Ionic Liquids - New Materials With Wide Applications (2010) IJCA 49A(05-06) 635-648

8/11/2019 Ionic Liquids - New Materials With Wide Applications (2010) IJCA 49A(05-06) 635-648

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Indian Journal of ChemistryVol. 49A, May-June 2010, pp. 635-648

Ionic liquids: New materials with wide applications

Nageshewar D Khupse*& Anil Kumar

Physical Chemistry Division, National Chemical Laboratory, Pune 411 008, IndiaEmail: [email protected] (NDK)/ [email protected] (AK)

Received 7 April 2010; accepted 21 April 2010

Ionic liquids have emerged as possible substitutes for volatile organic solvents and have found many applications in avariety of research areas. In this review, an effort has been made to discuss the special properties of ionic liquids that renderthese unique solvent media useful in chemical transformations, electrochemical applications, extractions, etc.

Keywords: Ionic liquids, Physicochemical properties, Solvent properties, Viscosity, Chemical processes,Electrochemical devices

Ionic liquids have recently emerged as green andenvironment friendly solvents for their use in theindustrial manufacture of chemicals. In the past

decade, ionic liquids have been increasingly used fordiverse applications such as organic synthesis,catalysis, electrochemical devices and solventextraction of a variety of compounds.1-3Ionic liquidsare composed of cations and anions having lowmelting point (


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INDIAN J CHEM, SEC A, MAY-JUNE 2010636

amount of AlCl3. These ionic liquids are known aschloroaluminates. Though one can have a more

efficient control over the nature of these ionic liquids,

their extremely hygroscopic nature restrict their use inorganic processes and one needs to take strict

precautions for using them under a dry atmosphere. Inspite of their hygroscopic nature these ionic liquids

have however been used as electrolytes in batteries.

Ionic liquids with [BF4] , [PF6] anions, etc., are air

stable and neutral. The ionic liquids can react

exothermically with Lewis acids and water. Ionicliquids containing [CF3SO3] or [Tf2N] anions

possess low melting points and are stable in water and

the medium containing Lewis acids. Some ionic

liquids under this category are noted to act as

moderately coordinating solvents.

Synthesis of Ionic LiquidsIonic liquids can be synthesized according to the

previously reported literature procedure and excellent

reviews are available on their synthesis and handling

techniques.8 The synthesis of ionic liquids has been

carried out in two steps: (1) formation of cation byquaternization reaction, and, (2) anion exchange

reaction. The characterization and their purities have

been determined by NMR.8The water content of the

pure and dried ionic liquids has been measured by

Karl Fischer coulometer analysis. The preparation of

ionic liquids, even in large quantities, presents nosignificant difficulties. Provided they are of sufficient

purity, most ionic liquids can be stored without

decomposition for extended periods although some

are relatively hygroscopic. Besides brief discussion on

the types of ionic liquids, one should note that it is

possible to synthesize numerous ionic liquids with

varying properties.

Physicochemical Properties

Melting point and thermal stability

This is the most significant characteristic property

of ionic liquids that can be correlated with thestructure and composition of ionic liquids. Selection

of both the cation and anion determines the melting

point of an ionic liquid. An ionic liquid with a cation

with low symmetry possesses lower melting pointsthan the one with high symmetry. Weak

intermolecular interactions and a good distribution of

charge in the cation favor low melting point of ionic

liquid.9

Most of the ionic liquids are stable at and above

400 C. The thermal decomposition depends on the

nature of anions rather than of cations.10

Further,thermal decomposition also decreases with an

increase in hydrophilicity of anions.

Viscosity

Viscosity of pure ionic liquids

Before we elaborate on the viscosity of mixtures of

ionic liquids, we summarize herein the viscosity

profiles of pure ionic liquids. Chloroaluminate ionic

liquids have been widely used in electrochemical

devices and also in carrying out chemical processes.

