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Parallel Developments in Aproticand Protic Ionic Liquids:Physical Chemistry andApplicationsC. AUSTEN ANGELL,* NOLENE BYRNE, ANDJEAN-PHILIPPE BELIERESDepartment of Chemistry and Biochemistry, Arizona StateUniversity, Tempe, Arizona 85287-1604

Received August 9, 2007

ABSTRACTThis Account covers research dating from the early 1960s in thefield of low-melting molten salts and hydrates,which has recentlybecome popular under the rubric of “ionic liquids”. It coversunderstanding gained in the principal author’s laboratories (initiallyin Australia, but mostly in the U.S.A.) from spectroscopic, dynamic,and thermodynamic studies and includes recent applications ofthis understanding in the fields of energy conversion and bio-preservation. Both protic and aprotic varieties of ionic liquids areincluded, but recent studies have focused on the protic classbecause of the special applications made possible by the highlyvariable proton activities available in these liquids.

In this Account, we take a broad view of ionic liquids(using the term in its current sense of liquids comprisedof ions and melting below 100 °C). We include, along withthe general cases of organic cation systems, liquids inwhich common inorganic cations (like Mg2+and Ca2+)strongly bind a hydration or solvation shell and thenbehave like large cation systems and melt below 100 °C.

We will try to organize a wide variety of measurementson low-melting ionic liquid systems into a coherent bodyof knowledge that is relevant to the ionic liquid field as itis currently developing.

Under the expanded concept of the field, ionic liquidstudies commenced in the Angell laboratory in 1962 witha study of the transport properties of the hydrates ofMg(NO3)2 and Ca(NO3)2. These were described as meltsof “large weak-field cations”1 and their properties cor-related with those of normal anhydrous molten salts viatheir effective cation radii, the sum of the normal radiusplus one water molecule diameter. This notion wasfollowed in 1966 with a full study under the title “A newclass of molten salt mixtures” in which the hydratedcations were treated as independent cation species. Thelarge cations mixed ideally with ordinary inorganic cationssuch as Na+ and K+, to give solutions in which suchcohesion indicators as the glass transition temperature,Tg, changed linearly with composition. The conceptproved quite fruitful, and a field (“hydrate melts” or“solvate melts”) developed in its wake in which asym-metric anions like SCN- and NO2

- were used withsolvated cations to create a very wide range of systemsthat were liquid at room temperature, while remainingionic in their general properties.

The validity of the molten salt analogy was givenadditional conviction by the observation that transitionmetal ions such as Ni(II) and Co(II) could be found in thecomplex anion states NiCl4

2- and CoCl42- when added

to the “hydrate melt” chlorides.2

Of special interest here was the observation3 of extremedownfield NMR proton chemical shifts of hydrate protonswhen the hydrated cation was Al3+. These lay muchfurther downfield than did the protons in strong mineralacids at the same concentration, leading to the design ofsolutions of highly acidic character from salts that wouldusually be considered neutral. Indeed it was shown in ref3 that platinum can be dissolved much more rapidly in ahot ionic liquid formed from AlCl3·(H2O)6 + Al(NO3)3·(H2O)12 than in boiling aqua regia. This scenario ofvariable proton activity in solvent-free, nominally neutralionic liquids will be revisited in the context of protic ionicliquids at the end of this Account.

Most of the interest in the field lies with room tem-perature liquids in which the cations are indeed of organicorigin, and the story takes up naturally with these becauseof the manner in which the organic cation ionic liquidscan be put to use in providing excellent complex anionsfor spectroscopic study. A good example is that shown inFigure 1 from the work of Gruen and McBeth4 in whoselaboratory one of us first encountered melts such aspyridinium chloride, PyHCl, and first used them in the(unpublished) study of Ni(II) complexes in low-meltingsystems such as PyHCl–ZnCl2. We were impressed by thesharp changes in electronic spectra of Ni(II), reflecting

* E-mail: [email protected].

Austen Angell obtained his bachelor’s and Masters’ degrees from the Universityof Melbourne, Australia, and his Ph.D. from Imperial College of Science, Universityof London. He is currently Regents’ Professor of Chemistry at Arizona StateUniversity, Tempe, AZ, having moved to ASU in 1989 after 23 years at PurdueUniversity. His first work in the sub-100 °C ionic liquids field was done atMelbourne University. He has since worked on a wide range of problems relatedto liquids and glasses, including electrolytes for lithium batteries and most recentlyionic liquids for fuel cells and protein folding/preservation studies. He has beenespecially intrigued by the problem of supercooled water and related liquids inwhich the phenomenon of polyamorphism is encountered. Awards for researchhave included the MRS Turnbull Lecture (2006), ACS Joel Henry Hildebrand awardfor the study of liquids (2004), the Neville Mott award of the Journal of Non-Crystalline Solids (1992), and the Morey award of the American Ceramic Society(1990). He has also been the recipient of an Alexander von Humboldt seniorresearch award, and a Fulbright award. He has served on the editorial boardsof a number of major journals including the Journal of Physical Chemistry, theJournal of Chemical Physics, and Annual Review of Physical Chemistry. In1998, he was honored with a special issue of the Journal of Physical Chemistry.

