physicochemical properties of imidazo-pyridine protic ionic liquids

www.rsc.org/MaterialsARegistered Charity Number 207890

Dr. Ghandi’s lab group at Mount Allison University has

been collaborating with several industries, including

KnowCharge in New Brunswick, Canada, to use novel

ionic liquids for several long term applications.

Title: Physicochemical properties of imidazo-pyridine protic ionic

liquids

This work introduces a new class of protic ionic liquids with

potential applications in fuel cells and a wide range of batteries,

in particular when high thermal stability is needed.

As featured in:

See S. Nazari et al.,

J. Mater. Chem. A, 2013, 1, 11570.

Journal ofMaterials Chemistry A

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aDepartment of Chemistry and Biochemistry

Brunswick, E4L 1G8, Canada. E-mail: kghanbDepartment of Chemistry, Dalhousie Univ

CanadacInstitute for Research in Materials, Dalhou

4R2, Canada

† Electronic supplementary informationand crystallographic data in CIF or10.1039/c3ta12022h

Cite this: J. Mater. Chem. A, 2013, 1,11570

Received 23rd May 2013Accepted 5th July 2013

DOI: 10.1039/c3ta12022h

www.rsc.org/MaterialsA

11570 | J. Mater. Chem. A, 2013, 1, 1

Physicochemical properties of imidazo-pyridine proticionic liquids†

Shidokht Nazari,a Stanley Cameron,b Michel B. Johnsonbc and Khashayar Ghandi*a

A new class of protic ionic liquids (PILs) were prepared by reacting imidazo-[1,2a]-pyridine (ImPr) with

benzene-1,2-dithiol (BDT), oxalic acid (Ox), phthalic acid (Phth), pimelic acid (Pim), and sulfuric acid.

[ImPr][HSO4] was determined to be the most thermally stable PIL with a decomposition temperature

of 326 �C and could potentially be used as an electrolyte in fuel cells and lithium ion batteries.

X-ray crystallography on oxidized [ImPr][BDT] indicated the formation of the first reported disulfide PIL.

[ImPr][Pim] and [ImPr][Phth] showed fragile behaviour. A Walden plot indicated ionic behaviour close to

ideal for [ImPr][Phth].

1 Introduction

In the past several years, ionic liquids have received consider-able attention as a greener alternative to conventional solvents.They provide a new approach to sustainable chemistry due totheir stability, non-ammability, and negligible vapour pres-sure.1 These unique properties, their catalytic behaviour,2 andtheir tunability3 make them great solvents or catalysts in manyorganic syntheses.

Ionic liquids can be divided in two broad categories: aproticionic liquids (AILs) and protic ionic liquids (PILs). AILs can besynthesized by transferring any group other than a proton to abasic site on the basic parent molecule. PILs were formed byproton transfer from a Brønsted acid to a Brønsted base. AILs'different properties and applications have been studied andreviewed more widely4–6 compared to PILs.7,8 One of the mostapplicable features of PILs is their high proton conductivity,even in anhydrous conditions at elevated temperatures; thismakes them great candidates as proton-conducting electrolytesin fuel cell applications.9–11

However, this type of PIL application largely depends on thedegree of ionization, which may be limited by incompleteproton transfer, aggregation, or the formation of ioncomplexes.12 The ionic conductivity is also dependent on ionmobility and the number of charge carriers, which depends ondensity, molecular weight, and the size of the ions.13–17

, Mount Allison University, Sackville, New

[email protected]; Tel: +1 506 961 0802

ersity, Halifax, Nova Scotia, B3H 4R2,

sie University, Halifax, Nova Scotia, B3H

(ESI) available. CCDC 949411. For ESIother electronic format see DOI:

1570–11579

In this work, new PILs with imidazolium-[1,2a]-pyridinecations with anions bearing thiolate and carboxylate moietieswere synthesized using a “green chemistry” method. They werethen characterized, and some of their physicochemical prop-erties were investigated.

There are a number of different techniques to determine theionicity of protic ionic liquids, including NMR,18–20 changes inthermal properties as a function of stoichiometry,19 IR spec-troscopy,18,20 and Walden plots.21 Among these techniques, theWalden plot, based on the classic Walden rule, is a convenientmethod to quantitatively assess the ionicity of PILs. TheWaldenrule relates22 the ionic mobility represented by equivalentconductivity (L0

m) to the uidity (h�1) of the medium. In theideal case, when the ion–ion interaction is negligible, the slopeof the plot should be one.24 Highly dilute aqueous KCl solutionis used to establish this ideal case.

PILs have applications in both biological systems7 andchromatography7,23 and they can be used as both media andcatalysts in organic reactions.24 PILs can also act as hyperpolarmedia because of their high dielectric conductivities caused byextended hydrogen bonding.25 The polarity of ILs decrease withincreasing distance between ions due to a decrease in theeffective charge density between them. The-best known exampleof this group of PILs is ethylammonium nitrate (EAN), whichhas many similarities to water, including high polarity.26–28

The thermal stability and physicochemical properties of PILs,including their ionic conductivity and polarity, rmly depend onthe nature of the anion and cation as well as the length of the alkylchain, alkyl branching, and the number of hydroxyl groupsinvolved in their structure.7,15 The correlation between the struc-ture and properties of PILs and the tunable behaviour of some ofthe ammonium,13,14,20,29 imidazolium,14,30–32 and heterocyclicamines has previously been reported.7

This will be the rst time that phthalic acid and pimelic acidhave been used as the source for anions in protic ionic liquids.

This journal is ª The Royal Society of Chemistry 2013

http://dx.doi.org/10.1039/c3ta12022h

http://pubs.rsc.org/en/journals/journal/TA

http://pubs.rsc.org/en/journals/journal/TA?issueid=TA001038

Paper Journal of Materials Chemistry A

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A thiolate/disulde-based ionic liquid was rst reported byZhou et al.,29 and has been suggested by a variety of researchersas an electrolyte for organic dye-sensitized solar cells.29,30 To ourknowledge, PILs with thiolate/disulde anions have not previ-ously been synthesized. Carboxylate anions such as formate,acetate, propionate and oxalate have been reported in thesynthesis of imidazolium-based and ammonium-basedPILs.14,25–29 The extensive hydrogen bonding in these systemsmake them highly polar.

