Influence of the hydrocarbon chain length of imidazolium-based ionic liquid on the dispersion and stabilization of double-walled carbon nanotubes in water

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Influence of the hydrocarbon chain length of

imidazolium-based ionic liquid on the dispersion and

stabilization of double-walled carbon nanotubes in water

Kenza Raiah, Abdelkader Djalab, Amel Hadj-Ziane-Zafour, Brigitte Soula,

Anne-Marie Galibert, Emmanuel Flahaut

To cite this version:

Kenza Raiah, Abdelkader Djalab, Amel Hadj-Ziane-Zafour, Brigitte Soula, Anne-Marie Galibert, et

al.. Influence of the hydrocarbon chain length of imidazolium-based ionic liquid on the dispersion and

stabilization of double-walled carbon nanotubes in water. Colloids and Surfaces A: Physicochemical

and Engineering Aspects, Elsevier, 2015, vol. 469, pp. 107-116. �10.1016/j.colsurfa.2015.01.015�.

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To link to this article : DOI : 10.1016/j.colsurfa.2015.01.015

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http://dx.doi.org/10.1016/j.colsurfa.2015.01.015

To cite this version :

Raiah, Kenza and Djalab, Abdelkader and

Hadj-Ziane-Zafour, Amel and Soula, Brigitte and Galibert,

Anne-Marie and Flahaut, Emmanuel Influence of the hydrocarbon chain

length of imidazolium-based ionic liquid on the dispersion and

stabilization of double-walled carbon nanotubes in water. (2015)

Colloids and Surfaces A: Physicochemical and Engineering Aspects,

vol. 469. pp. 107-116. ISSN 0927-7757

Any correspondence concerning this service should be sent to the repository

administrator:

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Influence

of

the

hydrocarbon

chain

length

of

imidazolium-based

ionic

liquid

on

the

dispersion

and

stabilization

of

double-walled

carbon

nanotubes

in

water

Kenza

Raiah

_,

_Abdelkader

_Djalab

_,

_Amel

_{Hadj-Ziane-Zafour}

_,

_Brigitte

_Soula

_,

Anne-Marie

Galibert

_,

_Emmanuel

_Flahaut

a_{LaboratoiredeGénieChimique,UniversitéSaadDahlab,RoutedeSoumaa,B.P.270Blida,Algeria}

b_{UniversitédeToulouse,UPS,INP,InstitutCarnotCirimat,118,RoutedeNarbonne,F-31062Toulousecedex9,France} c_{CNRS,InstitutCarnotCirimat,F-31062Toulouse,France}

h

i

g

h

l

i

g

h

t

s

•Synthesisofimidazolium-basedionic

liquids from natural compounds

(fattyacids).

•Stabilization of 50mg/L double

walledcarbonnanotubesinwaterup to20days.

•Dispersibilityincreasedwith

increas-ing the length of the hydrocarbon chain.

g

r

a

p

h

i

c

a

l

a

b

s

t

r

a

c

t

Keywords:

Carbonnanotubes

Imidazolium-basedionicliquids Aqueoussuspensions Stabilization

a

b

s

t

r

a

c

t

Imidazolium-based ionic liquids with a long hydrocarbon chain 1-methyl-1-ethanol-2-alkyl-imidazoliumiodide([M-E-Cn-Im]I,n=13,15and17)wereusedforthedispersionofDWCNTsinwater.

DWCNTssuspensionsobtainedwerestableformorethanamonth,andnosedimentationwasobserved. Thestabilityofthesuspensionswasinvestigated(measurementofopticaldensity,zetapotential, parti-clesize,viscosity,andTEMimages).MonitoringoftheabsorbancebyUV/visspectrophotometryfor20 daysshowedthatatlowconcentration(1mM),thebestsuspensionwasobtainedwiththeionicliquid ([M-E-C15-Im]I).Athigherconcentration(10mM),thedispersionefficiencyincreasedwiththelengthof

thehydrocarbonchain.Thiscouldbeexplainedbythehydrophobicinteractionbetweenthehydrophobic moietiesoftheionicliquidandtheCNTs.Therefore,wewereabletostabilizeDWCNTsusingalow con-centration(1mM)ofimidazolium-basedionicliquidssynthesizedfromnaturalcompounds.Thiswork highlightsthepotentialofimidazolium-basedionicliquidsforthepreparationofaqueoussuspensions ofDWCNTsathighconcentrationwithalimitedamountofaddedsurfactant(50mg/LofDWCNTswith 50mg/Lofionicliquid).

