There is no doubt that ionic liquids have become a major subject of study for modern chemistry. We have become used to ever more publications in the field each year, although there is some evidence that this is beginning to plateau at approximately 3500 papers each year. They have been the subject of several major reviews and books, dealing with different applications and aspects of their behaviours. In this article, I will show a little of how interest in ionic liquids grew and developed. . . . . . . . Keywords Ionicliquid Greenchemistry Electrochemistry Synthesis Catalysis Industrialapplications Structure Dynamics The beginnings important sub-class of ionic liquids after being rediscovered by Hiroyuki Ohno (Hirao et al. 2000); Walden’sinterestin A brief article such as this cannot cover every aspect of re- these molten salts was the relationship between their molecu- search using ionic liquids and it will inevitably reflect the lar size and their conductivity. Unfortunately, apart from a idiosyncrasies and personal experiences of the author. Even brief mention in a study of the parachor of some fused salts those subjects covered can only receive the briefest mention. in 1927 (Sugden and Wilkins 1929), the potential of this If I have left out your contributions to the field, I apologise, no breakthrough went unnoticed for a long time. slight is intended. I do attempt to direct the reader to reviews Nearly 40 years later, another group independently from which they can learn more. Other histories of ionic liq- recognised the potential benefits of lowering the melting uids, with different perspectives, are available in the literature points of the molten salts with which they were working. (Wilkes 2002;Angell etal. 2012). Hurley and Weir were mixing 1-alkylpyridinium halides with Like the selection of a single source from the many tribu- ‘true inorganic salts’, such as metal halides (Hurley and Weir taries of a great river, picking a single beginning for a research 1951) to make solutions from which the metals could be area is somewhat arbitrary. There are several beginnings to the electroplated. During this study, they discovered that the 1- story of ionic liquids in which they were discovered indepen- ethylpyridinium bromide-aluminium chloride ([C py]Br- dently. The earliest of these was when Paul Walden was AlCl ) 2:1 M ratio mixture was liquid at room temperature. searching for molten salts that were liquid at temperatures at They created a phase diagram for this system, containing two which he could use his equipment without special adaptations. eutectics at 1:2 (45 °C) and 2:1 (− 40 °C) molar ratios and a He discovered that [EtNH ][NO ] has a melting point of 12 °C maximum at the 1:1 molar ratio (88 °C), which they attributed 3 3 (Walden 1914). This was also the first example of a protic to the formation of bromochloroaluminate species in the melt. ionic liquid (PIL) (Greaves and Drummond 2008; Greaves The electrodeposition of metals remains an important part of and Drummond 2015), which were later to become an ionic liquid research (Zhang and Etzold 2016). The 2:1 molar ratio of [C py]Br-AlCl was later picked 2 3 up by Bob Osteryoung’s group to study the electrochemis- This article is part of a Special Issue on ‘Ionic Liquids and Biomolecules’ try of two iron(II) diimine complexes, ferrocene and edited by Antonio Benedetto and Hans-Joachim Galla. hexamethylbenzene at room temperature (Chum et al. 1975) having noted that this had not been possible in * Tom Welton molten Na[AlCl ] (m. pt. 175 °C) due to decomposition email@example.com of the solutes. However, they did note the drawback of this ionic liquid as being it is only at this composition that it is Department of Chemistry, Imperial College London, Exhibition liquid at room temperature. This drove the group to seek a Road, London SW7 2AZ, England 692 Biophys Rev (2018) 10:691–706 system that was liquid at room temperature over a wider The end of the beginning range of compositions, namely 1-butylpyridinium chlo- ride-aluminium chloride ([C py]-AlCl ) (Robinson and In the 1980s, interest in ionic liquids began to slowly spread, 4 3 Osteryoung 1979) which they used to study solute electro- with new researchers, such as Ken Seddon and myself, who chemistry and later characterised using Raman spectrosco- had not previously worked with high temperature molten salts py (Gale et al. 1978). coming into the field. Also, the range of investigations began In a separate tributary, other low-melting systems with or- to broaden. ganic cations were being used. George Parshall used [EtNH ][NO ] reappeared in the literature in 1981, in a 3 3 [Et N][GeCl ](m. pt.68°C) and[Et N][SnCl ](m. pt. short note by Evans et al. about the thermodynamics of its 4 3 4 3 78 °C) as solvents for platinum catalysed hydrogenation reac- solutions of krypton, methane, ethane, and n-butane and par- tions (Parshall 1972). While he did not continue with this ticularly Bhydrophobic bonding^ (Evans et al. 1981). It is work, he did inspire John Yoke to return to a previous obser- interesting to see that in this paper, the authors noted that vation (Yoke et al. 1963)that[Et NH][CuCl ] was an ‘oil’ (a [EtNH ][NO ] had the potential to be used as a non-aqueous 3 2 3 3 word often used by inorganic and organometallic chemists solvent for biochemical systems and the importance of its when something that they expected to be a solid was not) at network of hydrogen bonds in controlling its solvent proper- room temperature to investigate a number of ammonium and ties. This group went on to study the activity of alkaline phos- phosphonium chlorocuprate systems (Axtel et al. 1973). In phatase in water-[EtNH ][NO ] mixtures and found that at 3 3 another series of papers, Warren Ford explored molten lower concentrations, [EtNH ][NO ] had a beneficial effect 3 3 tetraalkylammonium tetraalkylborides (Ford et al. 1973)par- on the enzyme’s activity but that at 80% [EtNH ][NO ](v/ 3 3 ticularly triethylhexylammonium triethylhexylboride, which v), it was inactive (Magnuson et al. 1984). had the lowest viscosity of these. With remarkable prescience Colin Poole picked up on this work and recognised the for future interest in ionic liquids, this included the study of possibility of using [EtNH ][NO ] as a stationary phase in 3 3 their effects on the rates of organic reactions (Ford et al. 1974) gas-liquid chromatography (Pacholec et al. 1982). This then and their toxicity and antimicrobial activity (Rosenthal et al. led his group to investigate a number of different ionic liquids 1975). in this role (Poole et al. 1986), so initiating the study of appli- Analysis of the references cited in the papers of those work- cations of ionic liquids in analytical chemistry, which has be- ing with chloroaluminate ionic liquids and those working on come a highly active part of modern ionic liquids research (Ho the other systems suggests that these different groups were et al. 2014), and eventually led to ionic liquids being unaware of each other’s work. This had a number of contrib- commercialised as stationary phases for gas chromatography. uting factors. Those