2 Experimental2.1 Chemicals

Imidazo-[1,2a]-pyridine (98%), pimelic acid (98%), oxalic acid(99%) and phthalic acid (99%) were purchased from SigmaAldrich and used as received. Benzene-1,2-dithiol (97%) waspurchased from Alpha Aesar and used as received. All proticionic liquids were synthesized with solvent-free methods.

2.2 Synthesis and characterization

Imidazolium-[1,2a]-pyridine benzene 1-thiol, 2-thiolate[ImPr][BDT] and imidazolium-[1,2a]-pyridine benzene-1thiol, 10

thiolate,2,20 disulde [ImPr][BDS]. 0.071 g (0.5 mmol) benzene-1,2-dithiol was added to 0.05 mL (0.5 mol) imidazo [1,2a]-pyri-dine in a glovebox with a nitrogen atmosphere ([O2] < 1 ppm).The reaction vessel was sealed and transferred to an ultrasonicbath at 5–8 �C (Brasonic-50/60 Hz). The reaction mixture wassonicated for 5 minutes and a pale yellow precipitate formed.The reaction vessel was then returned to the glovebox, openedto the atmosphere, and le overnight to ensure reactioncompletion. The resulting white powder was obtained inquantitative yields and found to have a melting point of 65–68�C (the melting point of benzene-1,2-dithiol is 22–24 �C; imi-dazo-[1,2a]-pyridine has a melting point of �75 � 1 �C). Thefollowing characteristics were determined for the product: MS-ESI: m/z 119.01 (positive ion), 140.7 (negative ion); 1H NMR(CDCl3): d ¼ 8.08 (d, 1H; aromatic H), 7.54 (m, 3H; aromatic H),7.27 (m, 1H; aromatic H), 7.07 (m, 3H; aromatic H), 6.69 (m, 1H;aromatic H), 3.07 (S, 2H, thiol/NH). The product was recrystal-lized from toluene by slow evaporation in the glove box. X-raycrystallography shows formation of oxidized product ([Impr]-[BDS]) due to molecular oxygen dissolved in the toluene. [Impr]-[BDS] is stable when it is exposed to the air for several months.

Imidazolium-[1,2a]-pyridine hydrogen sulfate [ImPr][HSO4].0.05 mL H2SO4 (95%) was added to 0.1 mL imidazo-[1,2a]-pyridine dropwise to a pre-cooled (5–8 �C) vessel in an ultra-sonic bath (Brasonic-50/60 Hz). The reaction mixture wasremoved from the bath and stirred for two minutes and thenagain placed in the ultrasonic bath for 15 minutes. A purple/brown solid sample formed and was washed with diethyl etherto remove any remaining reactants. The resulting solid wasevacuated and heated in an oil bath set to 40 �C overnight. Themeasured melting point was 75–80 �C (the melting point ofH2SO4 is 10 �C; imidazo-[1,2a]-pyridine has a melting point of�75 � 1 �C). The yield is $90%. The following characteristicswere determined for the product: MS-ESI: m/z 119.01 (positive


ion), 194.4 (negative ion); 1H NMR (DMSO-d6): d¼ 5.01 (s broad;NH), 8.95 (d, 1H; aromatic H), 8.40 (d, 1H; aromatic H), 8.21 (d,1H; aromatic H), 7.97 (m, 2H; aromatic H), 7.52 (d, 1H; aromaticH); 13C NMR (D2O): d ¼ 139.54, 133.70, 128.69, 121.95, 117.16,115.06, 111.84 ppm.

Imidazolium-[1,2a]-pyridine hydrogen pimelate [ImPr][Pim].0.32 g pimelic acid was added to 0.2 mL imidazo-[1,2a]-pyridineand heated to 80 �C for one hour under constant stirring. Agreen viscous liquid formed and was washed by separatoryfunnel with diethyl ether (2�) to remove any remaining reac-tants. The yield is $90%. The product is liquid and does notfreeze. The m.p. of pimelic acid is 103–105 �C; imidazo-[1,2a]-pyridine has a melting point of �75 � 1 �C. The followingcharacteristics were determined for the product: MS-ESI: m/z119.01 (positive ion), 159.01 (negative ion); 1H NMR (DMSO-d6):d ¼ 8.59 (d, 1H; aromatic H), 7.98 (s, 1H; aromatic H), 7.62 (m,1H; aromatic H), 7.25 (m, 2H; aromatic H), 6.94 (m, 1H;aromatic H), 2.23 (m, 2H; alkyl chain), 1.52 (m, 2H, alkyl chain),1.47 (m, 1H, alkyl chain); 13C NMR (D2O): d ¼ 181.07, 139.37,133.24, 128.393, 122.341, 116.93, 114.851, 111.76, 35.567, 28.11,24.75 ppm.

Imidazolium-[1,2a]-pyridine hydrogen phthalate [ImPr]-[Phth]. 0.32 g phthalic acid was added to 0.2 mL imidazo-[1,2a]-pyridine and heated to 70 �C for one hour under constant stir-ring. A brownish viscous liquid was obtained was and waswashed with diethyl ether. It turned into white/yellow solid asthe non-ionic compounds were washed away. The measuredmelting point was 96–100 �C (phthalic acid decomposes at 210–211 �C; imidazo-[1,2a]-pyridine has a melting point of �75 � 1�C). The yield is $90%. The following characteristics weredetermined for the product: MS-ESI: m/z 119.01 (positive ion),165.01 (negative ion); 1H NMR (DMSO-d6): d ¼ 8.67 (d, 1H;aromatic H), 8.07 (s, 1H; aromatic H), 7.82 (m, 4H; aromatic H),7.70 (d, 2H; aromatic H), 7.58 (m, 4H; aromatic H), 7.44 (m, 1H;aromatic H), 7.07 (m, 1H, aromatic H); 13C NMR (D2O): d ¼172.81, 138.90, 133.34, 132.96, 130.72, 129.30, 128.26, 121.79,116.97, 114.74, 111.50 ppm.