∗ Correspondingauthorat:CNRS,InstitutCarnotCirimat,F-31062Toulouse, France.Tel.:+33561556280.

E-mailaddress:flahaut@chimie.ups-tlse.fr(E.Flahaut).

1. Introduction

Carbonnanotubes(CNTs),describedassuchforthefirsttime in 1991, are still of great interest for thescientific community duetotheirexceptionalintrinsicproperties(electrical,mechanical andthermal),openingaverywidefieldofapplicationsforthese

nanomaterials [1–3]. Though carbonnanotubes have promising applications, their insolubility in water and most organic sol-ventsischallenging.Thisinsolubilityisexplainedbybothintrinsic hydrophobicityandtheirstrongagglomerationcausedbyvander Waalsinteractionsandentanglement[4].Severalstudieshavetried tosolvethisproblembycovalentfunctionalizationmethods[5–7]

orbyadsorptionofamphiphilicagents[8–12].Covalent function-alizationusingoxidizingacidsathightemperaturecausesdefects onthewallsofthecarbonnanotubesandtheirlengthdecreases, thereforetheirelectricalandmechanicalpropertiesaredegraded

[13,14].Non-covalentfunctionalizationbyadsorptionofsurfactant orpolymersanduseofmildultrasonictreatments,centrifugation andfiltrationdoesnotdisturbthep–pstackingorthelengthsof CNTs,thus,maintainingintrinsicpropertiesoftheCNTs[15–17].

Differentkindsofsurfactantshavebeenusedsuccessfullyfor thedispersionofCNTs,suchastheanionicSDS(sodiumdodecyl sulphate)[18,19],cationicCTAB(cetyltrimethylammonium bro-mide)[20,21]andnon-ionicTritonX-100[22].Howeverthenature ofthesurfactantanditsconcentrationhaveanimportantroleinthe dispersionofCNTsinwaterandorganicmedia[23,24].Previous workhasshownthationicliquidshaveabetterdispersing effi-ciencycomparedtoconventionalsurfactants[25–28].Somestudies have been conducted on imidazolium-based ionic liquids: Kim etal.[29]preparedstablesuspensionsusinganimidazolium-based ionicliquid(1-butyl-3-methylimidazoliumtetrafluoroborate)and SWCNTs.SWCNTssuspendedbyimidazolium-basedionicliquid formagel,calledBuckygel[30].Non-covalentfunctionalization ofoxidizedsingle-walledcarbonnanotubesbypoly imidazolium-basedionicliquidswasalsousedfor thesynthesis ofgels[31]. SWCNTsdispersedin imidazolium-basedionicliquids[32]have beenusedascathodeinhigh-powerlithiumbatteries.AMWCNTs suspensionwith1-ethyl-3methylimidazoliumtetrafluoroboratein dimethlformamidewasusedas animmunosensorof myeloper-oxidasedetectioninhumanserum[33].Amagneticallysensitive materialwaspreparedwithSWCNTsandamagneticionicliquid (1-butyl-3-methylimdazolium[FeCl4]),whichfindsapplicationsin

biomedicalresearch(suchastargeteddrugdeliveryandcontrolled release,proteinseparation,cancertreatment)[34].Amethodwas developedtodecorateMWCNTswithnoblemetalparticles(gold) by themeansof ionicliquid 1-hexadecyl-3-methylimidazolium bromide[27].Yang etal.[35] performeda quantitative charac-terizationbyUV/visspectrophotometryofSWCNTssuspensions prepared inionic liquid 1-N-butylmethylimidazolium hexafluo-rophosphate.