Hussey and his school11,12contributed significantly to

determine several physico-chemical properties of this

early class of ionic liquids. A compilation on the

viscosities of a number of air-and moisture-stableionic liquids is available in the literature.13The major

problem in handling this class of ionic liquids is withregard to their sensitivity to water and chloride

impurities, and hence, measurement should make with

utmost care.14

The presence of small amounts of

chloride impurities could lead to a dramatic increasein the viscosity of ionic liquids. The variation of

viscosity of pure ionic liquids changes with a

variation in the structure of ionic liquids. The effect of

cation is smaller as compared to anion effect. The

[NTf2] anion offers low viscosity compared to

several other anions, while [PF6] anion contributes

significantly to the viscosity increase of ionic liquids.

The [EtSO4] anion shows increase in viscositycompared to the [BF4] anion.

The fact that the viscosity of ionic liquids is

sensitive to the presence of small amounts ofimpurities is responsible for the disagreement in the

values. However, the same fact can be employed to

our advantage, since the addition of small amounts of

cosolvents can reduce the viscosity to suit our needs,

without compromising on the promising capabilities

of ionic liquid media. For example, the viscosity of

[BMIM][BF4] increased by 25 % in the presence of

0.5 mol kg-1 of chloride content. The presence of

water had a stronger but inverse effect, i.e., theviscosity decreased greatly in the presence of small

amounts of water for not only hydrophilic but also

hydrophobic ionic liquids. In view of the potential

applications of such systems, a critical analysis of the

published data related to binary mixtures is important

while applying it for further studies. Drastic

discrepancies in the literature values of ionic liquids[BMIM][BF4] and [BMIM][NTf2] are observed. The

observed viscosity values for [BMIM][BF4] is given

in literature as 92 (ref. 15), 115 (ref. 16), 180 (ref. 17),


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KHUPSE & KUMAR:IONIC LIQUIDS: NEW MATERIALS WITH WIDE APPLICATIONS 637

219 (ref. 18), 116 (ref. 19), etc., and for

[BMIM][NTf2] as 44 (ref. 20), 52 (ref. 21), etc.

The physico-chemical properties of ionic liquids

vary with structure of cations and anions. The effectof anions on viscosity of imidazolium based ionic

liquids can be broadly depicted as [BMIM][NTf2] [BP][NTf2] > [HP][NTf2] at25 C. It is important to understand the solute-solventinteractions in ionic liquids that cause such drasticvariations with temperature. The temperature-dependence of the polarity parameters orthermosolvatochromism for all the ionic liquids wasthen studied at 298353 K. Depending on the cationicand anionic species, the polarity values showed eithera direct or an inverse relation with the change in

temperature. The ETN

value decreases withtemperature for the pyridinium-based (Fig. 8) andpyrrolidinium-based ionic liquids (Fig. 9), but

increases with temperature for the phosphonium ionicliquids (Fig. 10). This indicates that the choice of thecation can determine the response of polarity to achange in temperature. The betaine dye systems areknown to show a negative solvatochromism due todifferential solvation of more polar ground state(dipole moment of ground state G = 15 D) ascompared to the less polar excited state (dipolemoment of excited state E = 6 D). This explains theblue shift of absorption maxima for betaine dye withincreasing solvent polarity. In polar solvents, theground state is stabilized due to stronger solute-solvent interaction as compared to the excited state.

When the temperature is increased, the ground statesolvent interactions are weakened, thus reducing theenergy gap between the ground state and the excitedstate of the betaine molecule. As a result, an increasein temperature should cause a red shift in theabsorption maximum of betaine dye in polar solvents.This is precisely the case observed for the pyridiniumand pyrrolidinium-based ionic liquids. The oppositeeffect of temperature in the case of the phosphonium-based ionic liquids may then be explained alongsimilar lines. The blue shifts in the solvatochromismindicate that the phosphonium ionic liquids solvatethe excited state of betaine dye more efficiently as

compared to the ground state. The greaterstabilization of the excited state is also reflected inlowerET

Nvalue at 303 K in phosphonium-based ionicliquids as compared to the pyrrolidinium ionic liquidswhen the temperature increases.

Fig. 8 Temperature dependent ETN parameters for the

pyridinium-based ionic liquids. {[BP][BF4](), [OP][BF4](),

[BP][NTf2](), [HP][NTf2]() and [OP][NTf2]()}. [Figuretaken from ref. 54].