Imidazolium-[1,2a]-pyridine hydrogen oxalate [ImPr][Ox].0.18 g oxalic acid was added to 0.2 mL imidazo-[1,2a]-pyridineand heated to 80 �C for one hour with constant stirring. A white/yellow solid was obtained and washed with diethyl ether (2�) toremove any unreacted starting materials remaining. Themeasured melting point was 96–100 �C (m.p. and decomposi-tion temperature of oxalic acid is reported to be in the range of127–150 �C; imidazo-[1,2a]-pyridine has a melting point of �75� 1 �C). The following characteristics were determined for theproduct: MS-ESI: m/z 119.01 (positive ion), 201.0 [2M + Na]+

(negative ion); 1H NMR (DMSO-d6): d¼ 8.69 (d, 1H; aromatic H),8.09 (s, 1H; aromatic H), 7.75 (m, 2H; aromatic H), 7.48 (m, 1H;aromatic H), 7.10 (m, 1H; aromatic H), 6.57 (s, 4H, OH); 13CNMR (D2O): d ¼ 168.65, 139.42, 133.62, 128.60, 122.04, 117.13,115.01, 111.76 ppm.

2.3 Crystal structure determination of [ImPr][BDS]

The product was re-crystallized in toluene by slow evaporationin the glovebox. A red crystal was obtained aer three weeks.

J. Mater. Chem. A, 2013, 1, 11570–11579 | 11571


Fig. 2 Ortep drawing of [ImPr][BDS] crystal. Blue, yellow, green and black atomsare nitrogen, sulfur, hydrogen and carbon respectively. Each benzene-1,2-thio-late/disulfide ion is surrounded by one imidazolium-[1,2a]-pyridine ion and twomolecular imidazo-[1,2a]-pyridine ions.

Fig. 1 Chemical structure of the reactants used to synthesize the ionic liquids inthis work: imidazo-[1,2a]-pyridine (ImPr) as an organic base, and oxalic acid (Ox),benzene-1,2-dithiol (BDT), sulfuric acid, phthalic acid (Phth) and pimelic acid (Pim)as organic acids in the synthesis of imidazolium pyridine-based protic ionic liquids.

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Crystal data for C33H28N6S4 at 296 K: triclinic, P�1 (2) a ¼10.3434(3) A, b ¼ 11.2882(4) A, c ¼ 13.4642(2) A, a ¼ 81.722(5)�,b ¼ 83.227(4)�, g ¼ 89.615 (5)�. Z ¼ 2, m (MoKa) ¼ 3.420 cm�1,30 194 reections were measured, and 9385 were unique (Rint ¼0.087). The transmission factor ranged from 0.632 to 0.963. Thestructures were solved by direct methods (SHLEX97), andexpanded using Fourier techniques (DIRDIF99). For the struc-tural models, the hydrogen atoms were rened subject tosimilarity restraints placed on chemically equivalent bonddistances and angles. The non-hydrogen atoms were renedanisotropically, while hydrogen atoms were rened using theriding model. Figures were drawn using Ortep.

2.4 Raman spectroscopy

Raman spectra were recorded using a Nicolet NXR 950 spec-trometer equipped with an NXR Genie germanium detector,and a high power output Nd:YVO4 laser operating at 1064 nm.The [ImPr][BDT] sample was sealed between two glass slides inthe glove box and exposed to the laser radiation at roomtemperature with 64 scans. The signal collection was taken at 90degrees. Raman frequencies were in the range of 100–3700cm�1, and the resolution was 8 cm�1. The Raman power forsample S1, S2 and ImPr were 1.5 W, 0.5 W and 1 W respectively.

2.5 Thermal analysis

The thermal stability and phase behaviour of the samples wereinvestigated by thermogravimetric analysis (TGA) and differ-ential scanning calorimetry (DSC) respectively.

Thermogravimetric analysis (TGA, SDT Q600 from TAInstruments) was performed from room temperature tomaximum 475 �C with a heating rate of 10 �C min�1 and anargon ow of 50 mLmin�1. Samples were added to a tared Al2O3

crucible, the mass recorded (16.6 to 21.6 mg), and the temper-ature ramped to 475 �C.

Differential scanning calorimetry (DSC, TA Q200 from TAInstruments) thermograms were recorded over differenttemperature ranges, depending on the ionic liquid, using aheating and cooling rate of 10 �C min�1 under helium ow at arate of 25 mL min�1. Indium and water were used as standardsto calibrate the DSC temperature. Indium was also used forenthalpy calibration. The samples were hermetically sealed inaluminum pans, heated below their decomposition tempera-tures, and then cooled to�150 �C for liquid samples and to�20�C for solid samples. Sample masses ranged from 4.9 to 8.6 mg.

2.6 Viscosity

A BrookField-LV viscometer was used to measure the dynamicviscosity at different temperatures for the [ImPr][Pim] sample.The [ImPr][Phth] sample is in a liquid meta-stable state aermelting (the DSC trace does not show any crystallization pointsin three cycles). This allowed us to measure its viscosity.Viscosity was measured based on the rotational friction asfollows: a spindle is inserted into the samples and rotates; thiscreates friction (a shear force) between the layers of the liquid,and the viscometer measures this shear force by calculating theamount of torque required to turn the spindle at a known speed.

11572 | J. Mater. Chem. A, 2013, 1, 11570–11579

Three replicates were used for measurements. The uncertaintyof our viscosity measurements is less than 10%.

2.7 Ionic conductivity

The ionic conductivity of [ImPr][Pim] and [ImPr][Phth] weremeasured at different temperatures using a SUNTEX Conduc-tivity Meter-SC170 with a cell constant of 1 cm�1. The conduc-tivity probe was calibrated with a 0.01 M KCl solution to a valueof 1413 mS cm�1 (at 25 �C).

3 Results and discussion3.1 Crystal structure determination of oxidized [ImPr][BDS]

As is illustrated in the Ortep drawing obtained from crystallog-raphy X-ray diffraction of the red crystal (Fig. 2), only one of thethiol groups is deprotonated and the proton is transferred to thenitrogen of imidazo-[1,2a]-pyridine (N4). The crystal structureindicates the formation of a disulde bond in the system; thislikely resulted from the oxidation of the thiol group by residualmolecular oxygen in the toluene used in the recrystallization.



Scheme 1 Formation of a disulfide bond from a thiolate group via a radicalmechanism with molecular oxygen.


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The Ortep structure of the molecule shows that each anion issurrounded by an imidazolium[1,2a]-pyridine cation and twoimidazo[1,2a]-pyridines. This is close to the usual observationfor imidazolium-based PILs, with each anion hydrogen-bondedto three surrounding imidazolium cations.33 The N(4)-H unitforms a strong hydrogen bond to N(2), with a distance of 1.8 A.The S–H close the edge of the unit cell could also form a weakhydrogen bond to S–H in another unit cell (Fig. 3).