Nolene Byrne received her B.E.(Mat)(Hons) in 2003 then a Ph.D. in 2006 fromMonash University, Australia, in the area of improved electrolytes for lithium metalbatteries. She is currently in a postdoctoral role at Arizona State University,Tempe, AZ. Her research involves the use of ionic liquids as media for improvedbiopreservation and electrochemical devices.

Jean-Philippe Belieres did his undergraduate studies at the Université JosephFourier in Grenoble, France, and his Master degree in electrochemistry at theInstitut National Polytechnique de Grenoble, France. He received his Ph.D. inChemistry from Arizona State University, and he is currently a Research AssistantProfessor in the Chemistry and Biochemistry department at ASU. His researchhas been focused on fuel cells, lithium batteries, and ionic liquids.

Acc. Chem. Res. XXXX, xxx, 000–000

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distinct and different structures, which were produced bysmall changes in solution composition.

The knowledge that these liquids could be used formolten salt studies at low temperatures quickly becameuseful when the new science of high-pressure physicalchemistry using the diamond anvil high-pressure cell(DAC) burst on the scene. Little was known at that timeof the effect of pressure on ion coordination.

The DAC was then only available for room-temperaturestudies, and a room-temperature molten chloride for theapplication of this newly invented high-pressure techniquewas needed. Even these days, ambient-temperature ionicliquid chlorides are not common, but Angell and Abke-meier5 found that ethanolaminium chloride could serveas the ambient-temperature noncrystallizing componentthat induced ambient-temperature stability on mixtureswith ethylammonium chloride. This chloride ion meltproved ideally suited for the study of transition metal ionspectra up to 3.5 GPa, pressure being calibrated in therange to 0.3 GPa by observing the glass transition spec-troscopically and relating it to independently determinedTg values from high-pressure differential thermal analysis(DTA) studies.

Ni(II) electronic spectra of good quality, shown inFigure 2, were obtained, and the conversion from NiCl4

2-

at ambient pressure to NiCl64- at higher pressures was

observed. Impressively simple, the change with pressureof the equilibrium constant

dlnKeq/dP )-!V/RT (1)

for the process NiCl42- + 2Cl- f NiCl6

4-, yielded a !Vwithin 10% of the volume of 2 mol of chloride ion.5

Role of Complex Anions in the Development ofAmbient Temperature Air- and Water-StableIonic LiquidsWhile the latter study of transition metal spectra usedorganic cation ionic liquids as a tool for the study ofcomplex ions, Hodge and co-workers6 used complexanions in organic cation salts for the study of ionic liquidproperties, indeed properties that have found recentapplications.

The dramatically different effects of complexation onthe liquid viscosity are well illustrated by a 1976 study6

that used the lower-melting R-methyl derivative of pyri-dinium chloride as the source of the Lewis base chloride.Data seen in Figure 3 show that the cohesive energy ofan ionic liquid is lowered most strongly when chlorideions are complexed by FeCl3 to produce the large singlycharged FeCl4 anion. This observation leads to the predic-tion that, among ionic liquids of a given cation, thetetrachloroferrate salt should be the most fluid and alsomost conductive, even more so than the well-studied ionicliquids containing AlCl4

- anions.7 The veracity of thisexpectation is demonstrated in Figure 4 for salts of thebutylmethylimidazolium cation taken from a very recentstudy by Xu et al.8 For the 25 °C conductivity, thetetrachloroferrate is half an order of magnitude above anyothers in the group. The increase of ionic mobility bycomplexation seen here is no different in principle fromthe use of BeF2 additions to alkali fluoride melts utilizedin the molten salt reactor technology that had providedmuch of the driving force, and also funding, for moltensalt studies in the 1950–1970 period. There, the additionof 1 mol of BeF2 replaced two high charge intensityfluoride anions with a doubly charged anion of muchlarger dimensions, for a net decrease in cohesion.