Fig. 9 The ETN-T plots for the pyrrolidinium-based

ionic liquids, {[BMPyrr][NTf2](), [HMPyrr][NTf2]() and[OMPyrr][NTf2] ()}. [Figure taken from ref. 54].

Fig. 10 Temperature dependent ETN parameters for the

phosphonium-based ionic liquids. {[TBP][Ala] () and[TBP][Val] ()}. [Figure taken from ref. 54].


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The weakening of stabilizing excited state-solventinteractions should lead to an increase in energy gapbetween the ground state and excited state of betainemolecules. This causes the observed blue shift in thethermosolvatochromism. The results present aninteresting contrast to Suppans generalization thatabsorption bands which give solvatochromic blueshifts are expected, on this basis to showthermochromic red shift. The present line ofreasoning implies that while pyridinium- andpyrrolidinium-based ionic liquids are perceived aspolar solvent by the betaine molecule, thephosphonium-based ionic liquids acts as nonpolarsolvents for the same probe molecule. It would beinteresting to validate this implication further byanalyzing the reasons for the apparent contrast. Since

the betaine dye is neither a hydrogen bond donor nora Lewis acid, it is reasonable to assume that there isno direct interaction of the molecule with the anionsof the ionic liquids. The betaine dye is capable ofexhibiting strong dipole-dipole, dipole-induced dipoleand H-bond acceptor and dispersion interactions(due to large polarizability). The cation-betainemolecule interaction should then be the primaryinteraction, while the influence of the anion isaccounted for only in an indirect manner. An increaseof solvent polarity for probe (2) leads to abathochromic shift of max. This is consistent withseries of -* transitions, which go from a relatively

charge diffused ground state to an excited state,wherein electronic charges are more concentrated andcharge centers are more separated. Hence, more polarsolvents stabilize the electronic ground state with theeffect of shifting max to lower energy. As thetemperature increases, the solute-solvent interactionsare weakened. In the case of dye (2), this entails thatthe relative stabilization of the excited state decreasesas the temperature increases.

Increase in excited energy is reflected bydecreasing * value with temperature for all the ionicliquids studied. It is difficult to discuss the variationin and parameters along similar lines since these

parameters are determined by the combination ofresponses of two or more solute probes. A thoroughcomputational investigation into the interestingvariations in thermochromic behavior of the dyemolecules with the structure of the ionic liquidswould be desirable in the future.

Applications

Chemical processes

Ionic liquids are found to be promising solvents inmany of the organic reaction such as Diels-Alder,

Bails-Hillman, Heck Reaction, esterification,isomerization reactions and many couplingreaction.55,56Pressure, temperature and concentrations

of reactants govern the progress of the reaction.However, it has been observed that viscosity can also

play an important role in reaction kinetics. The ratesof Diels-Alder reaction increase with an increase inviscosity up to 1 cP. A sharp downfall in the rates ofthese reactions is witnessed in solvents possessingviscosities greater 1 cP. The increase in the rates wasattributed to the gain of vibrational mode at the

expense of translational modes up to 1 cP, thusfacilitating the bond formation.

57 However, the

reactions slow down considerably in viscous solventsowing to the diffusion problems in highly viscousenvironment thus causing retardation in the reaction

rates. Reports are available on the rate of reactionbeing faster in the ionic liquids possessing higherviscosities than in the ones with lower viscosities.

58

This means that the high viscosity favors the abovereaction. In another study, the rate constants for adiffusion-controlled reaction involving neutralreactants were measured in ionic liquids with different

viscosities at varying temperatures. The overallbimolecular rate constant, k2, was noted to increasewith an increase in the viscosities of ionic liquids.However, the data reported in the work appeared to beinsufficient for conclusive evidence of the role of

viscosity in Diels-Alder reactions.In our laboratory, we discovered that the k2values

for a number of Diels-Alder reactions in ionic liquids

with varying viscosities were inhibited in the ionic

liquids with high viscosities. The observations wereattributed to the diffusion problems arising out of high

viscosities. However, efforts to correlate k2 values

with different properties such as surface tension,

solvent properties, etc., of ionic liquids, were not

successful. The results were interpreted in terms of

the restricted diffusion of reactants in the encounter-

controlled regime. The order for the rates of reaction

of cyclopentadiene with the three acrylates used wasas follows: methyl acrylate > ethyl acrylate > butyl

acrylate. The difference may arise due to the different

microviscosity experienced by each of these acrylatemolecules, which will be a complex function of the

viscosity of the medium, the molecular volume of the

reacting moiety and the viscosity of the acrylate itself.