The unit cell for C33H28N6S4 is shown in Fig. S12 and 13 inthe ESI.† In some cases of imidazolium-based ionic liquids, p–p stacking interactions were observed and reported between theimidazolium rings. From the three-dimensional arrangement ofthe crystal, initially it seems there are parallel displaced p–p

stacking interactions between benzene-1,2-dithiol aromaticrings or the same p–p stacking interactions between the imi-dazolium rings, however, the distance between the aromaticrings in both cases is too large to form p–p stacking interac-tions (>3.3–3.8 A).34,35

Since formation of the disulde bond caused two aromaticrings to form an angle of 89.11� (C(6)–S(1)–S(2)–C(7)), the stericeffect hinders the tight packing of the aromatic rings andtherefore prevents the p–p stacking interactions.

The three dimensional arrangement of the crystal in Fig. 4shows C–H/p interactions between the CH of the imidazoliumring and the aromatic ring of the anion.33

A proposed mechanism for the oxidation of benzene-1,2-ditholate to disulde is illustrated in Scheme 1.

The molecular oxygen takes one electron from the thiolategroup to form an oxygen radical anion and radical thiol group.

Fig. 3 Simplified view of the [ImPr][BDS] crystal.

Fig. 4 Crystal structure of [ImPr][BDS] showing interactions between the CH ofimidazolium rings and the aromatic ring of benzene-1,2-thiolate/disulfide. Thedistances are in angstroms.


Consequently two radical thiol groups can form a dimer tocreate the disulde bond. The two oxygen radical anions willeventually produce hydrogen peroxide and molecular oxygen inthe solvent.

3.2 Raman spectroscopy

Since Raman spectroscopy can be carried out with minimalsample preparation, it is preferred over IR spectroscopy foroxygen-sensitive solids such as [ImPr][BDT] where preparationinvolves exposure to air.

The Raman spectra in Fig. 5 are from [ImPr][BDT] beforeformation of the disulde bond. One was recorded once thefresh sample was prepared (S1) and the other one when thesample had been le in the glove-box overnight (S2). Thespectrum was compared with the Raman spectrum of imidazo-[1,2a]-pyridine. For both S1 and S2 samples, there is a peak at3040 cm�1 corresponding to the NH+ peak which shows protontransfer from benzene-1,2-dithiol to imidazo-[1,2a]-pyridine.18

The SH peak at 2420 cm�1 was observed for S2, and the lack ofthis peak for S1 suggests complete deprotonation of benzene-1,2-dithiol.36,37

Considering that the [ImPr][BDT] is not stable this PIL isdifferent with the other PILs in that unlike the other ones it isnot of practical importance.

Fig. 5 Raman spectra of [ImPr][BDT] (a) S1(yellow line) corresponds to thesample which was freshly made (b) the S2 (blue line) corresponded to the samplethat was kept overnight in glovebox after synthesis. The results are compared toimidazo-[1,2a]-pyridine as starting material.

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Table 1 pKa1 and pKa2 of Brønsted acids and Brønsted bases in this study

BDT H2SO4 Ox Pim Phth ImPr

pKa1 6 (ref. 38) �3 1.29 4.50 2.98 6.79 (ref. 39)pKa2 9.4 1.99 4.27 5.43 5.28 —

Fig. 6 Thermogravimetry analysis (TGA) thermograms of synthesized PILs in therange of 20–475 �C with a heating rate of 20 �C min�1.


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3.3 Proton transfer

The pKa1 and pKa2 of the selected Brønsted acids as well as pKa

of imidazo-[1,2a]-pyridine are shown in Table 1. It is unlikelythat the second protonation happens as the pKa2 of theBrønsted acids are relatively large.

The exception is sulfuric acid which has a signicant acidicpKa2. The IR spectrum of the sample shows a peak at 1053 cm�1,which is assigned to the symmetric stretching band of HSO4

�.40

The symmetric stretching band of SO42� is reported at 982

cm�1,40 but it was not observed in this work, suggesting that thedeprotonation of HSO4

� does not occur in this system.The pyridine nitrogen on imidazo-[1,2a]-pyridine will lose

basicity aer one proton transfer to the imidazo nitrogen due toits resonance structure, as shown in Scheme 2.39 Therefore it isquite unlikely that two protons would transfer to one moleculeof imidazo-[1,2a]-pyridine.

As a 1 : 1 acid to base mole ratio was used to synthesize thisPIL, a second proton transfer is quite unlikely as the hydrogensulfate anion formed in the [HSO4][ImPr] ion pair is less acidic(pKa1 ¼ 1.99) than the H2SO4 (pKa2 ¼ �3). So the ImPr basemolecule prefers to accept a proton from sulfuric acid than fromhydrogen sulfate.

3.4 Thermal stability

The thermal stability of ionic compounds (including ionic liquids)can vary with different cation and anion combinations.39–49 Themost thermally stable PILs reported are those with bis-((tri-uoromethyl)sulfonyl)-imide (TFSI) anions and a variety of cationssuch as alkyl ammonium, imidazolium and some heterocycliccations due to its bond strength resulting from resonance stabi-lization.7,38 Carboxylate anions, particularly formate with an alkylammonium cation, usually have lower thermal stability resultingfrom their thermal decomposition to CO2.7,39

All synthesized PILs in this study have same imidazoliumpyridine cation, but the thermal stability will be different

Scheme 2 Resonance structure of the imidazo-[1,2a]-pyridine after acceptingone proton from Brønsted acid.

11574 | J. Mater. Chem. A, 2013, 1, 11570–11579

depending on the nature of the anion. The thermogravimetricanalysis of the synthesized PILs is shown in Fig. 6. All vesamples are heated up to 475 �C. Then the decrease in masspercentage is monitored.

The decomposition temperatures of [ImPr][Pim] and [ImPr]-[Phth] are both about 175 �C, where decomposition temperatureis dened as more than 10% weight loss in the TGA curve. Theone-step weight loss process observed in the TGA thermogramsof these two PILs conrms the formation of protic neutral salts,since excess acid or base starting material would show a two-step process with the initial step corresponding to the excess ofbase or acid in the system (although this could also be due toloss of moisture) and the second step corresponding to thePIL.14 A two-step TGA process was observed for [ImPr][Ox] and[ImPr][HSO4] where the initial mass loss is frommoisture in thesample. This initial loss is corroborated with DSC, which showsan endotherm at 100 �C in the rst cycle and is absent in thesubsequent cycles.