working with the different systems had In 1988 using the Surface Force Apparatus (SFA) technique, very different backgrounds and interests; those working on the Horn and co-workers showed that [EtNH ][NO ]formed 3 3 chloroaluminate systems were mostly electrochemists, while layers of ions at a charged mica surface (Horn et al. 1988). those working with other systems were more concerned with This important result went unnoticed by the mainstream ionic synthesis and catalysis. Also, these systems had not yet been liquid community for nearly 20 years. recognised to be part of a unified concept. Finally, in those Early in the 1980s, John Wilkes’ group introduced 1,3- days before electronic searching of the literature, searches that dialkylimidazolium cations into ionic liquids for the first time we now complete in a few minutes would have taken several in the form of 1-alkyl-3-methylimidazolium chloride alumin- days of work. The first confluence of these steams of work ium chloride ionic liquids ([C C im]Cl-AlCl ,where n =1– n 1 3 was in Chuck Hussey’s seminal 1983 review article on Room 4), with [C C im] being preferred because it gave ionic liq- 2 1 Temperature Molten Salt Systems (Hussey 1983). In this re- uids with the best transport properties (Wilkes et al. 1982). view, we can see that it was the chloroaluminate systems that The [C C im]Cl-AlCl system was shown to be liquid at room 2 1 3 had attracted on-going interest, with most work focussed on temperature across the composition range from 1:2 to 2:1 mol structures, halogenoaluminate equilibria and physical proper- ratio (Fig. 1). These cations went on to become by far the most ties and electrochemistry of the ionic liquids and their solu- popular for making ionic liquids. However, their introduction tions. This had been maintained by a handful of enthusiasts also led to a controversy over the role of hydrogen bonding in and their sponsors, who also worked on more well-established the structures of these ionic liquids. The competing ideas were higher temperature systems. The chloroaluminate systems that interionic interactions in these ionic liquids were either via were extremely difficult to handle due to their severe sensitiv- hydrogen bonding (Tait and Osteryoung 1984)orthatthey ity to water, which required special equipment such as an had a stacked structure with anions positioned above and inert-atmosphere glove box and created a barrier to entry into this part of field. Supelco Ionic Liquid GC Columns. https://www.sigmaaldrich.com/ analytical-chromatography/gas-chromatography/columns/ionic-liquid- literature.html Biophys Rev (2018) 10:691–706 693 The birth of a new field There is no definition of the concept of a field of research, but there is no doubt that ionic liquids are one. It seems to me that this began sometime at the end of the 1980s and during the 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 1990s. Undoubtedly, interest in ionic liquids was increasing, -50 but perhaps more importantly by the end of the twentieth -100 century those using different types of ionic liquids and with -150 different academic backgrounds had become aware of each Mole fracon AlCl other’s work. Ionic liquids were also being noticed by those Fig. 1 Phase diagram of [C C im]Cl-AlCl 2 1 3 who did not work with them. Another contribution from Wilkes on Air and water stable below the plane of the imidazolium ring (Fannin et al. 1984; 1-ethyl-3-methylimidazolium based ionic liquids in 1992 is Dieter et al. 1988). This debate was resolved by recognising seen by most as ushering a new phase in the development of that the imidazolium ring protons can indeed act as hydrogen ionic liquids (Wilkes and Zaworotko 1992), even though the bond donors, but only in the presence of sufficiently strong existence of such ionic liquids had been predicted previously hydrogen bond acceptors (Lungwitz and Spange 2012;Hunt (Cooper and Angell 1983) and other air and moisture stable 2017), and that stacked structures were also present (Elaiwi ionic liquids had been in use elsewhere (Poole et al. 1989). et al. 1995), particularly for ionic liquids with large anions that The claimed stability of these, thermal as well as hydrolytic, are poor hydrogen bond acceptors. was later shown to be somewhat overplayed (Wang et al. Other interests at this time focused upon the 2017; Maton et al. 2013). Notwithstanding this, these papers chloroaluminate species present in these ionic liquids do appear to have initiated a period of growth in the number (Hussey et al. 1986; Dymek et al. 1988; Abdul-Sada et al. and range of ionic liquids to have been made (Figs. 2 and 3), 1989) the impurities arising from their reactions with water for example the introduction of the [NTf ] ion (Bonhote et al. (Zawodzinski and Osteryoung 1990; Trulove and Osteryoung 1996), which in turn allowed researchers to broaden the range 1992) and how these could be removed (Zawodzinski et al. of cations used (Sun et al. 1997). Phosphonium-based ionic 1987; Abdul-Sada et al. 1987;Noelet al. 1991). The fact that liquids have also become an important sub-class of ionic liq- solutions of HCl in acidic compositions (those containing ex- uids (Fraser and MacFarlane 2009). cess AlCl ) of these ionic liquids are superacidic (Smith et al. This period also saw a growth in the interest in ionic liquids 1989) became of great importance for the application of ionic as solvents for chemical reactions, without necessarily being a liquids in the oil refining industry (Harris et al. 2009). Wilkes component of the reaction itself. This began with a study of also reported at this time that acidic compositions could act as Diels-Alder reactions, which are well known to be solvent a combination of solvent and catalyst for Freidel-Crafts reac- dependent, in [EtNH ][NO ] (Jaeger and Tucker 1989). This 3 3 tions (Boon et al. 1986). The inherent Lewis acidity of these took a little while to get off the ground and it was a decade and other ionic liquids has remained an important area for later before two papers appeared within a month of each other study to this day (Brown et al. 2007). from the Welton (Fischer et al. 1999) and Seddon (Earle et al. Interest in other solutes in these ionic liquids was dominat- 1999) groups on the use of dialkylimidazolium-based ionic ed by inorganic chemistry and the coordination chemistry of liquids for these reactions. Diels-Alder reactions are also transition metal species (Hussey 1988). known to be catalysed by Lewis acids, which was exploited Fig. 2 Some commonly used cations for ionic liquids Temp / C 694 Biophys Rev (2018) 10:691–706 Fig. 3 Some commonly used anions for ionic liquids in a study of these in chloroaluminate ionic liquids (Lee 1999). be selected, caught the imagination of many. While no one Alongside this development, in 1992, the alkylation of sodium person was responsible for this, it is true that Seddon was a p-naphthoxide with benzyl chloride/bromide in molten key figure in advertising the promise that they held. He was tetraalkylammonium and phosphonium salts was reported convinced that ionic liquids would affect every corner of