DiCrescenzoetal.[26]haveshownthattheionicliquidwith alonghydrocarbonchain1-hexadecyl-3-vinyl-imidazolium bro-mide formed a stable homogeneous aqueous dispersions with SWCNTsabove its critical micelleconcentration. Liu et al. [25]

reportedthattheyobtainedstableaqueoussuspensionsof MWC-NTswithionicliquid-typeGeminiimidazolium([Cn-C4-Cn-Im]Br2, n=12,14)evenatlowconcentration(1mM).Thebinarymixture of 1-tetradecyl-3-methylimidazolium chloride/ethylammonium nitratesuccessfullydispersedMWCNTs[36].Theionicliquidwith polycyclicaromaticfractionandalonghydrocarbonchain– [n-(N-carbazole)alkyl]-3-methylimidazoliumbromideyieldedgood CNTssuspensionsinwater(10months,at1mM),showingthe com-binedeffectsofbothahydrocarbonchainandacarbazolemoiety

[37].Theadsorptionoftheimidazolium-basedionicliquidonto SWCNTswasexplainedbytheirinteractionwithlowvanderWalls forces. Therefore,the electronicpropertiesof SWCNTremained unmodified[38].Madriaetal.[39]haveshownthat 1-methyl-3-alkoxyalkyland1-methyl-3-fluoroalkylimidazoliumarerelatively nontoxictohumanhealth.Riduanetal.[40]investigatedtheuse ofimidazoliumsaltsinseveralareasofbio-applications,including antitumor, antimicrobial,antioxidant and bioengineering. Imid-azolium ionic liquids can be regarded as chemical compounds

exhibitingapotential,slighttoxicitytothegrowthand develop-mentoftheearlydevelopmentalstagesofhigherlandplants[41]. Ionicliquidsproducedfromthebiocompoundscholineandamino acidswerefoundtohavelowtoxicitytohumansandtothe envi-ronment[42].

Inthiscontext,thegoalofthisworkwastodisperseand sta-bilize DWCNTsin water, using imidazolium-basedionic liquids synthesizedfromnaturalmaterialnamelyfattyacids.We investi-gatedtheabilityof1-methyl-1-ethanol-2-alkyl-imidazolineiodide todispersetheDWCNT,aswellastheeffectofthelengthofthe hydrocarbonchainandconcentration.Inordertocomparethe effi-ciencyof thethree synthesizedionic liquidswe monitoredthe concentrationofCNTsversustime(opticaldensitymeasurements); wemeasuredthevaluesofzetapotentialandviscosityandalso used staticlight diffusiontoestimatethesize ofagglomerates. Finally,transmissionelectronmicroscopyobservationswerealso performedinordertoevidencede-bundlinginthepresenceof sur-factant.

2. Materialsandmethods

2.1. Materials

Double-walledcarbonnanotubes(DWCNT)weresynthesized attheCIRIMATby catalyticchemical vapourdeposition(CCVD)

[43].AfterextractionbytreatmentwithaconcentratedHCl aque-oussolution(removalofcatalystsupportandunprotectedmetal nanoparticles),theCNTwererecovered,washedandkeptinasmall volumeofdeionizedwater.TheDWCNTthuspreparedarecalled “rawwettubes”.Myristicacid(99%),palmiticacid(99%)andstearic acid(99%),N(2-hydroxyethyl)ethylenediamine,ethyliodide,were purchasedfromSigma–Aldrich.

2.2. Synthesisandcharacterizationofionicliquids

Imidazolium-basedionicliquids([M-E-Cn-Im]I,n=13,15or17)

weresynthesizedbythereactionbetweenthecorresponding nat-uralfattyacidwithN(2-hydroxyethyl)ethylenediamine(Fig.1). Allthereagentswereinthesamemolarratio.Themixturewas heatedintoluenefor8hwithstirringandthewaterformedwas removedazeotropicallyusingaDean–Starkapparatus.Resulting productwascooledandfilteredandthesolventevaporated.The solidwasprecipitatedbyadding1-butanolandthenrecrystallized fromethanol.Theprecipitatedsolidandethyliodidewereheated byrefluxinginisopropanolfor4h,theresultingproductwas fil-teredandthesolventevaporated.Thesolid wasprecipitatedby addingpetroleumether[44,45].Weobtainedagoodyield(91.5%)in agreementwiththosealreadypublished((89%)[46],(92.1%)[44]). Thecharacterizationofthesynthesizedproductswascarriedoutby FT-IR,GC–MS,1_HNMRand13_{CNMRanalysis,andbymeasurement}

ofCMC(criticalmicelleconcentration).

