera in hand, was walking the beach during a break in the conference, and we chatted about many of the same contentious topics still being discussed today. Are ionic liquids new? Are they green/safe/toxic? Who is using them, how much do they cost, and when will they be commonplace in industry? Throughout the next decade, many of his stories continued to cover these key questions for the field and prompt research into answering the questions.

“An Introduction to Ionic Liquids” is at times a personal view of the field and at others a discussion of commonly accepted history, which perhaps can be misleading. Overall it is a high-level introduction that whets the appetite for more information. At its best, it encourages a drive to develop a deeper understanding of the field in order to find out why there has been so much excitement among practitioners, as well as so much disappointment.

The book is quite ambitious, and its 281 pages and 14 chapters cover an incredible range of chemistry, and each chapter could easily be a stand-alone book. This can be understood just by reviewing the titles of each chapter: “Introduction,” “History,” “Synthesis of Ionic Liquids,” “Properties of Ionic Liquids,” “Ionic Liquids as Designer Solvents,” “Green Credentials of Ionic Liquids,” “Electrochemistry,” “Catalysis,” “Inorganic Chemistry,” “General Organic Reactions,” “Named Organic Reactions,” “Biotechnology,” “Analysis,” and “Applications.”

The first two chapters might actually be some of the more contentious. Two areas where there has been much debate in the field are, first, definitions, nomenclature, and abbreviations, and, second, the history of the field. The introduction, for example, suggests that ionic liquids consist exclusively or almost exclusively of ions; however, the entire field of “protic ionic liquids” including the purported “first ionic liquid” (see below) might not fit such a definition. Similarly, the classes of cations and anions are now quite dated given the dozens of new cation and anion classes that have been reported recently.

The history chapter is disappointing to me, and the author falls into the trap of using modern terms and thinking to reinterpret older chemistry. Key dates in the history of the field are all selected in hindsight, while in fact they had little to do with the research understanding and strategies developed in the field from the mid-1990s onward. For example, as indicated above, the description of protic ionic liquids often does not fit into what some define as an ionic liquid; this debate is ongoing. The birth of ionic liquids being attributed to a 1914 paper where a protic salt was prepared is quite widely cited in the field, but the identification of a salt liquid at room temperature would seem to have little to do with the development of research strategies based on the notion that liquid salts could be used as solvents, materials, or even pharmaceuticals. However, to the author’s credit, this is exactly how many scientists and engineers in the field describe ionic liquid history.

The remaining chapters are based more on science than perception and thus serve as a reasonably short introduction to some of the early work carried out in the field. Nonetheless, these chapters can only give a feel for where the field has been, and they certainly should not be expected to be complete.

A good example of the field of ionic liquids and the problems anyone will have trying to provide complete coverage can be found in chapter 14. The number of applications is growing, and knowledge is being developed in areas that simply do not fit into convenient categories, such as synthesis, electrochemistry, and so on. These are placed in vignettes in the last chapter and might serve as a fitting juxtaposition to chapters 1 and 2 where the definitions and history provided are focused on the much older view of what ionic liquids are and from where they arose.

The applications of ionic liquids are growing into novel areas far afield from the green-solvent push in the mid-1990s. Many of these applications arose from a greater experimental and theoretical understanding of ionic liquids at an ionic level as complex materials and much less from the macroscopic observation that a salt could be a liquid at room temperature.

Overall, this book is easy to read, and I would recommend it for those new to the field who want to get a flavor of what is developing in the area. I would caution that one should always verify for oneself what is considered “common knowledge” and remember that there is a tremendous amount of very good work that could not possibly fit into an overview of this type. Freemantle has brought us the introduction—the field must provide the sustaining chapters.