The fluorinated anion salts (containing BF4- and PF6

-)that are most frequently cited in connection to practicalapplications of the ambient-temperature ionic liquids, areto be understood in the same terms (unfortunately, thetetrachloroferrate anion is slowly hydrolyzed by water, andthe applications of ambient temperature ionic liquidsgenerally require the presence of water-stable anions).Fluorinated species in addition to being water-stable havethe advantage of being unpolarizable and so minimize theeffect of van der Waals contributions to the liquid cohe-sion. The importance of this characteristic of fluorinatedanions to ionic liquid properties was first emphasized ina 1983 study of binary solutions of ionic liquid halides withlithium halides by Cooper.9 There it was predicted thatthe replacement of iodide anions by the equivalentnumber of BF4

- anions would lead to an increase ofambient temperature conductivity, which was then proventrue (unfortunately only in a “note added in proof” in ref9) by some 2 orders of magnitude.9,10 Cooper’s furtherextensive syntheses and measurements on viscosity andconductivity of a family of tetraalkylammonium tetrafluo-roborate salts remained unpublished (for over 2 decades)until 2003.8

In the meantime, Cooper and O’Sullivan11 reported, in1992, the first systematic study of ambient-temperaturewater-stable ionic liquids, using triflate and related anionscombined with imidazolium type cations. This announce-ment, presented at one of the Electrochemical Society’sInternational Molten Salt Conferences and eventuallypublished in its proceedings,11 was immediately followedby a communication by Wilkes and co-workers12 who hadbeen working on similar salts, and the field of high-fluidity, water-stable ionic liquids was launched.

FIGURE 1. Electronic (visible) spectrum of Ni2+ dissolved at10-4mol/L in pyridinium chloride.

Parallel Developments in Aprotic and Protic Ionic Liquids Angell et al.

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Ionicity of Ionic Liquids: Relation to VaporPressure and ConductivityThe “ionicity” of aprotic ionic liquids is the property thatis responsible for their characteristic low vapor pressures.Low vapor pressure is the single most important propertyof this class of liquid, the property that, more than anyother, is responsible for high level of interest in the field.However there is very little written about what exactly isinvolved in achieving high ionicity.13 If the ions createdin forming the ionic liquid were to remain locked togetherin pairs, the liquids would not be found to be of very lowvapor pressure, and their conductivities would also bepoor. They would just represent an unusual group of polarliquids. Indeed there are cases of aprotic ionic liquids inwhich for one reason or another the ions are largely pairedup, and these are characterized by a high vapor pressureand a poor conductivity (see below). To obtain the lowvapor pressures for which ionic liquids are generallyreputed, the ions must distribute themselves in a uniformmanner each ion surrounded by a symmetrical shell of

the opposite charge. This is the distribution that mini-mizes the electrostatic potential energy, developing theliquid equivalent of the ionic crystal Madelung energy.14

It is the difficulty of overcoming this electrostatic freeenergy to produce individual ion pairs that is responsiblefor the low vapor pressures of “good” ionic liquids.

An example of an ion-paired aprotic salt (unsym-metrical charge shell) is shown in Figure 5, which is a plotbased on the classical concept, due to Walden, that ionicmobility is determined by the viscous friction of the liquiddragging on the ion as it attempts to move under theelectrochemical force exerted by the DC electric field. This

FIGURE 2. Electronic spectra (d–d transitions) of Ni(II) in a chloride environment both at 1 atm pressure and variable temperature down intothe glass state (panel a) as a function of pressure up to 0.3 GPa at 24.5 °C and as a function of both up to 0.91 GPa, showing conversion fromtetrahedral to octahedral coordination at high pressures and low temperatures. Reprinted with permission from ref 5. Copyright 1973 AmericanChemical Society.

FIGURE 3. Variation of Tg of the simple chloride glass formerR-methylpyridinium chloride on chloride complexation with variousLewis acids. Adapted from ref 6 by permission.

FIGURE 4. Fluidity data for a number of salts of the butylmethylim-idzolium cation, showing that the tetrachloroferrate is more fluid thanany of the fluorinated anion salts, in accord with its lowest Tg seenhere (log !-1 ) 10-13) and in the Figure 6 legend. Reproduced withpermission from ref 8. Copyright 2003 American Chemical Society.


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salt, methoxymethyl-dimethyl-ethyl-ammonium tetrafluo-roborate (MOMNM2E-BF4 or [N1-O-1,211][BF4]) is seen,among the aprotic cases, to be that most clearly belongingto the “poor ionic liquids” domain15 in Figure 5. It wouldhave a high vapor pressure relative to others because itnever developed the full Madelung potential. The precisereason why the cation and anion like to associate in thissalt is not fully established, but it is probably due to theelectron depletion of the methylene group between O andN+ yielding acid protons that H-bond to BF4fluorines insome lock-and-key manner.

The only “poorer” ionic liquid than [N1-O-1,211][BF4] inFigure 5 is a “protic” ionic liquid in which the drive totransfer the proton and create two ions is very weak. Proticionic liquids as a class are given extensive discussion in a

later section as they are among the most interesting andversatile of ionic liquids.

Figure 5 can be used to classify liquids between thegroups “ionic” (whose members lie near the diagonal line),“subionic” (whose members lie below the line in theregion labeled as “poor” ionic liquids), and superionic(whose members lie above the line by virtue of havingsome ionic species, usually small and low-charged, thatcan “slip through the cracks” in a nanoscopic sense andthus escape the full viscous drag of the surroundingmedium).