The decomposition temperature of [ImPr][Ox] was deter-mined to be 165 �C and that of [ImPr][HSO4] was 336 �C, whichmakes [ImPr][HSO4] the most thermally stable PIL synthesizedin this study. The decomposition temperature of [Impr][HSO4]is slightly higher than 1-methyl-imidazolium sulfate (Td ¼ 320�C)40 and much higher than 1-ethyl-imidazolium sulfate (Td ¼200 �C).40

The [ImPr][BDT]/[ImPr][BDS] sample shows a two-stepdecomposition process: the rst one is due to the decomposi-tion process of [ImPr][BDT] and the second process is attributedto the decomposition of [ImPr][BDS]. The decompositiontemperatures of [ImPr][BDT] and [ImPr][BDS] are 126 �C and260 �C, respectively. This was established based on an experi-ment on a fully oxidized ionic liquid ([ImPr][BDS]).

Alkyl ammonium-based PILs with carboxylate anionsincluding formate, acetate, and malonate have been reported tohave decomposition temperatures in the range of 38–120 �Cbased on different cation and anion combinations.7 Thedecomposition temperature of the imidazolium-[1,2a]-pyridine-based synthesized PILs with carboxylate anions ([ImPr][Pim],[ImPr][Ox] and [ImPr][Phth]) show better thermal stability thanthose reported with alkyl ammonium-based PILs.




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3.5 DSC analysis

Thermal properties of the synthesized PILs are summarized inTable 1. All PILs showmelting points below 100 �C except [ImPr]-[Ox], which can be classied as a fused salt due to its highermelting temperature.13 Generally the melting of the ILs dependson the crystal lattice strength, which corresponds to the inter-molecular forces and ion interactions. The melting point alsodepends on the packing efficiency, which can result from sterichindrance or minimum hydrogen bonding.7,13

[ImPr][Pim] was the only ionic liquid that showed no melttransition (see ESI-Fig. S5† for DSC curve). This is rationalizedby its molecular structure containing a long alkyl chain anion(Fig. 1) exhibiting poor packing of the crystal. [ImPr][Ox] is theonly ionic liquid that shows a melt transition above 100 �C(Fig. 7). This could be due to the hydrogen bonding or greaterelectrostatic forces between its cations and anions (see ESI-Fig. S2† for DSC thermogram).

The shi observed between the crystallization and meltingtemperature of samples is understood to be based on a non-equilibrium thermodynamic state during a heating/coolingcycle (heating and cooling cycles were combined in one plot inall DSC traces of samples).30

The DSC trace of [ImPr][BDT]/[ImPr][BDS] shows twomelting points but one crystallization temperature, corre-sponding to the second melting point (see ESI-Fig. S4† for DSCcurve). The rst melt is related to [Impr][BDT], which can bepartially oxidized at higher temperatures and shows an endo-therm at 43 �C. The oxidized version of the sample ([ImPr][BDS])were crystallized in the cooling cycle but not the non-oxidizedone. The DSC of the [ImPr][BDT] sample also was repeated whenthe sample was exposed to the air for more than three weeks(see ESI-Fig. S9† for DSC curve). One endotherm at 43 �C wasobserved, conrming the DSC assignment of the rst peak ofthe un-oxidized sample.

No crystallization was observed for [ImPr][Phth], due to thefact that it remains in the liquid state to �150 �C (Fig. 7). Thisobservation could be due to the fact that there is sufficientdisorder in the liquid state to hinder formation of solid state on

Fig. 7 DSC thermograms of (a) [Impr][Phth] showing glass transition tempera-ture (Tg) (observed in second cycle) and melting temperature (Tm) (observed infirst cycle) (b) [Impr][Ox] showing Tm and Tc [endotherm down].


cooling.14 Subsequently, this super-cooled viscous liquidundergoes a glass transition at 2 �C in second cycle.

The glass transition temperature was detected at a muchlower temperature for the [ImPr][Pim] ionic liquid, which hasan anion that contains a long alkyl chain.

The glass transition state is represented by a cohesive energyof the sample. This energy is decreased by repulsive Pauli forcesdue to the overlap of closed electron shells, while it is increasedthrough the attractive Coulomb, van der Waals, and hydrogen-bonding interactions.13,14

Low Tg values resulting from low cohesive energies normallyare indicative of some desirable physicochemical propertiessuch as low viscosity and high ionic conductivity. Therefore theglass transition temperature of ionic liquids can be tuned bymodifying their anion or cation, as Tg increases slowly withincreasing alkyl chain length, and increases by a larger amountwith the substitution of a hydroxyl group.7,13,14 Tg observed for[ImPr][Pim] is higher than those reported for imidazolium-based protic ionic liquids with formate and acetate anions,probably due to the longer alkyl chain length.

The phase transition Tp in Table 2 is a solid–solid phasetransition or polymorphism perhaps due to different congu-rations of hydrogen bonding within the samples. This transi-tion was observed in the [ImPr][BDT] cooling cycle at �3 �C.These processes involve less energy than the melting and crys-tallization phase transitions.

The enthalpy of transition can be estimated by using the peakintegration. The entropy of the rst order transition can be esti-mated by eqn (1). DHt is the enthalpy of transition, DSt is theentropy of transition, and Tt is the temperature of transition.Enthalpy and entropy associated with crystallization andmeltingare given in Table 3. The enthalpy energy values are close to somereported solid imidazolium-based protic ionic liquids.30

DHt ¼ TtDSt (1)

It was shown earlier that [ImPr][Ox] and [ImPr][Phth] havehigher melting points than [ImPr][HSO4]. There are manyfactors affecting the melting point, including H-bonding, pi-stacking interactions, charge dispersions, and intramolecularhydrogen bonding.41,42 To understand the reason behinddifferences in the melting point of these PILs it is necessary todo MD simulations as well as X-ray diffraction studies.

Table 2 Thermal properties of the synthesized PILs including melting point (Tm),crystallization temperature (Tc), glass transition temperature (Tg), solid–solidphase transition (Tp) and decomposition temperature (Td)

Name Tm/�C Tc/�C Tg/�C Tp/�C Td/�C

[ImPr][BDS][ImPr][BDT] 43 (26a) 21 (nd) nd �3/11b 260 (126)[ImPr][Pim] nd nd �47 nd 175[Impr][Ox] 130 102 nd nd 165[ImPr][phth] 88 nd 2 nd 175[ImPr][HSO4] 62 60 nd 36 322

a Melting point observed for non-oxidized [ImPr][BDT]. b Exothermicand endothermic solid–solid transitions.