TheFouriertransmissioninfra-red(FT-IR)measurementswere madeonPerkinElmerSpectrometeroperatingwithUATR (Univer-salattenuationtotalreflectance),spectralrangebetween4000and 650cm−1,at4cm−1ofresolution.Thegaschromatographycoupled

withmassspectrometry(GC–MS)analysiswasperformedwitha PerkinElmerClarus500,RTX-5MSequippedwithacapillaryGC column(60m,0.25mm,0.25mm),andthesystemwasheatedfrom 100to290◦Cwithaheatingrateof8◦C/minandkeptat290◦Cfor

10min.Thetemperatureoftheinjectoranddetectorwere250◦C

and200◦C,respectively.Thecharacterizationsby1HNMRand13C

NMRwerecarriedoutusingaBrükeradvance(500MHz)equipped with a TCI probe type (Triple CryoprobeInverse); the samples weredissolvedinCDCl3.Thedeterminationofthecriticalmicellar

Fig.1. Generalschemeforthesynthesisreactionsoftheionicliquids.

concentration (CMC) was performed by conductivity measure-ments(Hannainstruments,EC215).

TheFT-IRspectraofthethreeionicliquidsareshowninFig.2. Thepresenceofthebandat2922cm−1 correspondstoCH₃ and

CH2groups.Thepresenceofastrongbandat3392cm−1indicates

theformationofprimaryamine.Inaddition,theappearanceofa bandat1642cm−1indicatestheformationoftheamide(C N)and

isanevidenceoftheformationoftheimidazolinering[46].Mass spectradatawereanalysedusingtheNISTMSSearchlibrary,and thefollowingspectralprofileswereobtainedwithMS(m/z%):74 (99),84(92),87(77),43(61),56(58),44(45),128(43),41(39),55 (32)and141(26)(Fig.3)[46].

Thecharacterizationby1_{H NMRhasgiventhespectrashow}

in Fig. 4. Identification of the peaks is: ı 3.04ppm (t, 2H, C N C CH2 N ),ı3.38ppm(t,2H, CH2 C OH),ı3.71ppm

(t,2H, C N CH₂ ),ı2.21ppm(t,2H, N C CH2 ).InFig.5are

shown the13_{CNMR spectra,indicatingthepresenceofpeaksı}

173.80ppm( N C N )andı60.48ppm( C OH).Themain dif-ferencebetweenthethreespectraisaround3ppmandcorresponds toimpurities(residuesofreactants);thesignalsexpectedforour compoundsarewellidentified,although1_{HNMRdoesnotgiveany}

informationaboutthealiphaticchainlength.

Fig.6showsthevariationoftheelectricalconductivityasa func-tionoftheconcentrationforthethreeionicliquids.Thesecurves allowedustodeterminethevaluesoftheCMCforeachionicliquid. TheCMCwascloseto0.8mMforthethreecompounds(theCMC istheconcentrationatwhichtheslopechanges,itwasdetermined usingthefirstderivative).

2.3. Dispersionofcarbonnanotubes

DWCNTsuspensionswerepreparedindeionizedwateralone. Foreachionicliquid,astocksolutionwasalsopreparedin deion-izedwater,undermagneticstirringfor4h,inordertoensurethe dissolutionofeachsurfactant.Themixtureofthetwostock dis-persion/solution(DWCNTandsurfactant)wasdesignedtoobtain 10mlsuspensionsofDWCNTataconcentrationof50mg/L,and thethreeionicliquidsatconcentrationsequalto1,1.5and10mM. DWCNTsuspensions obtainedweremixedinanultrasonicbath (BioblockT570,35KHz,160W)for10min,thenusinganultrasonic probe(VibraCellModel75042,20kHz,500W)for30minat25% amplitudewithapulseof5sonand5soff,inordertoavoid over-heating.Ourchoiceofahighconcentrationof50mg/Lisjustified byfurtherworkrelatedtotheenvironmentalimpactassessmentof suchsuspensionswhichwillberequiredinordertocomparethe performanceoftheseImidazolium-baseddispersingagentswith

ourearlierworkwithDWCNTsinvolvingcellulosederivativessuch ascarboxymethylcellulose[47,48]orxanthanderivatives[17].