sci- (Badri and Brunet 1992), followed by the room temperature ence and would take every opportunity to tell this to an ever dialkylimidazolium ionic liquids a few years later (Earle et al. wider audience in, what I used to jokingly call, BThe Ken 1998). Seddon, ionic liquids are going to save the world roadshow^. Probably arising partly from the previous interest of He would then follow this up by inviting people to QUILL those working with ionic liquids in inorganic solutes as well (the Queen’s University Ionic Liquids Laboratory), the first as its undoubted technological importance, the use of ionic university–industry collaborative research centre dedicated liquids as solvents for transition metal catalysis became a to ionic liquid research to initiate their research using ionic major theme at this time. Throughout the 1980s, Knifton liquids. He was certainly responsible for bringing many new had been investigating hydrogenation (Knifton 1981)and people to the field. hydroformylation of in a variety of quaternary Group 15 Also at about this time, Jim Davis introduced the term Task halides (Knifton 1988), although at temperatures that were Specific Ionic Liquid (Wierzbicki and Davis 2000; Visser et al. higher than usually associated with the term ionic liquid. 2001). These ionic liquids differed from the concept of ionic A key breakthrough using a room temperature ionic liquid liquids as ‘designer solvents’ (Freemantle 1998)by containing came with the nickel catalysed dimerization of alkenes in a covalently tethered functional group to one or both of the ethylaluminium chloride containing ionic liquids (Chauvin ions of an otherwise ordinary ionic liquid to imbue the et al. 1990), which eventually led to the Institut Français du resulting salt with a capacity to interact with dissolved sub- Pétrole’s Difasol process (Plechkova and Seddon 2008). This strates in specific ways (Davis 2004). These ionic liquids have work was the first to take explicit advantage of the solubility been studied for their potential for a wide range of applications of the catalyst and the insolubility of the reaction products in (Lee 2006;Sawantet al. 2011). the ionic liquid. Exploiting this behaviour in biphasic systems The growth in the number of participants in the ionic liquid became a principal driver for much of the future work in field was supported by another important change. In 1999, catalysis in ionic liquids (Wasserscheid and Keim 2000; ionic liquids became commercially available in high qualities Dupont et al. 2002). and at accessible prices from Solvent Innovation GmbH (SI), a company founded by Claus Hilgers and Peter Wasserscheid as a supplier of ionic liquids. While ionic liquids had been An explosion of interest—promises made, available commercially before this, the quality and price of applications delivered Solvent Innovation’s products made ionic liquids accessible to researchers in quantities that they could make use of. Other By the end of the twentieth century, ionic liquids were coming suppliers came into the market in the following years. This to the attention of a wider audience, e.g. the journalist Michael meant that you did not need to be an expert in the synthesis of Freemantle wrote the first of what became a series of reports in ionic liquids in order to be able to use these in your research. I Chemical & Engineering News (Freemantle 1998). Judging also hope that I played a small part in this growth by writing a by how often this article is cited in the introductions to papers, review article that summarised the field up to 1999 (Welton his ‘designer solvent’ concept, based on the idea that there are 1999). These together with the work of many others led to the millions of cation-anion combinations that will give ionic liq- well-known explosion of interest in the potential applications uids from which the ideal ionic liquid for any application can of ionic liquids. It is impossible to mention all of their Biophys Rev (2018) 10:691–706 695 potential applications in a paper such as this, but there are 2011), they also have the potential to be useful for inorganic some that should be included. (Freudenmann et al. 2011) and material synthesis (Torimoto Probably the first and largest of these was the application of et al. 2010). ionic liquids to ‘clean’ and/or ‘green’ technologies and specif- In 1999, Joan Brennecke caused much excitement when ically the growing green chemistry movement. The most vo- she reported combining ionic liquids with supercritical CO cal advocates of ionic liquids as green solvents at this time to form a biphasic system for separations (Blanchard et al. were Ken Seddon (Seddon 1997) and Robin Rogers 1999;Jutz et al. 2011). This led to a large amount of work (Huddleston et al. 1998) who led a NATO Advanced on the solubility of CO and other gases (Lei et al. 2014)in Research Workshop, on Green Industrial Applications of ionic liquids and eventually to their possible application in Ionic Liquids held in April 2000 in Heraklion, Crete (together carbon capture (Zeng et al. 2017). Of course, the ability of with Sergei Volkov) (Rogers et al. 2003) and then 1 year later the ionic liquids to act as CO capture agents can be combined asymposium on Green (or Greener) Industrial Applications with their ability to be excellent solvents for synthesis and of Ionic Liquids held during the ACS National Meeting in San catalysis for the conversion of CO to higher value products Diego (Rogers and Seddon 2002). The claim to greenness for (He et al. 2014; Wang and Wang 2016). Ionic liquids have also ionic liquids at that time rested on their lack of measurable been used for electrochemical valorisation of CO (Alvarez- vapour pressure (Earle and Seddon 2000), which was later Guerra et al. 2015). This latter approach is an example of a shown to be untrue for many ionic liquids (Earle et al. wider interest in the use of ionic liquids for electrocatalysis 2006). This was an example of a growing problem in the field; (Zhang et al. 2016). results that had been obtained for just a few (sometimes even Research into other catalytic reactions in ionic liquids con- one) ionic liquids were described as generic properties of all tinued apace (Parvulescu and Hardacre 2007; Zhang and Zhang ionic liquids and then later found not to be. Many questioned 2011). Perhaps stimulated by the Nobel Prize in the area, many whether ionic liquids could really be described as green sol- groups worked on the various palladium catalysed coupling vents at all (Clark and Tavener 2007;Jessop 2011;Cevasco reactions (Welton and Smith 2004). It was in this context that and Chiappe 2014). My own opinion is that the very concept the formation of N-heterocyclic carbenes (NHCs) in of a green solvent is somewhat naïve, and we should really be imidazolium-based ionic liquids first came to our attention asking whether ionic liquids can contribute to more sustain- (see below) (Mathews et al. 2001). While work continued on able production and use of chemicals, which they clearly can liquid-liquid biphasic catalysis, in 2003, an important do (Welton 2015). Whatever one’s stance in