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Table 3 Melting temperature (Tm) and enthalpy (DHm), and crystallization temperature (Tc) and enthalpy (DHc) for synthesized PILs

Sample Tm/�C DHm/kJ mol�1 DSm/kJ mol�1 Tc/�C �DHc/kJ mol�1 �DSm/kJ mol�1

[ImPr][Pim] nd — — nd — —[ImPr][Phth] 88 27.96 0.32 nd — —[ImPr][Ox] 130 25.43 0.19 102 22.12 4.6[ImPr][BDS]/[ImPr][BDT] 43/26a 8.30/7.60a 5.1/3.42a nd/21 8.99 2.3[ImPr][HSO4] 62 10.60 5.8 60 6.26 9.58

a It is the melting point observed for non-oxidized version of [ImPr][BDT] and its corresponding DHm and DSm.


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The glass transition temperature of [ImPr][Phth] is greaterthan [ImPr][Pim]; this indicates a higher cohesive energy in[ImPr][Phth]. Glass transition were observed for [ImPr][Pim]and [ImPr][Phth] and were higher than those that were reportedfor the imidazolium-based ionic liquids (although no imida-zolium ionic liquid has been made with these anions) evenincluding those with formate and acetate anions; this indicateshigher cohesive energy in the new synthesized PILs. This meansadded aromatic ring increases the cohesive energy. This is a newnding that has not been known before, to the best of ourknowledge.

3.6 Viscosity measurement

[ImPr][Pim] was the only liquid sample among the synthesizedPILs at room temperature. As was shown earlier in the DSCtrace, [Impr][Phth] can stay in the liquid phase for long timeaer melting (DSC did not reveal a crystallization temperature,only a glass transition temperature), which allowed us to dosome physical measurements even below its melting point. As isshown in Fig. 8, the viscosity of [ImPr][Pim] and [ImPr][Phth]obviously decrease as the temperature rises (see ESI-Tables S1and 2† for viscosity data for [Impr][Phth] and [Impr][Pim]).

The substitution of a hydroxyl group on the cation or anionof PILs can signicantly increase the viscosity, likely due toextended hydrogen bonding, and ion–ion interactions.7,14

Therefore the viscosity of the [ImPr][Pim] at room temperatureis higher than those with those imidazolium-based PILs withother types of anions.7

Fig. 8 Log of viscosity of [ImPr][Pim] and [ImPr][Phth] as a function of 1000/T(K�1). The lines are fits of Vogel–Fulcher–Tammann equation to the experimentaldata.

11576 | J. Mater. Chem. A, 2013, 1, 11570–11579

A higher viscosity of [ImPr][Phth] in comparison to [ImPr]-[Pim] was expected. The greater cohesive energy discussedearlier, caused by the presence of the aromatic ring in the anionincreases the intermolecular p–p interactions, and hence theviscosity.

The term “fragile” is attributed to compounds that experi-ence decreasing viscosity as the temperature rises, at a fasterrate than predicted by the Arrhenius relationship.48–52 Thefragility of glass-forming liquids can be investigated by plottinglog(viscosity) versus (Tg/T) in the low temperature region whereTg/T is less than 1. Such a plot is usually known as an Angell plot(Fig. 9 and 10).48,53 In such plots, the viscosity dependence oftemperature in strong glass formers such as SiO2 obeys Arrhe-nius law (linear curve in Fig. 9 and 10 and eqn (2)).54

logðhÞ ¼ �Aþ B

T(2)

while fragile liquids that deviate from Arrhenius behaviour canbe described by Vogel–Fulcher–Tamman equation:

logðhÞ ¼ �AVFT þ BVFT

T � Tg

(3)

A, B, AVFT, BVFT are adjustable parameters in eqn (2) and (3).The Angell plot of [ImPr][Pim] and [ImPr][Phth] in Fig. 9 is

compared to the multiple data obtained from some other typesof PILs.7 However, since there is only one measurement formany of these PILs, typically at ambient temperature, singledata points have been used. The synthesized PILs in this studyshow fragility behaviour similar to other PILs.

Fig. 9 Fragility plot for the selected synthesized PILs in this study compared tosome reported imidazolium, heterocyclic and alkyl ammonium protic ionic liquids.



Fig. 10 Arrhenius plot of two PILs in present work compared with series of datapresented by Angell et al.49 The glass temperatures in degree Kelvin are indicatedin parentheses.

Fig. 11 Changes in ionic conductivity of selected synthesized PILs bytemperature.


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Most PILs have fragilities from intermediate to high. Angellet al. found this by comparing the result to known liquids withstrong and intermediate fragility (Fig. 10).49 The fragility of theselected PILs in the present work are compared in the same wayand the results show very fragile behaviour.

3.7 Density measurement

The density of ionic compounds depends on how closely theycan pack together. Therefore, the densities of PILs varied withsmall changes in structure of the cation and anion due todifferent sizes and shapes of the ions, and ion–ion interactions.Density decreases with bulkier groups attributed to an increasein steric hindrance; so by increasing the alkyl chain length ofthe cation or anion, the densities decrease slightly.13 Thedensity of [ImPr][Pim] is lower than those for [ImPr][Phth] forthe same reason (ESI-Fig. S11†).

On the other hand, the higher density of the [ImPr][Phth]sample might be attributed to p–p interactions in its structure.Fig. S11† also shows that densities slightly decrease with risingtemperature. It was already known that the density of lactam-based PILs decrease slightly with larger-ringed cations,51 andthis has been attributed to decreased interactions between theanion and cation. The slightly larger density which has beenobtained for imidazolium [1,2a]-pyridine-based PILs comparedto imidazolium-based PILs can be related to the p–p stackinginteractions causing better packing system.9,55

Fig. 12 Walden plot of selected synthesized PILs, where L is the equivalentconductivity and h�1 the fluidity. The solid line represents “good” ionic liquidsbehavior based on classification of Angell et al.24

3.8 Ionic conductivity measurement

Fig. 11 shows the log of conductivity versus temperature for[ImPr][Pim] and [ImPr][Phth] as selected PILs in this studyrepresents how ionic conductivity increases with the tempera-ture (see ESI-Tables S3 and 4† for conductivity data of [Impr]-[Phth] and [Impr][Pim]).