2.4. Characterizationofsuspensions

DWCNTsuspensionswerecharacterizedbyUV/visabsorption (Varian,Cary300).Theabsorbancemeasurementswereperformed from400to900nm,immediatelyafterpreparationandfollowing 20-day sedimentation. Zetapotential measurements were per-formedwithzetasizer (ModelZEN3691,MalvernInstruments), usingsuspensionscentrifugedat1600rpmfor30min.The super-natant wasseparated andthen lefttosettledown for24h.No sedimentationcouldbeobservedwithin24h.Therateofviscosity versusshearratewasdeterminedwitharheometer(AntonPaar Physica,Germany),operatingwithaconcentriccylinder,and con-nectedtoawaterbathinordertoensureaconstanttemperature of25◦C.Sizemeasurementsofdispersedcolloidswereperformed

bythetechnique ofstaticlight diffusion(SZ-100,Horiba Scien-tific),thesizesweremeasuredfor48hatregulartimeintervals. DWCNTsuspensionswerealsocharacterizedbytransmission elec-tronmicroscopy(JEOL1400TEM),operatedat120kV.Thesamples werepreparedbypipettinganddroppingafewdropsof suspen-sionon400meshLaceyCarboncoppergrids,purchasedfromAgar Scientific.

3. Resultsanddiscussion

Characterization of DWCNT suspensions was performed by UV/vis spectrophotometry. Although individual CNTs may be UV–visactiveinthemeasurementrange(400–900nm),thereis noabsorptionbandforbundledCNTs[12,25].Fig.7representsthe absorbanceoftheDWCNTsuspensionsat50mg/Lwiththethree ionicliquids([M-E-Cn-Im]I,n=13,15and17)ataconcentrationof

10mM,aswellaspreparedindeionizedwateronly(nosurfactant). Althoughwedidnotdeserveanymaximumabsorbanceband cor-respondingtoCNTs,theeffectoftheadditionofasurfactantwas ratherclearintermsofincreasedabsorbance.Astheionicliquids donothaveanyabsorptionbandwithintheanalysedrange,the absorptioncanonlybeattributedtoCNTs.

Thevariationoftheabsorbanceasafunctionoftime(20days) fortheDWCNTsuspensionspreparedwiththethreeionicliquids atamolarconcentrationof1mM,1.5mMand10mMisshownin

Fig.8.Theabsorbancemeasurementswereperformedat500nm

[22,49].Accordingtoourdata,at1and1.5mMconcentrations,we obtainedanalmostidenticalabsorbancewiththetwoionicliquids [M-E-C15-Im]Iand[M-E-C17-Im]I.Thelowconcentrations(1mM

Fig.2.FT-IRspectraof(A)[M-E-C13-Im]I,(B)[M-E-C15-Im]Iand(C)[M-E-C17-Im]I.

Fig.3.Massspectraof(a)[M-E-C15-Im]I,comparedto(b)datafromNISTMSSearchlibrary(asanexample).

Fig.5.13_{CNMRspectraof[M-E-C}

15-Im]I(asanexample).

Fig.6. Variationoftheelectricalconductivityasafunctionoftheconcentrationofionicliquid.