this debate, there development was made when two groups applied silica support- is no doubt that this concept brought many rushing into the ed ionic liquids in catalytic processes. Christian Mehnert et al. field, to the extent that the RSC journal Green Chemistry was supported their ionic liquids by covalently bonding cations to forced to limit the papers about ionic liquids that it would the silica surface (Mehnert et al. 2002), whereas accept (Welton 2011). Peter Wasserscheid, Rasmus Fehrmann and their co-workers A result from early in this period of growth that has proven introduced their supported ionic liquid-phase (SILP) concept to inspire many researchers to join the field was the discovery by physisorption (Riisager et al. 2003). The SILP process is that some ionic liquids could be used to dissolve and regener- not limited to soluble transition metal complex catalysts and ate cellulose (Swatloski et al. 2002). This led to a rapid in- can also be used with nanoparticle catalysts, which have also crease in the interest in the potential application of ionic liq- received a great deal of attention (Scholten et al. 2012). uids in cellulose processing, but it has transpired that this has These were not the first examples of supporting an ionic been difficult to achieve, and it has not yet been achieved liquid for catalysis (Mehnert 2005). That honour goes to commercially (Gericke et al. 2012;Wangetal. 2012). Richard Carlin and Joan Fuller, who incorporated palladium This work also led to interest in the use of ionic liquids for into a gas permeable ionic liquid–polymer gel composed of the processing of biomass, which has become a major area of [C C im][PF ]-poly(vinylidene fluoride)–hexafluoropropylene 4 1 6 activity. At first, this work focussed on using the same ionic copolymer and used it for the hydrogenation of propene (Carlin liquids that had been identified as capable of dissolving cellu- and Fuller 1997). The SILP innovation was the development of lose to dissolve the whole of wood (Kilpelainen et al. 2007), a practically applicable means to support catalyst solutions of but more recently has included the selective extraction of lig- ionic liquids in which the catalyst remains active and stable nin from the biomass (Brandt et al. 2013). There has also been over extended periods in a continuous gas-phase process. considerable interest in the production of biomass-derived In the year 2000, the potential of ionic liquids as solvents products from the biomass polymers in ionic liquids (Zhang for enzyme, particularly lipase, catalysed reactions was et al. 2017). This continues to be a highly active area of re- rediscovered independently by different groups (Cull et al. search and will probably remain so for some time to come. 2000; Erbeldinger et al. 2000; Lau et al. 2000). This is an While most attention this century has been on the applica- example of an area in which it is absolutely not possible to tion of ionic liquids to organic synthesis (Hallett and Welton treat all ionic liquids as if they are the same. Some ionic 696 Biophys Rev (2018) 10:691–706 liquids provide a very hostile environment for biocatalysts, flammable organic solvent electrolytes are ignited. Hence, it is whereas others enhance their activities (Weingaertner et al. not surprising that non-volatile and non-flammable ionic liq- 2012). This latter possibility led to rapid growth in the area uids have been proposed as safer replacements for these (Kragl et al. 2000; Sheldon et al. 2002) and much subsequent (Navarra 2013). The advantages of ionic liquids go beyond activity on the potential applications of ionic liquids in this simple non-flammability to include wide electrochemical win- technologically important area, with a wide variety of enzyme dows, stability to the various electrode materials and good and reaction types being studied (Itoh 2017). Whole cell bio- discharge ability and cycle ability. Proof of concept perfor- catalysis, largely in ionic liquid/water biphasic systems mances show that ionic liquids may provide realistic solutions (Pfruender et al. 2004), has also been a highly active area for to the conflicting demands of commercial batteries. However, research (Fan et al. 2014). as yet, no ideal ionic liquid system has been developed Other applications rely mainly on the physical nature of (Balducci 2017). ionic liquids, rather than their chemistry. One of these is lubri- Protic ionic liquids have been used for several energy tech- cants. Ionic liquids were first reported as very promising high- nologies, but as anhydrous proton conductors, they are partic- performance lubricants to replace other synthetic oils as early ularly well suited to fuel cells (Diaz et al. 2014). Again prom- as 2001 (Ye et al. 2001). Their non-volatility means that they ise is shown, but no ideal ionic liquid has yet been identified. can be used under vacuum (Liu et al. 2002), which is a real This story repeats for applications of ionic liquids for Dye problem for conventional lubricant oils. As well as being used Sensitised Solar Cells (Zakeeruddin and Graetzel 2009)and as the lubricant itself, ionic liquids have been used as an addi- supercapacitors (Eftekhari 2017). For all of these applications, tive for conventional lubricants (Phillips and Zabinski 2004). In the usual advantages of ionic liquids as electrolytes make this role, it is not necessary for the ionic liquids to have a high them attractive, but typical problems associated with ionic solubility in the carrier, so they may even be used with quite liquids, such as high viscosity and disappointing mass and non-polar lubricants. Finally, they can be applied as thin films charge transport, still need to be overcome whilst maintaining (Yu et al. 2006) for use in miniaturised devices. While studying thermal and electrochemical stabilities. This is likely to re- the behaviour of ionic liquids as thin film lubricants, quire new candidate ions to be developed. Susan Perkin discovered the remarkable quantized nature of In addition to their use for separations in analytical chem- friction in these systems (Smith et al. 2013). Although ionic istry, noted above, ionic liquids have found application in liquids do show great promise as lubricants, there are still many separations and extractions for materials as different as those obstacles to be overcome, such as their thermal and chemical from the nuclear industry (Sun et al. 2012) and bioactive com- stabilities (Zhou et al. 2009;Zhou and Qu 2017). pounds (Ventura et al. 2017). The use of ionic liquids in solid- It would be remiss to ignore the potential for ionic liquids phase microextraction has shown the potential for commercial to make an impact in applications for energy generation and application (Ho et al. 2011). storage (Watanabe et al. 2017; MacFarlane et al. 2014). The Of course, the first commercial application of ionic liquids, possibility of using these as battery electrolytes was one of the BASF’s BASIL (Biphasic Acid Scavenging utilising Ionic first applications considered after the discovery of the Liquids) process, was to solve a separation problem (Volland