Cohesive Energy of Ionic LiquidsThe salts that develop the full Madelung potential are, bythe above understanding, those that will have the largestcohesive energy (for given ionic sizes). Generally they willbe the cases that have ions of comparable size anduniform shape so that they pack well and occupy a smallvolume. However salts that conform to this descriptionwill in general find it easy to crystallize into some highlattice energy structure and will have high melting points.They will therefore not be among the group known as“ionic liquids”. To qualify as ionic liquids there must besome factor that lowers the lattice energy and therebylowers the melting point. The “good” ionic liquids thenmust be those with some optimum departure from highsymmetry, achieved by judicious (or serendipitous) choiceof ion shape and interaction factors.

Those that achieve a sufficiently low melting point willthen fail to crystallize and, on further cooling, willterminate in the glassy state. The temperature at whichthe liquid vitrifies, Tg, provides a second measure of theliquid cohesion. The plot is expected to have a minimumwhere decreasing Coulomb attractions balance increasingvan der Waals attractions. Such a minimum is indeedfound8 in the vicinity of a volume that corresponds to acharge concentration of !4 M (Figure 6).

The case of the “poor” ionic liquid [N1-O-1,211][BF4], inwhich the cations and anions are largely paired, is ofinterest in this connection. This aprotic ionic liquid wasseen, in Figure 10 of ref 8 to have the lowest cohesion, bythe Tg criterion, of any aprotic IL. This is fully consistentwith its poorly developed Madelung potential. It is onlyunderlain by the protic cases, I–IV, discussed below.

Aprotic Ionic Liquid Applications: LithiumBattery ElectrolytesDespite the interest widely expressed in ionic liquid-basedlithium battery electrolytes, we omit reference to ourrecent studies in this area. This application is limited bya fundamental problem, discussed in ref 9 and illustratedin Figure 3, where it is seen that all small cations, includingLi+, get trapped as anionic complexes (by outcompetingthe large solvent cations) and hence must lose mobility.This problem is reduced by use of fluorinated anions9 butnot overcome.

FIGURE 5. Walden Plot for ionic liquids and other molten salt andhydrate systems. Position of ideal line is fixed by data for KCl in 1M aqueous solution. “Good” ionic liquids cluster on the ideal line.For exceptions, see text. Reprinted with permission from ref 15.

FIGURE 6. Glass transition temperatures of protic ILs in relation tothe plot for aprotic salts of weakly polarizable anions from ref 8.Protic ILs are large shaded symbols (see ref 8 for details). Adaptedfrom ref 8.


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Protic Ionic Liquids: Properties andApplicationsIn contrast to the systems on which most of the work inthe ionic liquid field has been performed (representedhere by data in Figure 4), the protic ionic liquids areformed very simply by proton transfer from an inorganicacid to a Bronsted base. The base is usually organic incharacter, but there are also inorganic examples. The salthydrazinium nitrate was reported in 1971 to have amelting point of only 70 °C,16 and hydrazinium formateand hydrazinium acetate are both molten below 100 °Cand can easily be studied through room temperaturedown to their glass transition temperatures.16 The electri-cal conductance of hydrazinium formate at room tem-perature (measured in 1970 but only reported in the openliterature in 200317) is higher than that of any aprotic ionicliquid and is among the highest known. Many eutecticmixtures of ammonium salts are liquid below 100 °C, andthese have recently been shown to be very interesting inapplication as fuel cell electrolytes,18 as will be discussedfurther below. And of course there are the hydroniumcompounds such as hydronium triflate, which is congru-ently melting, Tm)35 °C, and has a very high electricalconductivity. Triflic acid and other superacids will pre-sumably protonate many inorganic molecules that wouldnot normally be considered as having cationic forms, sothere are likely to be many additional inorganic proticionic liquids recognized in the future. However the organicbases are far more versatile for protic ionic liquid formation.

There are organic bases not only from the primary,secondary or tertiary amines, but also with nitrogen atomsin heterocyclic rings (which may or may not be resonancestabilized).19 There are also protonatable phosphorusanalogs available, and sulfur-, selenium-, and iodine-basedexamples also exist.

An important aspect of protic ionic liquids is that theproperties of the ionic liquid formed on proton transferdepend very strongly on the relative strengths of the acidand base between which the proton is transferred.20 Whilethe concentration of protonated species is always veryhigh, of order 5–10 M in most cases, the properties mayvary from strongly acidic to strongly basic in character,as is readily seen by simple tests with proton sensitiveprobes, such as the common indicator ions. For instance,a wide pH range litmus paper registers bright pink incontact with triethylammmonium triflate but dark bluein contact with triethylammonium acetate. The systematicand quantitative measurement of basicity in these solvent-free liquids is a recent achievement.

