Parallel Developments in Aprotic and Protic Ionic Liquids Angell Et Al
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ohio1/yar-yar/yar-yar/yar01207/yar2248d07z xppws 23:ver.3 10/17/07 12:31 Msc: ar-2007-001842 TEID: lmh00 BATID: ar85a19 Acc. Chem. Res. XXXX, xxx, 000–000 We will try to organize a wide variety of measurements 31 1 Parallel Developments in Aprotic on low-melting ionic liquid systems into a coherent body 32 of knowledge that is relevant to the ionic liquid field as it 33 2 and Protic Ionic Liquids: is currently developing. 34 3 Physical Chemistry and Under the expanded concept of the field, ionic liquid 35 studies commenced in the Angell laboratory in 1962 with 36 4 Applications a study of the transport properties of the hydrates of 37 5 C. AUSTEN ANGELL,* NOLENE BYRNE, AND Mg(NO3)2 and Ca(NO3)2. These were described as melts 38 6 JEAN-PHILIPPE BELIERES of “large weak-field cations”1 and their properties cor- 39 7 Department of Chemistry and Biochemistry, Arizona State related with those of normal anhydrous molten salts via 40 8 University, Tempe, Arizona 85287-1604 their effective cation radii, the sum of the normal radius 41 9 Received August 9, 2007 plus one water molecule diameter. This notion was 42 43 10 followed in 1966 with a full study under the title “A new 11 ABSTRACT class of molten salt mixtures” in which the hydrated 44 12 This Account covers research dating from the early 1960s in the cations were treated as independent cation species. The 45 13 field of low-melting molten salts and hydrates,which has recently large cations mixed ideally with ordinary inorganic cations 46 14 become popular under the rubric of “ionic liquids”. It covers + + 15 understanding gained in the principal author’s laboratories (initially such as Na and K , to give solutions in which such 47 16 in Australia, but mostly in the U.S.A.) from spectroscopic, dynamic, cohesion indicators as the glass transition temperature, 48 17 and thermodynamic studies and includes recent applications of Tg, changed linearly with composition. The concept 49 18 this understanding in the fields of energy conversion and bio- 50 19 preservation. Both protic and aprotic varieties of ionic liquids are proved quite fruitful, and a field (“hydrate melts” or 20 included, but recent studies have focused on the protic class “solvate melts”) developed in its wake in which asym- 51 21 - - because of the special applications made possible by the highly metric anions like SCN and NO2 were used with 52 22 variable proton activities available in these liquids. solvated cations to create a very wide range of systems 53 23 that were liquid at room temperature, while remaining 54 ionic in their general properties. 55 24 In this Account, we take a broad view of ionic liquids The validity of the molten salt analogy was given 56 25 (using the term in its current sense of liquids comprised additional conviction by the observation that transition 57 26 of ions and melting below 100 °C). We include, along with metal ions such as Ni(II) and Co(II) could be found in the 58 27 the general cases of organic cation systems, liquids in 2- 2- complex anion states NiCl4 and CoCl4 when added 59 2+ 2+ 28 which common inorganic cations (like Mg and Ca ) to the “hydrate melt” chlorides.2 60 29 strongly bind a hydration or solvation shell and then Of special interest here was the observation3 of extreme 61 30 behave like large cation systems and melt below 100 °C. downfield NMR proton chemical shifts of hydrate protons 62 + when the hydrated cation was Al3 . These lay much 63 Austen Angell obtained his bachelor’s and Masters’ degrees from the University further downfield than did the protons in strong mineral 64 of Melbourne, Australia, and his Ph.D. from Imperial College of Science, University acids at the same concentration, leading to the design of 65 of London. He is currently Regents’ Professor of Chemistry at Arizona State University, Tempe, AZ, having moved to ASU in 1989 after 23 years at Purdue solutions of highly acidic character from salts that would 66 University. His first work in the sub-100 °C ionic liquids field was done at usually be considered neutral. Indeed it was shown in ref 67 Melbourne University. He has since worked on a wide range of problems related 3 that platinum can be dissolved much more rapidly in a 68 to liquids and glasses, including electrolytes for lithium batteries and most recently ionic liquids for fuel cells and protein folding/preservation studies. He has been hot ionic liquid formed from AlCl3·(H2O)6 + Al(NO3)3· 69 especially intrigued by the problem of supercooled water and related liquids in (H2O)12 than in boiling aqua regia. This scenario of 70 which the phenomenon of polyamorphism is encountered. Awards for research variable proton activity in solvent-free, nominally neutral 71 have included the MRS Turnbull Lecture (2006), ACS Joel Henry Hildebrand award for the study of liquids (2004), the Neville Mott award of the Journal of Non- ionic liquids will be revisited in the context of protic ionic 72 Crystalline Solids (1992), and the Morey award of the American Ceramic Society liquids at the end of this Account. 