chloroaluminate ionic liquids in the second half of the last et al. 2003). BASF used methylimidazole to replace century (Reynolds and Dymek 1985). However, significant triethylamine as a proton scavenger in the synthesis of progress was facilitated by the introduction of the less reactive alkoxyphenylphosphanes (Scheme 1). This led to a salt by- systems later. This coincided with demand for new battery product, 1-methylimidazolium chloride, with a melting point technologies to enable miniaturisation for small devices and of 75 °C, which is liquid at the temperature of the reaction and length of power output and ease of recharging for transport. separates spontaneously as a second liquid phase under the Even the most cursory search of the term lithium battery reaction conditions. This improvement led to the design of a will quickly find images of these bursting into flames as their jet stream reactor for the new all-liquid BASIL™ process, Scheme 1 Synthesis of alkoxyphenylphosphanes Biophys Rev (2018) 10:691–706 697 which gave an increased productivity for the process of a However, as is the nature of all liquids, this short-range order 4 −3 −1 factor of 8 × 10 to 690,000 kg m h . At the end of 2004, breaks down at greater distances from any given ion. This was BASF SE started a dedicated BASIL plant using this technol- shown very nicely by neutron diffraction studies of ogy. The announcement of this success led to a change in the [C C im]Cl and [C C im][PF ] by Chris Hardacre et al. 1 1 1 1 6 attitude of many sceptics (Bionic liquids will never be used in (2003). This and other studies showed that there were also industry^) as to the practical utility of ionic liquids. significant differences in these structures with the [C C 1 1 The discovery that ionic liquids could act as entrainers im]Cl structure dominated by hydrogen bonding interactions (separation agents) to break azeotropic mixtures to give im- between the chloride ion and the ring protons, while these proved separations by distillation provided another possibility were largely absent in the [C C im][PF ]structure. 1 1 6 for their use (Jork et al. 2004). While this first example fo- The next conceptual advance came from a molecular sim- cussed on drying ethanol or THF, many other systems were ulation study of [C C im][PF ]and [C C im][NTf ] ionic liq- n 1 6 n 1 2 rapidly added to the list for which ionic liquids could be used uids in the now-famous ‘Portuguese Flag’ representations (Pereiro et al. 2012). This application combines the ability of (Fig. 4) (Canongia Lopes 2006). These show nanometre- ionic liquids to interact more strongly with some compounds scale structuring in ionic liquids with alkyl side chains longer rather than others, to favour the separation, with their non- than or equal to C-4 showing aggregation of the alkyl chains volatility, so that they do not contaminate the distillate. in nonpolar domains, excluded from the charged domains. As These advantages have also led to ionic liquids being applied the length of the alkyl chain increases, the nonpolar domains for other separations of volatile organic compounds (Salar- become larger and more connected. Depending on the relative Garcia et al. 2017). size of the charged and non-polar regions, the structures An excellent recent example of this is the full production change from a continuous network of ions with isolated non- scale plant use of an ionic liquid to remove mercury from polar regions to a continuous non-polar phase with isolated natural gas (Abai et al. 2012). The mercury is present in tiny islands of charge. These results were shortly afterwards con- concentrations in the gas stream, but the enormous volume of firmed experimentally (Triolo et al. 2007), although it did take natural gas production leads to large absolute amounts of mer- some time before these diffraction results were fully under- cury passing into the production plant. The combination of a stood (Annapureddy et al. 2010). Of course, not all ions are chlorocuprate(II) ionic liquid system with the SILP technolo- the same and differentiating between the cations and anions gy (see above) led to the ability to remove the mercury with can give further insight that enables rational ion selection to these high throughput volumes. give a range of structures (Shimizu et al. 2010). When the One final application that is generating much interest is the alkyl chains become long enough, ionic liquid crystals can possible application of ionic liquids in pharmaceutical indus- be formed (Goossens et al. 2016). Theory and experiment tries. Davis introduced the first ionic liquid derived from an have moved forward together to grow our understanding of active pharmaceutical ingredient (API) as early as 1998 ionic liquid structures (Hayes et al. 2015; Greaves and (Davis et al. 1998). This work has taken two routes: (i) using Drummond 2013; Russina et al. 2017). the kind of ions used for ionic liquids as the counterion of an This area is an example of a great good fortune that we API that can aid its pharmacokinetics or (ii) two APIs can be have experienced, but largely taken for granted, that interest delivered as a single ionic liquid (Egorova et al. 2017). in ionic liquids has developed at the same time that the role It seems that Ken Seddon and the other advocates of ionic that theoretical and computational chemistry can play has liquids were right and today there are myriad areas of science been transformed by the power of modern computing. This in which ionic liquids have been applied. They have even has led to many advances being theory led. I lack the expertise found applications in space (Nancarrow and Mohammed to review the literature of theory and simulation of ionic liq- 2017)! uids, so I will just direct the reader to reviews of quantum chemical (Izgorodina et al. 2017;Hunt 2017), molecular dy- namics (Pádua et al. 2007; Dommert et al. 2012), and other Deeper understanding methods (Dong et al. 2017). Understanding how ionic liquid ions interact with each While many came to work with ionic liquids due to their other is key to understanding how these structures arise and exciting applications, others sought a deeper understanding how their properties arise from these. These interactions arise of their behaviours, structures and how these arise from a combination of Coulomb forces, hydrogen bonds, pi-pi (Weingaertner 2008). interactions, and dispersion forces (Fumino and Ludwig Perhaps the first question that is asked about any material is 2014). what is its structure. For ionic liquids, this was greatly in- Being composed of ions, Coulomb forces are the ubiqui- formed by what was already known for molten salts and the tous contributor to the interactions of ionic liquid. Other forces are more dependent upon the structures and chemistries of the basic fact that opposite charges attract and like charges repel. 