The Walden plot was used by Angell et al. to categorize ionicliquids as good, poor and non-ILs.24 Based on the Walden rule(eqn (4)), good ionic liquids have an increasing ionic conduc-tivity directly related to the increase in uidity. Those PILs withweak proton transfer from a Brønsted acid to a Brønsted base,


or associated ions will lie below the ideal line. The solid idealline corresponds to dilute aqueous KCl solution in which thesystem is known to be fully dissociated and to have ions of equalmobility.

Lmha ¼ C ¼ constant (4)

where Lm is molar conductivity, h is viscosity and a character-izes the degree of decoupling.

The Walden plot for [ImPr][Pim] and [ImPr][Phth] is shownby plotting the equivalent ionic conductivity (mS cm2 mol�1)versus uidity (Pa�1 s�1). As is shown in Fig. 12, [ImPr][Phth]shows almost ideal behaviour over a wide range of tempera-tures, while [ImPr][Pim] is a poor ionic liquid.

Aqueous pKa values can be used to estimate the degree ofproton transfer in PILs (an important factor that affects theionicity of PILs). It has been shown that a difference of 4 in thepKa between the acid and base in aqueous solution will besufficient to produce 99% proton transfer.12

Some alkylammonium PILs with an acetate anion haveshown an ionicity close to ideal behaviour in Walden plots. The

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Fig. 13 DpKa (pKa (base) � pKa (acid)) of the synthesized PILs in this study.


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high ionicity of the mentioned PILs corresponded to their highvalue for the DpKa.

The pKa1, pKa2 and DpKa (pKa (base) � pKa1 (acid)) values forthe selected synthesized PILs is shown in Fig. 13 (pKa (ImPr) ¼6.79). As it is shown in Fig. 13, the DpKa for [ImPr][Phth] islarger than value of DpKa for [ImPr][Pim] whichmeans relativelybetter proton transfer in the [ImPr][Phth] system. Since theDpKa value of [ImPr][Phth] is slightly less than 4, other factorsare likely involved in the high ionicity of this protic ionic liquid.

The ionicity of N-methylpyrrolidinium acetate PIL improvedto ideal behaviour by increasing the mole ratio of acid to base inthe system. This observation corresponded to the formation ofdimer hydrogen-bonded acid complexes, which are strongerconjugated acids than monomeric acids. As these complexspecies can be present to some extent at all PIL compositions,this phenomenon also can be extended to phthalic acid as anortho-dicarboxylic acid; these can make a dimeric complex aerone deprotonation or oligomeric complex by external hydrogenbonding.

Therefore the high ionicity of [ImPr][Phth] might correspondto dimerization of the phthalate anion by hydrogen bonding tomake dimer and trimer acid–base systems, which can behave ashighly ionized protic ionic liquids.

Although conductivity and viscosity measurements were notdone for [ImPr][HSO4], [ImPr][Ox] and [ImPr][BDT] samples,their ionicity based on DpKa might be predictable. As shown inFig. 13, the ionicity of synthesized PILs based on DpKa followsthis order: [ImPr][HSO4] > [ImPr][Ox] > [ImPr][Phth] > [ImPr]-[Pim] > [ImPr][BDT].

4 Conclusions

To our knowledge, this is the rst instance where imidazo-[1,2a]-pyridine was used as base for the synthesis of protic ionicliquids, with benzene-1,2-dithiol, oxalic acid, phthalic acid, andpimelic acid used as the anion counterparts of the PILs. Thethermal stability and phase transition of the samples werestudied by TGA and DSC and the results were correlated to thestructure of the anion or cation. The added aromatic ring to

11578 | J. Mater. Chem. A, 2013, 1, 11570–11579

either cation or anion increases the cohesive energy. The [ImPr]-[Pim] and [ImPr][Phth] as the selected protic ionic liquids inthis study showed very fragile behaviour in an Arrhenius plot.[ImPr][Phth] showed remarkable ionic behaviour close to ideal.

The combination of analyzing the data based on the Waldenplot and TGA data opens a new perspective on the application ofthese types of PILs as non-aqueous electrolytes for electro-chemical applications, with promise in fuel cells and as elec-trolytes in batteries (including Li-, Na-, and Mg-basedbatteries).56

Acknowledgements

This research was nancially supported by the Natural Sciencesand Engineering Research Council of Canada, the New Bruns-wick Innovation Foundation and Canada Foundation forInnovation. K.G. also acknowledges the Canada Foundation forInnovation, Atlantic Innovation Fund and other partners thatfund the Facilities for Materials Characterization managed bythe Department of Chemistry and the Institute for Research inMaterials at Dalhousie University. We thank James M. Ehrmanfrom Mount Allison University for his kind help with the SEMimages.

Notes and references

1 S. Sowmiah, V. Srinivasadesikan, M. C. Tseng and Y. H. Chu,Molecules, 2009, 14, 3780.

2 S. Katkevica, A. Zicmanis and P. Mekss, Chem. Heterocycl.Compd., 2010, 46, 158.

3 F. Shi, Y. Gu, Q. Zhang and Y. Deng, Catal. Surv. Asia, 2004, 8,179.

4 X. Liang, G. Gong, H. Wu and J. Yang, Fuel, 2009, 88, 613.5 S. Ahrens, A. Peritz and T. Strassner, Angew. Chem., Int. Ed.,2009, 48, 7908.

6 T. Welton, Chem. Rev., 1999, 99, 2071.7 S. Zhang, N. Sun, X. He, X. Lu and X. Zhang, J. Phys. Chem.Ref. Data, 2006, 35, 1475.

8 M. Earle, P. Wasserscheid, P. Schulz, H. Olivier-Bourbigou,F. Favre, M. Vaultier, A. Kirschning, V. Singh, A. Riisager,R. Fehrmann and S. Kuhlmann, Organic Synthesis, in IonicLiquids in Synthesis, Wiley-VCH Verlag GmbH & Co. KGaA,2007, pp. 265–568.

9 T. L. Greaves and C. J. Drummond, Chem. Rev., 2007, 108,206.

10 C. F. Poole, J. Chromatogr., A, 2004, 1037, 49.11 S. Y. Lee, A. Ogawa, M. Kanno, H. Nakamoto, T. Yasuda and

M. Watanabe, J. Am. Chem. Soc., 2010, 132, 9764.12 C. Brigouleix, M. Anouti, J. Jacquemin, M. Caillon-

Caravanier, H. Galiano and D. Lemordant, J. Phys. Chem. B,2010, 114, 1757.