The enhanced steric effect will also play an important

role in governing the reactions.The reaction of cyclopentadiene with methyl

acrylate showed Arrhenius behavior as evident from


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the temperature dependence of rate constants in

[BMIM][PF6] and [EMIM][BF4]. The activation

energy, Ea, was reported to be 63.4 kJ mol-1

and

57.7 kJ mol-1

for the reaction of cyclopentadiene withmethyl acrylate in [BMIM][PF6] and [EMIM][BF4]

respectively. However, any change in temperature led

to a change in the values of the ionic liquids, whichaffected the rate of the reaction. The intrinsic

activation energy,Eo, was reported to be 51.8 kJ mol-1

implying a difference (Ea-Eo) of 5.9 kJ mol-1

for

[EMIM][BF4] and of 11.6 kJ mol-1

for [BMIM][PF6].

These values were in agreement with the qualitative

prediction that the reactants would have to overcome

a higher barrier in a more viscous medium, leading

to a decrease in the rate of the reaction. A detailed

study of the isoviscosity relationships, Arrheniusparameters and determination of microviscosity aresome of the most challenging problems one needs to

address in this area.

The effect of a viscosity reducer in the presence

of ionic liquid was also checked on the rates of

reaction. For this purpose, the reaction of

cyclopentadiene with methyl acrylate was carriedout at 298 K in a mixture of [BMIM][BF4]

with dichloromethane (45 mol % of [BMIM][BF4]

in 55 mol % of dichloromethane). Here,

dichloromethane was used as a viscosity reducer,

with = ~18 cP as compared to the value of = 233 cP for [BMIM][BF4]. The resulting rateconstant, k2, was much higher than that in pure

[BMIM][BF4] or dichloromethane alone. Although a

simple correlation between a given solvent property

and the magnitude of a particular rate constant does

not conclusively imply a dependence of rate on that

solvent property, the evidences do merit further

investigation.

It is important to understand the structure-property

correlations with reference to their effects on the

chemical processes. The highly viscous nature of

ionic liquid can slow down the rate of a bimolecularDiels-Alder reaction by an order of magnitude as

compared to that in water. However, there have been

very few attempts to correlate the physico-chemical

properties of ionic liquids with the kinetic and

stereochemical outcome of the reactions.59

Towards

this end, an extensive collection of kinetic data

for a variety of organic reactions carried out in a

range of ionic liquids and subsequent

comparison of the results with theoretical models is

essential. In this context, our group studied the

kinetic studies of an intramolecular Diels-Alder

(IMDA) reaction of (E)-1-phenyl-4-[2-(3-methyl-2-

butenyloxy) benzylidene]5-pyrazolone in a series of

pyridinium-based ionic liquids. The IMDA reactionswere carried out in different pyridinium based ionicliquids. The alkyl substituents on the pyridinium

cation were varied to give different cations, viz.,

1-butyl, 1-hexyl and 1-octyl. The anions used were

[BF4] and [NTf2] . The use of two anions resulted in

a homologous series of ionic liquids, differing onlywith respect to the alkyl substituent on the pyridinium

cations. Changing the alkyl substituents, the anion or

the temperature, could thus vary the viscosity of the

ionic liquids (Fig. 11). The rate constants within the

[BF4] series of ionic liquids show a definite

correlation with the viscosity of the medium. The rateof reaction decreased from 4.45 10-5s-1in [BP][BF4]

to 1.68 10-5s-1in [3MOP][BF4] for a correspondingchange in viscosity from 175.4 cP to 66.1 cP at

308 K. The correlation between kand was not very

obvious for the [NTf2] series. Surprisingly, the

magnitude of k in the [NTf2] ionic liquids was very

close to that in [BF4] based ionic liquids. If the rateconstants are indeed dependent on the viscosity of the

medium, then the lower viscosity of the [NTf2] based

ionic liquids should have led to higher rates.