In order to assist the prediction of protic ionic liquidproperties, the authors20,21 have generalized the Gurneyproton energy level diagram22 not just to deal with protontransfers between acid and base species in an aqueousenvironment but, rather, to describe the energy changeswhen a proton moves between any proton donor andproton acceptor pair in the absence of any solvent at all.So far, for lack of alternatives, levels in the diagram relativeto that of the H3O+/H2O level have been based on the

known aqueous pKa values, but this is of course onlyapproximate because of the different dehydration energiesof acid and base species in passing from aqueous toanhydrous systems. The approximate values will in timebe replaced by energy levels based on NMR chemical shiftstudies23 and other direct protic ionic liquids measure-ments, for instance, vapor pressure changes,20 and par-ticularly on voltages obtained from appropriate electro-chemical measurements. The latter are just beginning.24,25

An example of the energy level diagram is provided inFigure 7.21 It is constructed, following Gurney, by groupingacid and conjugate base forms of each molecule or iontogether at a free energy level (in electronvolts) based onthe arbitrary assignment of 0 eV to the couple H3O+/H2O.The right-hand member of the couple is described aspossessing an unoccupied quantized proton energy level,while the left-hand member has this level “occupied”. Theproton in the occupied level of any couple can “fall into”the vacant level of any couple lower on the diagram. Whenthe occupied level of a couple lies on a neutral molecule,the fall of its proton to the unoccupied level of a coupleat a lower level leaves behind a negatively charged speciesand creates a new positive species from the neutralmolecule into whose unoccupied proton level it fell, andan ionic liquid is created if the melting point of the

FIGURE 7. The proton energy level diagram for predicting theionicities of ionic liquids resulting from the falling of a proton froman occupied level (on an acid molecule) to a vacant level (on abasic molecule or on a divalent anion, in the latter case thecountercation(s) must remain to balance the new anion charge).Reprinted with permission from ref 21. Copyright 2007 AmericanChemical Society.


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product is below 100 °C, as is frequently the case. Therelation between the level values is based on E ) –!G/F) 2.303RT!pKa/F.

We can now group the products of such protontransfers together into three protic electrolyte categories,acid, neutral, and basic. Acid protic electrolytes resultwhen the proton falls from an occupied level on asuperacid to a vacant level on a weak base, for examplefrom triflic acid to fluoroaniline or to water, as in commonaqueous acids. Neutral electrolytes arise when the protonfalls from a moderately strong acid like nitric or meth-anesulfonic acid to a strong base like ethylamine orcyclopentylamine. Provided the proton falls across a gapof 0.7 eV or more, the electrolyte is a “good” ionic liquid,meaning the proton resides on the base for at least 99%of the time. Finally, basic electrolytes, for example, theproduct of proton transfer from a weak acid such as aceticacid to a base such as ethylamine, will give a poor ionicliquid. To obtain a truly basic ionic liquid, the cation ofan initial proton transfer process yielding a product likeEAN must be combined with the product of second protontransfer process that yields a basic anion like OH- or NH2

-

by some metathetical reaction, for example,[CH3CH2NH3

+][Cl-] + NaOH f [CH3CH2NH3+][OH-] +

NaCl. Such compounds tend to be high melting and havebeen little explored to date. The properties of low meltingliquids produced by reducing the charge density of theanion by complexation, for example, with a weak Lewisacid, such as Al(OH)3, have yet to be explored.

The ionic conductances of some protic ionic liquidsare shown in Figure 8 along with those of aqueous LiClto show that for some salts like dimethylammoniumnitrate17 the conductivities at ambient temperature canbe as high as those found in aqueous solutions. The

conductivity of an aprotic ionic liquid with low glasstemperature, [N1-O-2,211][BF4], is included for comparison.

Inorganic Protic Ionic LiquidsThe smallest organic cation that can be obtained in anionic liquid of the organic cation type is the methylam-monium cation, CH3NH3

+. It is of interest to knowwhether, by passing to protic salts of the inorganic type,in which the cations are, for instance, ammonium orhydrazinium, even higher conductivities can be obtained.Hydrazinium nitrate and hydrazinium formate (havingmelting points below 100 °C)16 were earlier cited asexamples of inorganic ionic liquids, and the conductivityof hydrazinium formate was found to be extremely high,18

though the finding remained unpublished until veryrecently. The conductivity of ammonium bifluoride, al-though not strictly an IL (having a melting point of 125°C), is even higher.17 Now we can add the conductivitiesof a number of ammonium salt mixtures, whose eutectictemperatures fall well in the IL range, to the collection ofinorganic ionic liquids. The conductivity data for thesesystems18 are shown in Figure 9.

In Figure 9, the conductivities, while indeed very high,do not reach the values of the best organic cation ILsexcept in the case of the hydrazinium formate. The caseof ammonium bifluoride is exceptional, and it is possiblethat this unusual liquid, extensively used as a cleaning anda fluorinating agent in inorganic fluoride chemistry26 maybe a “dry” proton conductor.