698 Biophys Rev (2018) 10:691–706 Fig. 4 Snapshots of simulation boxes of [C mim][PF ]: a n 6 [C C im][PF ] CPK colouring; b 2 1 6 [C C im][PF ]same 2 1 6 configuration as in a with red/ green (charged/nonpolar) colouring; c [C C im][PF ]; d 4 1 6 [C C im][PF ]; e [C C im][PF ]; 6 1 6 8 1 6 f [C C im][PF ] 12 1 6 constituent ions of the ionic liquids and lead to strong differ- Dispersion forces also vary with the identity of the constituent entiation in their behaviours. From the first debates around ions of the ionic liquids and contribute greatly to the formation hydrogen bonding in ionic liquids (see above), it has been of the complex structures described above. clear that these are sometimes present and sometimes not. In molecular solvents, such strong cation-anion interac- For a hydrogen bond to form, the ions must contain both a tions as are found in ionic liquids often lead to the formation cation that is a sufficiently strong hydrogen bond donor and an of ion pairs (Marcus and Hefter 2006). Whether these form in anion that is a suitably strong hydrogen bond acceptor. The ionic liquids has been the subject of much debate. ability to change these independently has been noted as a key Masayoshi Watanabe and co-workers opened up this discus- driver of the designer solvent concept for ionic liquids sion with their observation that many ionic liquids had con- (Niedermeyer et al. 2013). Consequently there have been ductivities markedly lower than one would expect from the many experimental and theoretical investigations of hydrogen measured diffusivities of their constituent ions (Tokuda et al. bonding in ionic liquids (Dong and Zhang 2012). The doubly 2006). One potential explanation is that not all of the ions are ionic nature of cation-anion hydrogen bonds in ionic liquids taking part in the conduction processes, because of the concert- has proven a particular challenge (Hunt et al. 2015). ed motion of ions of different charge in ion pairs or larger Biophys Rev (2018) 10:691–706 699 clusters (MacFarlane et al. 2009). Another possible explanation demonstrated the effects of hydrogen bonding and is that the ions do not carry unit charge, because hydrogen dipolarity/polarizability on the rates (Crowhurst et al. 2006) bonding leads to charge transfer (Hunt et al. 2015). Both ex- and selectivities (Bini et al. 2008) of reactions in ionic liquids. perimentalists and theoreticians have investigated this phenom- For the majority of reactions studied, the LSERs can be enon in great detail (Hollóczki et al. 2014), with evidence for achieved using both ionic liquids and molecular solvents, both explanations. Of course, these are not mutually exclusive meaning that differences between these are quantitative in and both may be contributing to the observed conductivities. nature, rather than qualitative. However, it has been shown The question of whether ionic liquids contain ion pairs, or that it is sometimes not possible to achieve this, indicating a similar, aggregates is an ongoing one. While conductivity re- change in the mechanism of the reaction. sults such as those above do point towards at least the possi- For the reaction between the dimethyl-4-nitrophenylsulfo- bility ion pairs, other experiments categorically refute their nium ion [(p-NO PhS(CH ) ] with the chloride ion in mo- 2 3 2 existence (Lui et al. 2011). In a liquid that is entirely com- lecular solvents the reaction rate decreased with increasing posed of ions, with all ions surrounded by counterions and just chloride ion concentration (Hallett et al. 2009), due to a step- by random motion some of these being in closer contact with wise mechanism via ion pairs. In all of the ionic liquids stud- one of their counterions than others, it is not even straight- ied, the rate increased linearly with increasing chloride ion forward to define an ion pair. Barbara Kirchner and co- concentration, as one might expect from a simple nucleophilic workers have used the idea of using the lifetimes of these substitution that does not involve ion pairing. This shows that contacts to define an ion pair and on this basis have found no ion pairs were formed in the ionic liquid that were suffi- no evidence in molecular dynamics simulations for ion pairing ciently long-lived or in high enough concentration to be kinet- (Kirchner et al. 2015). ically relevant. This was used to propose that the constituent As well as interacting with each other, the ions of an ionic ions of the solution are highly electrostatically screened. liquid can interact with solute species. Interest in quantifying So far, we have only scratched the surface of understanding the interactions between these ionic liquids began to emerge how ionic liquid-solute interactions lead to changes in reactiv- as early as the 1980s (Zawodzinski and Osteryoung 1989). ity and this remains an important area of research (Keaveney Perhaps not surprisingly, interest in the chloroaluminate sys- et al. 2017). tems focussed on parameters arising from Lewis acid interac- As early as 2001 (Mathews et al. 2001), it had been tions (Mantz et al. 1997). Elsewhere, Michael Abraham et al. noted that the addition of palladium to imidazolium- (1991) applied his multiparameter model to the ionic liquids based ionic liquids could lead to the formation of com- that Poole had been using for his GC columns (Poole et al. pounds containing N-heterocyclic carbene ligands 1986). It was shown that the ionic liquids studied were all (Fig. 5), but that this did not always occur. In this case, it strongly dipolar and hydrogen bond acceptors. This work was necessary to add NaCl to solutions in [C C im][BF ] 4 1 4 was followed by studies of systematically selected ionic liq- uids, both chromatographically (Anderson et al. 2002) and spectroscopically (Crowhurst et al. 2003). These and other studies showed that ionic liquids are highly polar solvents in terms of their dipolarity/polarizability with a much wider range of hydrogen bond donor (cation) and hydrogen bond acceptor (anion) abilities (Poole 2004). Some studies sug- gested that the ionic liquids were stronger hydrogen bond donors than others. This apparent discrepancy was explained by the nature of the solute probe, with charged hydrogen bond acceptor solutes reporting higher values than uncharged sol- utes (Ab Rani et al. 2011). In a study of the effects of ionic liquids on Diels-Alder reactions, it was found that the strength of the hydrogen bond from the ionic liquid cation to a solute was determined by the ability of the cation to donate a hydro- gen bond moderated by the ability of the anion to accept a hydrogen bond (Aggarwal et al. 2002a). A derivative of this concept was later used to explain the solubility of cellulose in ionic liquids (Hauru et al. 2012). It is through their interactions with solutes that ionic liquids affect chemical reactions (Chiappe and Pieraccini 2005). Fig. 5 The molecular structure of a mixed phosphine-NHC palladium Linear solvation energy relationships (LSERs) have complex formed in [C C im][BF ] 4 1 4 700 Biophys Rev (2018) 10:691–706 for the complex to form. Soon after, Varinder Aggarwal possible to avoid the energy cost of nearest neighbour et al. noted the low product yields in the base-catalysed interactions, leaving the uncharged alkyls chains to popu- Baylis-Hillman reaction of methyl acrylate and benzalde- late the interface (Lovelock 2012). hyde in the presence of [C C im]Cl (Aggarwal et al. The use of ionic liquids in electrochemical applications has 4 1 2002b). They explained this by a side reaction of the benz- led to a great deal of interest in the behaviour of ionic liquids at aldehyde with [C C im] via a NHC formed by deproton- charged surfaces (Fedorov and Kornyshev 2014). This behav- 4 1 ation of the imidazolium cation. From these results, interest iour has turned out to be complex and is yet to be fully under- in this reactivity began to grow (Dupont and Spencer 2004) stood. This partly arises from our understanding of electro- and it became clear that a sufficiently basic anion was chemical phenomena being dominated by classical dilute elec- required to deprotonate the imidazolium ring to produce trolytes, which do not aid the understanding of systems com- the NHC. As the years progressed, more compounds incor- posed entirely of ions. porating NHCs were identified, for example 1,3-dialkyli- In2007, RobAtkinandGregWarrpublishedthere- midazolium-2-carboxylate (Gurau et al. 2011). Sometimes, sults of a study that used atomic force microscopy to the NHCs have been shown to be very positive, as in their show the layering of [EtNH ][NO ], [PrNH ][NO ], and 3 3 3 3 use to catalyse the Benzoin condensation (Kelemen et al. [C C im][CH CO ] on mica, silica, and graphite surfaces 2 1 3 2 2011). At other times, they have been a hindrance, as in (Atkin and Warr 2007). They showed that the degree of their reaction with cellulose during its processing in [C C layering, upto8or9layers or 5nm, for[EtNH ][NO ] 2 1 3 3 im][CH CO ] (Clough et al. 2015). was dependent upon the charge and roughness of the sur- 3 2 The observation of products that could be the result of face and the structure of the ionic liquid. This was shortly reactions with NHCs does not prove that these are present in followed by an X-ray reflectivity study of the the ionic liquids as free species. These products could arise temperature-dependent structures of three ionic liquids from concerted reactions of the imidazolium cation, as sug- with the tris(pentafluoroethyl)trifluorophosphate anion at gested by a recent computational investigation of the reaction a charged sapphire substrate (Mezger et al. 2008), which of CO with [C C im][CH CO ](Yanetal. 2017). also showed the formation of layers of ions. Using SFA, 2 2 1 3 2 Oldamur Hollóczki et al. have also suggested that the carbene Perkin and co-workers not only confirmed the layering of trap is required for the reaction to occur, or at least the pres- ions, but also demonstrated that as the alkyl chains of her ence of neutral electrophiles makes the formation of the NHC ionic liquids increased from [C C im][NTf ]to[C C 4 1 2 6 1 more likely (Hollóczki et al. 2013). Recent kinetic observa- im][NTf ], the layering changed from simple cation- tions of the Benzoin condensation have given the first strong anion monolayers to tail-to-tail cation bilayers (Perkin evidence of the spontaneous formation of NHCs in ionic liq- et al. 2011). Thinking in 2007 by Alexei Kornyshev uids (Daud et al. 2017). However, whilst this is evidence of the (Kornyshev 2007) on the nature of the double-layer and NHC being the catalyst for the reaction, it is not an absolute capacitance in ionic liquids gave a theoretical framework confirmation of this, because what the result shows is that the in which these results could be conceptualised. C -H bond of the imidazolium ring is not involved in the rate In 2013, Matthew Gebbie et al. (2013)used SFAwith [C determining step of the reaction, not that the NHC definitely C im][NTf ] to measure attractive forces between gold and 1 2 is. mica surfaces extending well beyond the measured surface Ionic liquid ions do not only interact with themselves layering, up to 35 nm. They used these results to argue that and solute species, they also interact with the materials ionic liquids should be considered to be dilute electrolytes, that they interface with. The importance of applications with the vast majority of ions being undissociated and so not such as SILP makes understanding the gas-liquid interface contributing to the electrostatic screening of the electrodes. particularly important. Hence, a number of techniques This led to immediate controversy (Perkin et al. 2013). have been used to probe this. Generally, it has been found While the experimental results are not in doubt, the explana- that the cations and anions usually share the surface layer tion of these and subsequent confirmations are still a hot topic without any particular layering, but that the alkyl chains (Gebbie et al. 2017). My own view is that so far the only type of the ions tend to aggregate at the surface, with the ion of organisation that has been considered at an electrode is the orientated such that the alkyl chain is presented as the displacement of ions into surface layers and other forms of surface (Santos and Baldelli 2010). For anions with ordering should be considered. For example, if the ions do not trifluoromethyl groups, these orient themselves to the sur- completely freely rotate in this extended layer and instead face, provided that the cation alkyl chain is sufficiently became orientated with respect to each other, this could con- short that its ionic head group is not immersed deep in the tribute to these longer range forces. liquid away from the surface. It has been proposed that the driving force for this behaviour is that the highly charged moieties of the ions are driven to the bulk where Biophys Rev (2018) 10:691–706 701 Scheme 2 Proposed complex formation in a DES. The ChCl- urea DES has a eutectic point at a 1:2 ratio What is in a name? between the communities working with these different sys- tems, which has perpetuated to the detriment of both. Also, Throughout the last century, there was much controversy over there is no scientific justification for believing that a salt with which materials should be included as ionic liquids and which a melting point of 90 °C is in any necessary way different should be excluded. Clearly, the term ionic liquid simply from one with 110 °C. I hope that the time has come for this means a liquid composed of ions. However, the constraint that constraint to be dropped. ionic liquids should be liquid at temperatures below 100 °C Strictly speaking, the term ionic liquid implies that the liq- was added around the turn of the century. I often find myself uid is only composed of ions, with no molecular species pres- accused of introducing this constraint in my 1999 review ent. This does not mean that materials that do have molecular (Welton 1999). However, this is most certainly not the case, constituents, the nearly ionic liquids, are not of interest. and let me repeat what I did say there: The most well-known of these systems are the Deep Eutectic Solvents, which were first introduced by Abbott BRoom-temperature ionic liquid, non-aqueous ionic liq- et al. (2003). The first of these was formed by mixing uid, molten salt, liquid organic salt, and fused salt have choline chloride and urea in a 1:2 molar ratio. The concept all been used to describe salts in the liquid phase. With was that the hydrogen bond between the donor molecule the increase in electronic databases, the use of keywords and the chloride ion is so strong that it generates an ‘ion’ as search tools is becoming ever more important. While whichismuchlarger(Scheme 2), breaking down the authors are free to choose any name that they wish for cation-anion interactions and lowering the mixture’smelt- their systems, I would suggest that they at least include ing point, so generating an ionic liquid-like material. It was the term ionic liquid in keyword lists. In this paper, I later shown that this was a somewhat naïve view of the allow the term ionic liquid to imply that the salt is low bonding in these systems (Ashworth et al. 2016) with melting.