In order to access the correlation between the rates

and viscosity in greater detail, temperature dependentstudies were carried out in different ionic liquids. Thevariation in temperature served to control the

Fig. 11 Plot of ln kfor the IMDA reaction against the viscosity,

, of pyridinium ionic liquids with [BF4] anion () and [NTf2]

anion () at different temperatures. [Figure taken from ref. 53a].


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viscosity of the medium and the results plotted are acompilation of the rates in different ionic liquids at

different temperatures. For the [BF4] series of ionic

liquids, the uniform variation in the experimental datashowed that the rate constants at the same viscosity

were nearly the same (independent of the means bywhich that viscosity value was attained) on changing

the alkyl substituent or the temperature. The rate

constants decreased uniformly with increasing

viscosity, thus indicating universal viscosity

dependence. It was obvious however, that thisuniversal trend failed to extend to the [NTf2]

series. The [NTf2] based ionic liquids also showed a

similar but independent trend with the changing

viscosity; in fact, the decrease in rates was sharper for

[NTf2] ionic liquids for a similar magnitude of

increase in viscosity. At a given value of viscosity, the

rate of the IMDA reaction in [NTf2] was much lower

than that in the [BF4] ionic liquid. The rate constant

decreased from ~13 10-5s-1to ~3 10-5s-1when the

viscosity increased from 50 cP to 100 cP for the

[BF4] based ionic liquids, i. e., a decrease to one-

fourth of the original rate constant. For a viscosity of

100 cP for [NTf2] ionic liquids, the rate decreased

to one-tenth of its value at 50 cP, i. e., from

~2.10 10-5s-1to ~0.21 10-5s-1The results indicate that in addition to viscosity, the

rates are also influenced by a solvent property thatvaried, independent of viscosity, on changing fromone anion series to another. Within the homologous

series, this effect might still be operative but was

either masked by a greater competing influence of

viscosity or it changed in proportion to the viscosity.

Electrochemical devices

Excellent reviews are published demonstrating the

applications of ionic liquids to electrochemicaldevices such as super capacitors, lithium ion batteries,

polymer-electrolyte fuel cells and dye-sensitized solar

cells. In an electrochemical device, an ionic liquid

acts as electrolyte.60-64

Ionic liquids should be resistantto any electrochemical reduction and oxidation. Twomain advantages of using ionic liquids in

electrochemical devices include non-volatility and

prevention of electrolytes from drying during the

operation. The transport properties, like viscosity,

conductance and diffusion gain significance when one

considers diffusion process or conductivity for

making an effective electrochemical device. However,

the fundamental requirement for an ionic liquid to be

useful in developing applications in electrochemistry

is that it should be able to offer a wideelectrochemical window. While aqueous systems

possess about 1.23 V as the electrochemical window,

propylene carbonate and acetonitrile can offer anelectrochemical window as high as 4 V. Almost many

the ionic liquids offer electrochemical window in therange of 4-5 V, while the most common and popular

ionic liquid, [BMIM][BF4], possesses an

electrochemical window as high as 6 V as shown in

Fig. 12. Another ionic liquid, [BMP][CF3SO3], is

reported to have an electrochemical window of 6 V.One notes that the selection of reference electrode

can be very important criteria in enhancing

electrochemical stability. Ionic liquids have been

found to be useful for electric double layer

capacitors.65,66

Several applications in this regard have

been critically examined. For example, many ionic

liquids composed of [EMIM] cation with a variety of

anions have been found effective. Another popular

ionic liquid, [BMIM][BF4], with quite a high

electrochemical window is also very useful for this

application. In general, the capacitance changes in the

order: aqueous solutions > ionic liquids > organic

solutions.