Protic Ionic Liquids As Fuel Cell ElectrolytesIt is astonishing that protic ionic liquids had been knownfor nearly a century before it was realized27,28 that theycould serve as the proton carriers in fuel cell electrolytes.In fact, they can serve very well in this role and can

FIGURE 8. Conductivities of various protic ionic liquids (and oneaprotic case, [N1-O-2,211][BF4]) compared with those of aqueous LiClsolutions to show how the aqueous solutions are no longer uniquewith respect to high ionic conductivity. II is dimethylammoniumnitrate; V is methoxypropylammonium nitrate. Reprinted from ref 17with permission of AAAS.

FIGURE 9. Arrhenius plot of the ionic conductivities of a collectionof inorganic protic salts and salt mixtures (hydrazinium formate,ammonium bifluoride, and various ammonium salt mixtures). Com-parison is made with ethylammonium nitrate and two phosphoricacids of different water content. Except for hydrazinium salts, theinorganic PILs are less highly conducting. High conductivity in solidstate is shown for two cases.


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provide electrolytes of a type that are simply not availablein systems in which water acts as acid or base in theproton transfer process.

It is even more astonishing that until 2006 it had notbeen realized that mixtures of inorganic salts (namely, theammonium salts) can be used as protic electrolytes forfuel cells running above 80 °C (hydrazinium salts couldbe used to much lower temperatures). Applied as fuel cellelectrolytes, the ammonium salts give more stable per-formance than electrolytes containing organic cations. TheIR-corrected polarization curves for some inorganic elec-trolyte fuel cells are shown in Figure 10, using thelogarithmic current (“Tafel plot”) form. The plateau cur-rent at the theoretical voltage can now be extended outto 50 mA·cm-2 in some inorganic systems to be describedin future articles, beyond the previously recognized limits.The increase in efficiency relative to the phosphoric acidcell is about 20%, and the implied reduction in generatedheat may be very important in some applications, whichdo not require high power.

While the ionic liquids offer the possibility of higherefficiency cells, the phosphoric acid (PA) cell has asignificant advantage in kinetics. The kinetic advantagelies in the higher conductivity of the phosphoric acid,which is a molecular liquid that exhibits not only consid-erable autoionization but also “dry” proton mobility. Thissuperprotonic component, which is illustrated in Figure11, is unfortunately not present in the protic ionic liquids,which mostly lie on or below the “ideal” Walden line.8 Itmay be necessary to compromise on power output for thesake of energy efficiency, though the NH4HF2 data inFigure 9 suggest that breakthroughs may eventuate.

In an initial report on the relation of fuel cell voltageoutput to other properties of the PILs,18 we noted arelation between the output voltage and the difference inpKa values for the acid and base components of theelectrolyte. The voltage increased strongly with increasing!pKa and reached the theoretical value when the !pKa

value for the electrolyte reached about 14. There is now

an urgent need to develop direct measures of the freeenergy of proton transfer for the anhydrous case.

In Situ Measures of Proton TransferEnergetics and Relative Proton Activities inIonic LiquidsTwo steps have now been taken toward direct protontransfer energy characterization. The first23 has been tostudy the NMR proton chemical shift for the proton thathas been transferred to the base nitrogen but is still beingcounterattracted by the conjugate base of the acid fromwhich it came. The stronger the “pull” from the conjugatebase (which means the smaller the free energy of transfer),the more deshielded will be the N–H proton. The moredeshielded the proton, the further downfield its resonancefrequency will lie relative to a standard reference, forexample, C(CH3)4. Thus the N–H chemical shift ("N–H)can serve as an in situ probe of the proton transfer energy.We find immediately that it does not enjoy a linearrelation with the value of !pKa but rather gives a relation-ship similar to that between the fuel cell open circuitvoltage (OCV) and the !pKa value.18 The second step hasbeen to develop a method of measuring directly thevoltage necessary to displace a proton from its normal siteon a nitrogen onto the anion from which it originallycame. This will be described elsewhere.29 These directmeasures have already found application in anotherrecently developing application of ionic liquids, namely,the stabilization of biomolecules discussed in the nextsection.

Proton Activity and Protein Stabilization inPILsAn exciting new role for ionic liquids may lie in the storageand manipulation of sensitive biomolecules. Fujita et al.30

have recently reported that the protein cytochrome c isremarkably stablilized in solution in a nontoxic dihydro-gen phosphate IL, and we have found31 that lysozyme in

FIGURE 10. IR-corrected polarization curves for hydrogen/oxygenfuel cells utilizing inorganic and organic cation ionic liquid electro-lytes. Reproduced from ref 18 by permission.

FIGURE 11. Walden plot showing the behavior of equivalentconductivity expected from precise correlation with liquid viscositysolid line, with examples of subionic behavior in some protic saltcases and superprotonic behavior in some acid + water systems.Reprinted with permission from ref 21. Copyright 2007 AmericanChemical Society.