^ many different types of H-bond formed. For example, the urea is found to form a H-bonded [urea(choline)] com- If it was not me, where did this come from? The first time that plexed cation in addition to the [Cl(urea) ] ion. Andy I was aware that the temperature of 100 °C was associated Abbott tells me that the name Deep Eutectic Solvent came with ionic liquids was at the NATO Advanced Research about because a referee of the first paper was insistent that Workshop on Green Industrial Applications of Ionic they could not be called ionic liquids. This does not seem Liquids held in April 2000 in Heraklion, Crete. It appears that to have done them any long-term harm as they have be- the purpose of doing this was to make it clear that the meeting come an area of a great deal of activity (Zhang et al. 2012). was not going to include high temperature inorganic molten Another group of materials that have received attention by salts in its programme. This led to an unfortunate schism the ionic liquid community are the lithium-glyme solvate ionic Scheme 3 Proposed complex formation in lithium glyme solvate ionic liquid. The lithium salt-glyme molar ratio is 1:1 702 Biophys Rev (2018) 10:691–706 distribution, and reproduction in any medium, provided you give appro- liquids (Scheme 3). These share conceptual similarities with priate credit to the original author(s) and the source, provide a link to the the DESs in that a complexing material, this time a glyme to a Creative Commons license, and indicate if changes were made. lithium salt, is added to make a complex ion that is larger, so breaking down cation-anion interactions and reducing the melting point of the mixture. These were first introduced by Watanabe and co-workers References for use as electrolytes for lithium-ion batteries (Ueno et al. 2012), for which they are gaining much attention (Watanabe Ab Rani MA, Brandt A, Crowhurst L, Dolan A, Lui M, Hassan NH, et al. 2017). They recognised from the beginning that while Hallett JP, Niedermeyer H, Perez-Arlandis JM, Schrems M, To T, long-lived [Li(glyme)] complex cations are formed, other Welton T, Wilding R (2011) Understanding the polarity of ionic liquids. Phys Chem Chem Phys 13:16831–16840 species are possible and that the precise composition Abai M, Atkins M, Cheun KY, Holbrey JD, Nockemann P, Seddon KR, depended upon competitive interactions between the glymes Srinivasan G, Zou Y (2012) Process for removing metals from hy- + + and the Li cations and between the counter anions and the Li drocarbons. World Pat., WO2012046057 cations. These glyme-based ionic liquids are related to a much Abbott AP, Capper G, Davies DL, Rasheed RK, Tambyrajah V (2003) Novel solvent properties of choline chloride/urea mixtures. Chem older material, 5 M lithium perchlorate-diethyl ether, which Commun 70–71 although no longer used, was used as a solvent for organic Abdul-Sada AK, Avent AG, Parkinton MJ, Ryan TA, Seddon KR, synthesis for some time (Heydari 2002) and was considered to Welton T (1987) The Removal of Oxide Impurities from Room- + + be a ‘fused salt’ containing both [Li(ether)] and [Li(ether) ] Temperature Halogenoaluminate Ionic Liquids. J Chem Soc Chem ions (Ekelin and Sillén 1953). Commun 1643–1644 Abdul-Sada AK, Greenway AM, Seddon KR, Welton T (1989) Upon the existence of [Al Cl ] in room-temperature Chloroaluminate ionic 3 10 liquids. Org Mass Spectrom 24:917–918 Where are we now Abraham MH, Whiting GS, Doherty RM, Shuely WJ (1991) Hydrogen bonding 17. The characterization of 24 gas-liquid chromatographic stationary phases studied by Poole and coworkers, including molten I started this paper with the metaphor of the history of ionic salts, and evaluation of solute stationary phase interactions. J liquids as separate small streams slowly joining as tributaries Chromatogr 587:229–236 of a great river that then flowed its way across the scientific Aggarwal A, Lancaster NL, Sethi AR, Welton T (2002a) The role of landscape. Now, with the many thousands of people who are hydrogen bonding in controlling the selectivity of Diels-Alder reac- tions in room-temperature ionic liquids. Green Chem 4:517–520 studying and researching in that great river, we can see that Aggarwal VK, Emme I, Mereu A (2002b) Unexpected side reactions of this is moving on apace and there is much to still be discov- imidazolium-based ionic liquids in the base-catalysed Baylis- ered and understood. However, I do notice that another change Hillman reaction. Chem Commun 1612–1613 is underway. It is now rare for individual scientists to be Alvarez-Guerra M, Albo J, Alvarez-Guerra E, Irabien A (2015) Ionic expert, or even interested, across the whole field. Meetings liquids in the electrochemical valorisation of CO2. Energy Environ Sci 8:2574–2599 on ionic liquids are most often now focussed on one or two Anderson JL, Ding J, Welton T, Armstrong DW (2002) Characterizing of their applications, or a subset of their physical properties ionic liquids on the basis of multiple solvation interactions. J Am and that great river is separating into separate channels as it Chem Soc 124:14247–14254 flows through its delta. This is probably to be expected of a Angell CA, Ansari Y, Zhao Z (2012) Ionic Liquids: Past, present and field of study that grows to this size and I consider myself future. Faraday Discuss 154:9–27 Annapureddy HVR, Kashyap HK, De Biase PM, Margulis CJ (2010) very lucky to have experienced so much of this and to have What is the origin of the Prepeak in the X-ray scattering of contributedtoit. imidazolium-based room-temperature ionic liquids? J Phys Chem B 114:16838–16846 Acknowledgements I would like to acknowledge all of the colleagues, Ashworth CR, Matthews RP, Welton T, Hunt PA (2016) Doubly ionic collaborators and co-workers who have made the field of ionic liquids hydrogen bond interactions within the choline chloride–urea deep such fun to be in. eutectic solvent. Phys Chem Chem Phys 18:18145–18160 Atkin R, Warr GG (2007) Structure in confined room-temperature ionic Compliance with ethical standards liquids. J Phys Chem C 111:5162–5168 Axtel DT, Good BW, Porterfield WW, Yoke JT (1973) BFused salts^ at room temperature. Spectroscopic and 0ther studies of liquid Conflict of interest Tom Welton declares that he has no conflict of Chlorocuprates(I). 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