The high viscous ionic liquid has also been mixedwith propylene carbonate to decrease the viscosityand to provide output data better than conventionalsystems. Another important area of application ofionic liquids is lithium batteries. The main interest inusing ionic liquids for this application has been to find

Fig. 12 Electrochemical window of [BMIM][BF4] measured by

linear sweep voltammetry using W, Pt electrodes which require ahigh conductivity and a wide electrochemical window.


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substitutes for organic solvents. Also, if ionic liquidsare used instead of organic solvents, the batteries willbe safer to use. Chloroaluminate ionic liquids have

been used from the very beginning in lithiumbatteries. In fact, this is the reason that one finds alarge number of data on physico-chemical andelectrochemical properties of chloroaluminates.Normally, a combination of two species has beenemployed for this work. Also, ionic liquids with[BF4] and [NTf2]

anions coupled with lithium salts

of the same anion have been frequently employed forthis purpose. For example, [EMIM][BF4] mixed withLiBF4 acts as a good substance for lithium batterywith LiCoO2 as a cathode material. Also, one getsgood results with LiCoO2 and Li-Al as cathode andanode materials, respectively with the same ionicliquid. In order to avoid cathodic reduction,pyrrolidinium and piperidinium based ionic liquidsare preferable. Interestingly, some additives like waterand SOCl2 in traces with ionic liquids have beenreported to be effective for this application. Also,ionic liquids based on guanidinium cation with

[NTF2] anion are found to be potential electrolytes

for electrochemical devices. Though exhaustivereports are available, reviewing the use of ionicliquids in lithium batteries, a greater need exists todevelop a model to predict the composition of amixture of ionic liquids with some solvent that will

provide the optimum results on lithium batteries. It isnow certain that ionic liquids with sufficientconductivity and cycle stability in the lithiumdeposition/dissolution process can be designed byintroducing appropriate functional groups. Ionicliquids are non-volatile due to their insignificantvapor pressure. This property of ionic liquid rendersthem a potential candidate for solar cell. An importantpoint has emerged in this regard. Ionic liquids basedon [HMIM] cation and [NTf2]anion are noted to beeffective in solar cells. Further, addition of a dye inionic liquid system has been noted to enhance the

efficiency of a solar cell. Ionic liquids have appearedto be potential substitute for smooth operation ofpolymer electrolyte membrane fuel cell at hightemperatures. A mixture of ionic liquids withadditives like amines is the most promising candidatefor this work. Proton conducting gelatinous

electrolyte with [BMIM][BF4] are thermallymechanically stable up to 300 C. It is now possible totransport hydrogen via fluorohydrogenate conductionin ionic liquids system. Quite interestingly, ionicliquids have found a useful role in actuators. The

research group of Asaka67 has shown that a dryactuator can be fabricated layer to layer casting usingbucky gel, which is a gelatinous room temperature

ionic liquids containing single wall carbon nanotubes.

The solid polymer electrolyte layer is sandwichedbetween polyaniline (PANI) and polypyrrole (PPY),etc. Unfortunately, the solid polymer electrolyte layerpossessing low ionic conductivity leads to slowresponse system. The electrolyte solution gives rise toswelling polymer electrolyte thereby leading to poor

performance of the system. Figure 13 depicts a crosssection of PPy double sided actuators with ionicliquids. [BMIM][PF6] showed PPy was stable and wasable to withstand continuous functioning for morethan 3600 cycles without degradation.

68

Extraction technology

Solvent extraction is used in nuclear reprocessing,

ore processing, the production of fine organic

compounds, processing of perfumes and other

industries. The use of ionic liquids to separate toxic

metal ions and organic molecules has beeninvestigated.69,70 At present, ionic liquids have been

used as extraction solvents for the separation of metal

ions by crown ethers. Hence, ionic liquids have got

much more attention towards their use in extraction

processes. Earlier, crown ethers found use in

designing novel solvent extraction systems that areselective for certain metal ions which are based on the

size of the crown-ether rings. Crown ether extract

metal ions from aqueous solutions by complexation.