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200 mg/mL concentration is still folded after 3 years inambient solutions with large ethylammonium nitrateconcentrations. The source and full extent of this stabilityis not yet properly understood.

It is well-known from standard biological concentrationsolution studies that each protein has pH range in whichits native state is most preferred and that deviations fromthat pH range can affect its folding energy and its stabilityagainst such undesirable behavior as aggregation, andrecently, fibrillization. It might be expected that the protonactivity of the solvent will exert a strong effect on thestability of proteins in the ionic liquid protective solution,and this is verified by our findings,32 which are shown inFigure 12. In Figure 12, the proton activity is representedby the "(N–H) while, for stability on the vertical axis, weuse a refoldability index pending the necessary long-termassessments.

The refoldability index is the percentage of the initialenthalpy of unfolding obtained after lowering the tem-perature to allow thermally denatured molecules to refoldand then remeasuring the unfolding enthalpy. The dif-ference between the two is a measure of the fraction ofthe protein lost to the aggregation process and is foundto be 3%. The stability index is then given by the ratio(second unfolding enthalpy)/(first unfolding enthalpy) andis 0.97, or 97%, under favorable circumstances. The “%refoldability” for hen egg lysozyme is shown as a functionof the N–H chemical shift ("(N–H)) determined for thesolution in which the lysozyme was dissolved, in Figure12. The presence of a "(N–H) range equivalent to about 2pH units in which the same high refoldability index isobtained is quite striking. The refoldability index variesas a function of water content at constant ionic liquidcontent. But when the "(N–H) of the solution is measured,it is found that the "(N–H) has also changed with watercontent, and it is evidently only the proton activity that isimportant. The solution in which the 3 years stability wasrecorded31 happened to have been formulated near the

center of the maximum stability zone of Figure 12 forlysozyme.

Different proteins have different favorable protonactivity stability ranges. We hope to establish theseoptimum conditions for many normally fragile biomol-ecules so that they can be preserved indefinitely atambient temperatures, thereby opening the door to un-refrigerated shipping and storage of drugs and otherbiologically important materials.

C.A.A. expresses deep appreciation for the support over the yearsof his efforts in ionic liquid studies (formerly “low-melting salts”)and in liquid and glassy state studies in general by the NSF-DMRSolid State Chemistry program, as well as by other agencies, mostrecently the DOD Army Research Office.

References(1) Angell, C. A. Electrical Conductance of Concentrated Aqueous

Solutions and Molten Salts: Correlation through Free VolumeTransport Model. J. Phys. Chem. 1965, 69, 2137.

(2) Angell, C. A.; Gruen, D. M. Octahedral-tetrahedral coordinationequilibriums of nickel(II) and copper(II) in concentrated aqueouselectrolyte solutions. J. Am. Chem. Soc. 1966, 88, 5192–5198.

(3) Sare, E. J.; Moynihan, C. T.; Angell, C. A. Proton MagneticResonance Chemical Shifts and the Hydrogen Bond in Concen-trated Aqueous Electrolyte Solutions. J. Phys. Chem. 1973, 77,1869–1876.

(4) Gruen, D. M.; Mcbeth, R. L. Tetrahedral NiCl4– Ion in Crystals andin Fused Salts - Spectrophotometric Study of Chloro Complexesof Nickel(II) in Fused Salts. J. Phys. Chem. 1959, 63, 393–395.

(5) Angell, C. A.; Abkemeier, M. L. Diamond Cell Study of Pressure-Induced Coordina-tion Changes for Ni(II) in Liquid Chloride Sol-vents. Inorg. Chem. 1973, 12, 1462–1464.

(6) Angell, C. A.; Hodge, I. M.; Cheeseman, P. A. In Internationalconference on molten salts; Pemsler, J. P.,Ed. ElectrochemicalSociety: 1976; pp 138.

(7) Carlin, R. T.; Wilkes, J. S. Complexation of metallocene dichloride(Cp2MCl2) in a chloroaluminate molten salt: relevance to homo-geneous Ziegler-Natta catalysis. J. Mol. Catal. 1990, 63, 125–129.

(8) Xu, W.; Cooper, E. I.; Angell, C. A. Ionic liquids: Ion mobilities, glasstemperatures, and fragilities. J. Phys. Chem. B 2003, 107, 6170–6178.

(9) Cooper, E. I.; Angell, C. A. Versatile organic iodide melts andglasses with high mole fractions of lithium iodide: glass transitiontemperatures and electrical conductivities. Solid State Ionics 1983,9–10, 617–622.

FIGURE 12. Refolding fraction (defined by the ratio of area under the DSC unfolding endotherms of second to first upscans after change ofproton activity represented by the proton chemical shift "(N–H) referenced to external tetramethyl silane. Insert shows the unfolding endothermsin the most stable domain compared with those on either side ("(N–H) values of 7.42 and 11.45). 1 ) EAHSO4, 2 ) TEATf; 3 ) EAN; 4 )HOEAN, 5 ) EAFm, and 6 ) DBAAc. Open circles are 1:1 binary mixtures of EAN–TEATf, EAN–TEATfAc, and EAN–EAFm.