13 C. Ke, J. Li, X. Li, Z. Shao and B. Yi, RSC Adv., 2012, 2, 8953.14 J. Stoimenovski, E. I. Izgorodina and D. R. MacFarlane, Phys.

Chem. Chem. Phys., 2010, 12, 10341.15 T. L. Greaves, A. Weerawardena, C. Fong, I. Krodkiewska and

C. J. Drummond, J. Phys. Chem. B, 2006, 110, 22479.




Publ

ishe

d on

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ownl

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Uni

vers

iteit

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27/0

9/20

13 1

4:08

:41.

View Article Online

16 J. P. Belieres and C. A. Angell, J. Phys. Chem. B, 2007, 111,4926.

17 A. Bagno, C. Butts, C. Chiappe, F. D'Amico, J. C. D. Lord,D. Pieraccini and F. Rastrelli, Org. Biomol. Chem., 2005, 3,1624.

18 Z. B. Zhou, H. Matsumoto and K. Tatsumi, Chem.–Eur. J.,2005, 11, 752.

19 P. Bonhote, A. P. Dias, N. Papageorgiou,K. Kalyanasundaram and M. Gratzel, Inorg. Chem., 1996,35, 1168.

20 M. S. Miran, H. Kinoshita, T. Yasuda, M. A. B. H. Susan andM. Watanabe, Chem. Commun., 2011, 47, 12676.

21 A. Noda, M. A. B. H. Susan, K. Kudo, S. Mitsushima,K. Hayamizu and M. Watanabe, J. Phys. Chem. B, 2003,107, 4024.

22 B. Nuthakki, T. L. Greaves, I. Krodkiewska,A. Weerawardena, M. I. Burgar, R. J. Mulder andC. J. Drummond, Aust. J. Chem., 2007, 60, 21.

23 V. Tricoli, G. Orsini and M. Anselmi, Phys. Chem. Chem.Phys., 2012, 14, 10979.

24 M. Yoshizawa, W. Xu and C. A. Angell, J. Am. Chem. Soc.,2003, 125, 15411.

25 C. Schreiner, S. Zugmann, R. Hartl and H. J. Gores, J. Chem.Eng. Data, 2009, 55, 1784.

26 N. B. Darvatkar, A. R. Deorukhkar, S. V. Bhilare andM. M. Salunkhe, Synth. Commun., 2006, 36, 3043.

27 E. Janus, I. Goc-Maciejewska, M. Łozynski and J. Pernak,Tetrahedron Lett., 2006, 47, 4079.

28 A. Zhu, T. Jiang, D. Wang, B. Han, L. Liu, J. Huang, J. Zhangand D. Sun, Green Chem., 2005, 7, 514.

29 H. Zhou, J. Yang, L. Ye, H. Lin and Y. Yuan, Green Chem.,2010, 12, 661.

30 S. Guo, Z. Du, S. Zhang, D. Li, Z. Li and Y. Deng, Green Chem.,2006, 8, 296.

31 S. K. Shukla, N. D. Khupse and A. Kumar, Phys. Chem. Chem.Phys., 2012, 14, 2754.

32 S. Gabriel and J. Weiner, Ber. Dtsch. Chem. Ges., 1888, 21,2669.

33 D. F. Evans, Langmuir, 1988, 4, 3.


34 R. Atkin and G. G. Warr, J. Am. Chem. Soc., 2005, 127, 11940.35 A. Pinkert, K. L. Ang, K. N. Marsh and S. Pang, Phys. Chem.

Chem. Phys., 2011, 13, 5136.36 H. Tian, E. Gabrielsson, Z. Yu, A. Hagfeldt, L. Kloo and

L. Sun, Chem. Commun., 2011, 47, 10124.37 V. Jovanovski, V. Gonzalez-Pedro, S. Gimenez, E. Azaceta,

G. Caba~nero, H. Grande, R. Tena-Zaera, I. Mora-Sero andJ. Bisquert, J. Am. Chem. Soc., 2011, 133, 20156.

38 J. Dupont, J. Braz. Chem. Soc., 2004, 15, 341.39 C. Janiak, J. Chem. Soc., Dalton Trans., 2000, 3885.40 E. A. Meyer, R. K. Castellano and F. Diederich, Angew. Chem.,

Int. Ed., 2003, 42, 1210.41 J. Coates, Interpretation of Infrared Spectra, A Practical

Approach, in Encyclopedia of Analytical Chemistry, JohnWiley & Sons, Ltd, 2006.

42 S. H. Cho, Y. J. Lee, M. S. Kim and K. Kim, Vib. Spectrosc.,1996, 10, 261.

43 W. L. F. Armarego and C. L. L. Chai, Aromatic compounds, inPurication of Laboratory Chemicals, Elsevier Inc., 2008.

44 W. L. F. Armarego, J. Chem. Soc., 1964, 4226.45 F. P. Daly, C. W. Brown and D. R. Kester, J. Phys. Chem., 1972,

76, 3664.46 N. Bicak, J. Mol. Liq., 2005, 116, 15.47 T. Maruyama, S. Nagasawa and M. Goto, Biotechnol. Lett.,

2002, 24, 1341.48 C. A. Angell, Proc. Natl. Acad. Sci. U. S. A., 1995, 92, 6675.49 W. Xu, E. I. Cooper and C. A. Angell, J. Phys. Chem. B, 2003,

107, 6170.50 M. A. B. H. Susan, A. Noda, S. Mitsushima and M. Watanabe,

Chem. Commun., 2003, 938.51 J. P. Guthrie, Chem. Biol., 1996, 3, 163.52 B. F. Pedersen, The crystal structure of potassium hydrogen

oxalate KHC2O4, Acta Chem. Scand., 1968, 22, 2953.53 M. Ikeda and M. Aniya, Materials, 2010, 3, 5246.54 L. Dagdug, L. S. Garcia-Colin and P. Goldstein, Rev. Mex. Fis.,

2005, 51, 144.55 Z. Du, Z. Li, S. Guo, J. Zhang, L. Zhu and Y. Deng, J. Phys.

Chem. B, 2005, 109, 19542.56 K. Ghandi and S. Nazari, US Pat., serial no. 61/852,934, 2013.

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physicochemical properties of imidazo-pyridine protic ionic liquids

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