The efficiency of such extraction processes is strongly

dependent not only on cations but also on counteranions. The extraction process is more favored by

hydrophobic counter anions in aqueous solutions than

Fig. 13 Cross-section of PPy double sided PVDFelectrochemical actuator using polymer in ionic liquids aselectrolyte. [Published from ref. 69, with copyright permission

from Elsevier Limited, UK].


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with hydrophilic anions. Due to low efficiency inusing crown ether for extraction, ionic liquids have

been used as extraction solvents to remove strontium

nitrate from aqueous phase into ionic liquids bycrown ether.71

The distribution coefficients for the

ionic liquid extraction systems are higher thanextraction systems based on organic solvents.

Successful extraction of organic acids from

aqueous solutions into an ionic liquid, 1-butyl-

3-methylimidazolium hexafluorophosphate, has been

reported. The solvation of ionic species (such ascrown-ether complexes, NO3 and SO4) in the ionic

liquids is more favored thermodynamically than in

conventional solvent extractions. The unique

solvation capability of the room-temperature ionic

liquid has not been used to full advantage.

In many nuclear plants, large volumes of liquid

waste are produced, called tank waste. This liquid

waste, which may be acidic or basic depending on the

treatment process, contains many long-lived

radioactive products, including137

Cs,129

I,90

Sr and99

Tc and a variety of transuranic elements, which are

normally -emitters.72 The 137Cs+ and 90Sr2+ in tankwaste are normally present at low concentrations in a

large volume of liquid and any treatment process for

this waste must therefore produce a significant

reduction in the volume of this liquid. So to extract

these

radioactive products, ionophores such ascalix[4]arene-bis(t-octylbenzocrown-6, BOBCalixC-6

and dicyclohexano-18-crown-6, dissolved in

hydrophobic solvents, including hydrophobic ionic

liquids have been used.73

Hence, electrochemistry can

play a useful role in this recycling process. Thisproposed electrochemical process for remediation of

the extraction solvent preserves both the ionophore

and ionic liquids.

ConclusionsFrom the above discussion, it is clear that ionic

liquids can be very effective solvent media forobtaining optimum output in several applications with

minimum possible environment pollution. It is hoped

that demerits of ionic liquids such as high viscosity

will be tackled by chemical or physical means to

enable them to emerge as very powerful solvents. It is

also hoped that simple semi-empirical models if

developed, will be able to help many chemical

engineers to predict thermal and transport properties

of ionic liquids on the basis of the information on the

structures of ionic liquids.

AcknowledgementNDK thanks CSIR, New Delhi, for providing a

research fellowship to carry out this work. AK thanks

DST, New Delhi, for providing financial assistance inthe form of many research grants-in-aid during which

this work was carried out.

Nomenclature

Abbreviation Name[MMIM] 1,3-Dimethyl imidazolium

[EMIM] 1-Ethyl-3-methyl imidazolium[BMIM] 1-Butyl-3-methyl imidazolium[HMIM] 1-Hexyl-3-methyl imidazolium[OMIM] 1-Octyl-3-methyl imidazolium

[BMPyrr] 1- Butyl-1-methyl pyrrolidinium[HMPyrr] 1-Hexyl-1-methyl pyrrolidinium

[OMPyrr] 1-Octyl-1-methyl pyrrolidinium[BP] 1-Butyl-pyridinium[HP] 1-Hexyl-pyridinium

[OP] 1-Hexyl-pyridinium[4-MBP] 1-Butyl-4-methyl pyridinium

[TBP] Tetrabutyl phosphoniumBF4 Tetrafluoroborate

PF6 HexafluorophosphateNTf2 Bis(trifluoromethane sulfonyl)imideC2H5SO4 Ethyl sulfateCF3CO2 Trifluoromethyl acetate

CF3SO3 Trifluoromethyl sulfate(C2F5SO2)2N Bis(pentafluoroethanesulfonyl)imide[Ala] Alanate[Val] Valinate

ClO4 Perchlorate Excess viscosity

Vf2 Volume fraction of ionic liquids

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Ionic Liquids - New Materials With Wide Applications (2010) IJCA 49A(05-06) 635-648