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(10) Angell, C. A.,In Molten Salts:From Fundamentals to Application:NATO-ASI; Gaune-Escarde, M.,Ed. Kluwer Academic Publishers:Delft, Kas, Turkey, 2002, pp 305–322.

(11) Cooper, E. I., O’Sullivan, E. J., In Proceedings of the 8th Interna-tional Symposium on Molten Salts; Electrochemical Society: 1992,Vol 92-16, pp 386.

(12) Wilkes, J. S.; Zaworotko, M. J. Air and Water Stable 1-Ethyl-3-Methylimidazolium Based Ionic Liquids. J. Chem. Soc., Chem.Commun. 1992, 965–967.

(13) Angell, C. A.; Xu, W.; Yoshizawa, M.; Hayashi, A.; Belieres, J.-P.;Lucas, P.; Videa, M. In Electrochemical Aspects of Ionic Liquids;Ohno, H., Ed. Wiley Interscience: 2005; pp 5–23.

(14) Kittel, C., Solid State Physics, 3rd ed.; Wiley, New York, 1967;Chapter 3.

(15) Angell, C. A.; Xu, W.; Yoshizawa, M.; Hayashi, A.; Belieres, J.-P. InIonic Liquids; Ohno, M., Ed.; 2003.

(16) Sutter, E. J.; Angell, C. A. Glass transitions in molecular liquids. I.Influence of proton transfer processes in Hydrazine-bassed solu-tions. J. Phys. Chem. 1971, 75, 1826–1831.

(17) Xu, W.; Angell, C. A. Solvent-free electrolytes with aqueoussolution-like conductivities. Science 2003, 302, 422–425.

(18) Belieres, J.-P.; Gervasio, D.; Angell, C. A. Binary inorganic ionicsalt mixtures as high conductivity electrolytes for >100 °C fuel cells.Chem. Commun. 2006, 46, 4799–xxxx.

(19) Yoshizawa, M.; Ogihara, W.; Ohno, H. Design of new ionic liquidsby neutralization of imidazole derivatives with imide-type acids.Electrochem. Solid-State Lett. 2001, 4, E25–E27.

(20) Yoshizawa, M.; Xu, W.; Angell, C. A. Ionic Liquids by ProtonTransfer: Vapor Pressure,Conductivity, and the Relevance of !pKa

from Aqueous Solutions. J. Am. Chem. Soc. 2003, 125, 15411–15419.

(21) Belieres, J.-P.; Angell, C. A. Protic ionic liquids: preparation,characterization, and proton free energy level representation. J.Phys. Chem. B 2007, 111, 4926–4937.

(22) Gurney, R. W. Ionic Processes in Solution; Dovers Publications:New York, 1962.

(23) Byrne, N.; Belieres, J.-P.; Angell, C. A., Manuscript in preparation,2007.

(24) Angell, C. A.; Belieres, J.-P.; Byrne, N., U.S. Patent Application 60/917,899, 2007.

(25) Kanzaki, R.; Uchida, K.; Hara, S.; Umebayashi, Y. Acid -baseproperty of ethylammonium nitrate ionic liquid directly obtainedusing ion-selective field effect transistor electrode. Chem. Lett.2007, 36, 684–685.

(26) Leroy, D.; Lucas, J.; Poulain, M.; Ravaine, D. Study of the ionicconduction of zirconium fluoride-based glasses. Mater. Res. Bull.1978, 13, 1125–1133.

(27) Susan, M. A. B. H.; Noda, A.; Mitsushima, S.; Watanabe, M.Bronsted acid-base ionic liquids and their use as new materialsfor anhydrous proton conductors. Chem. Commun. 2003, 938–939.

(28) Angell, C. A.; Xu, W.; Belieres, J.-P.; Yoshizawa, M., InternationalPatent Application PCT/US2004/013719.

(29) Tang, L.; Belieres, J.-P.; Byrne, N.; Friesen, C.; Angell, C. A.Manuscript in preparation, 2007.

(30) Fujita, K.; MacFarlane, D. R.; Forsyth, M. Protein solubilising andstabilising ionic liquids. Chem. Commun. 2005, 4804–4806.

(31) Byrne, N.; Belieres, J.-P.; Angell, C. A. Reversible folding-unfolding,aggregation protection, and multi-year stabilization, in high con-centration protein solutions, using ionic liquids. Chem. Commun.2007, 2714–2716.

(32) Byrne, N.; Belieres, J.-P.; Angell, C. A., Directed destabilization oflysozyme in protic ionic liquids reveals a new low energy reversiblyunfolding “pre-fibril” state. Nature, submitted for publication, 2007.

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