73 (1990). He has also been the recipient of an Alexander von Humboldt senior Most of the interest in the field lies with room tem- 74 research award, and a Fulbright award. He has served on the editorial boards of a number of major journals including the Journal of Physical Chemistry, the perature liquids in which the cations are indeed of organic 75 Journal of Chemical Physics, and Annual Review of Physical Chemistry. In origin, and the story takes up naturally with these because 76 1998, he was honored with a special issue of the Journal of Physical Chemistry. of the manner in which the organic cation ionic liquids 77 Nolene Byrne received her B.E.(Mat)(Hons) in 2003 then a Ph.D. in 2006 from can be put to use in providing excellent complex anions 78 Monash University, Australia, in the area of improved electrolytes for lithium metal for spectroscopic study. A good example is that shown in 79 batteries. She is currently in a postdoctoral role at Arizona State University, Figure 1 from the work of Gruen and McBeth4 in whose 80 Tempe, AZ. Her research involves the use of ionic liquids as media for improved biopreservation and electrochemical devices. laboratory one of us first encountered melts such as 81 pyridinium chloride, PyHCl, and first used them in the 82 Jean-Philippe Belieres did his undergraduate studies at the Université Joseph (unpublished) study of Ni(II) complexes in low-melting 83 Fourier in Grenoble, France, and his Master degree in electrochemistry at the Institut National Polytechnique de Grenoble, France. He received his Ph.D. in systems such as PyHCl–ZnCl2. We were impressed by the 84 Chemistry from Arizona State University, and he is currently a Research Assistant sharp changes in electronic spectra of Ni(II), reflecting 85 Professor in the Chemistry and Biochemistry department at ASU. His research has been focused on fuel cells, lithium batteries, and ionic liquids. * E-mail: [email protected]. 10.1021/ar7001842 CCC: $37.00 © XXXX American Chemical Society VOL. xxx, NO. xx, XXXX / ACCOUNTS OF CHEMICAL RESEARCH A PAGE EST: 8.4 ohio1/yar-yar/yar-yar/yar01207/yar2248d07z xppws 23:ver.3 10/17/07 12:31 Msc: ar-2007-001842 TEID: lmh00 BATID: ar85a19 Parallel Developments in Aprotic and Protic Ionic Liquids Angell et al. The dramatically different effects of complexation on 126 6 the liquid viscosity are well illustrated by a 1976 study 127 that used the lower-melting R-methyl derivative of pyri- 128 dinium chloride as the source of the Lewis base chloride. 129 Data seen in Figure 3 show that the cohesive energy of 130 an ionic liquid is lowered most strongly when chloride 131 ions are complexed by FeCl3 to produce the large singly 132 charged FeCl4 anion. This observation leads to the predic- 133 tion that, among ionic liquids of a given cation, the 134 tetrachloroferrate salt should be the most fluid and also 135 most conductive, even more so than the well-studied ionic 136 - 7 liquids containing AlCl4 anions. The veracity of this 137 expectation is demonstrated in Figure 4 for salts of the 138 FIGURE 1. Electronic (visible) spectrum of Ni2+ dissolved at butylmethylimidazolium cation taken from a very recent 139 -4 8 10 mol/L in pyridinium chloride. study by Xu et al. For the 25 °C conductivity, the 140 tetrachloroferrate is half an order of magnitude above any 141 others in the group. The increase of ionic mobility by 142 86 distinct and different structures, which were produced by complexation seen here is no different in principle from 143 87 small changes in solution composition. the use of BeF additions to alkali fluoride melts utilized 144 88 The knowledge that these liquids could be used for 2 in the molten salt reactor technology that had provided 145 89 molten salt studies at low temperatures quickly became much of the driving force, and also funding, for molten 146 90 useful when the new science of high-pressure physical salt studies in the 1950–1970 period. There, the addition 147 91 chemistry using the diamond anvil high-pressure cell of 1 mol of BeF replaced two high charge intensity 148 92 (DAC) burst on the scene. Little was known at that time 2 fluoride anions with a doubly charged anion of much 149 93 of the effect of pressure on ion coordination.