Fig.7.UV/visspectraofionicliquids[M-E-C13-Im]I,[M-E-C15-Im]I,[M-E-C17-Im]Iat10mMandDWCNTsuspensionswithionicliquids[M-E-C13-Im]I,[M-E-C15-Im]I,

Fig.8. UV/visabsorbanceintensityversustimeofDWCNTs(50mg/L)inionicliquids(a)[M-E-C13-Im]I,(b)[M-E-C15-Im]I,(c)[M-E-C17-Im]Iand(d)deionizedwateralone

ataconcentrationof(A)10mM,(B)1.5mMand(C)1mM,upto20days.

and1.5mM)arejust abovethecritical micelleconcentrationof thethreeionicliquids(CMCisequaltoca.0.8mM,evaluatedfrom electricalconductivitymeasurements)anddidnotleadtogood dis-persionandstability.Thisphenomenoncanbeexplainedbythefact thatthedispersionofthenanotubeswiththeamphiphilesisonly

possibleabovethelimitoftheCMC[50,51].Thus,thedispersing poweroftheionicliquidsclosetotheCMCwaslowcomparedto theconcentrationof10mM(10timesmore).

Attheconcentrationof10mMofionicliquid,theabsorbance of suspensions increased with increasing the length of the

Fig.9. Variationoftheviscosityversusshearrateforthethreeionicliquidsof1mMandDWCNT(50mg/L)suspensionswiththethreeionicliquids.

hydrophobicchain. Thebestvalue wasobtainedwiththeionic liquid[M-E-C17-Im]I.Similarlytheabsorbanceofthesuspension

preparedwith[ME-C15-Im]Iwashigherthantheoneprepared with[ME-C13-Im]I.Thethree ionicliquidshavea homologous structure.Theonlydifferenceliesinthelengthofthehydrocarbon chain(C=13,15and17).Theincreaseindispersingefficiencywith increasingthelengthofthehydrocarbonchaincanbeexplained bytheincreaseinhydrophobicinteractionbetweenthe hydrocar-bonchainsofionicliquidsandDWCNTs[25,52](generally,because CNTshaveaveryhydrophobicsurface,thehydrocarbonchainstend tobestronglyadsorbed[53,54]).Abetterinterfacebetweenthe hydrophobicchainandthenanotubesmeansahigherstabilityof theadsorptionandlesspossibleexchangeswiththesurrounding solvent.Inthiscomparison,thereisprobablynomodificationof thesterichindranceduetothesmalldifferencesintermsofcarbon atomsinthealkylchain.Takingintoaccountthefactthatthecharge iscomingfromexactlythesamechemicalgroupfortheseriesof ionicliquids,theelectrostaticrepulsionisnotexpectedtobe differ-ent.Ifthedifferenceinchainlengthwasenoughtoleadtodifferent ratesofcoverageontothenanotubes,thisshouldfinallyleadtoa

differentnumberofchargesandthiswouldfinallybeinfavourof theshorteralkylchainionicliquid(C13).Consequently,weassume thatthekeyparameteristhestabilityoftheinteractionbetween theadsorbedionicliquidandthenanotubes,whichisinfavourof thelongestalkylchain.

Themeasurementofzetapotentialisimportanttoexplainthe stabilityofthecolloidalparticlesinaqueoussuspension[28,55].WE measuredthezetapotentialofDWCNTsuspensionusingthethree ionicliquidsattheconcentrationof1mMandobtainedzeta poten-tialvalues largerthan40mV.AccordingtoDong etal.[37] the particleswithazetapotentiallargerthan15mVinabsolutevalue areconsideredtobestabilizedbyelectrostaticrepulsions.Ahigh positivevalueofzetapotentialindicatesthestrongadsorptionof theionicliquidtothesurfaceofDWCNTs,leadingtostable suspen-sionsduetoCoulombforcebetweentheCNTs[25].Therefore,ahigh stabilityofDWCNTsuspensionswasevidenced.Table1illustrates thevaluesofzetapotentialoftheDWCNTsuspensionswiththe threeionicliquids.Thelargestvalueofzetapotentialwasobtained withtheDWCNTssuspensionpreparedwiththeionicliquid [M-E-C15-Im]I,althoughitisquitesimilartothevalueobtainedfor

Fig.11.TEMmicrographofDWCNTsuspensionsindeionizedwaterat50mg/Lwithionicliquidat1mM(a)[M-E-C13-Im]I,(b)[M-E-C15-Im]I,(c)[M-E-C17-Im]Iand(d)

deionizedwateralone.Thescalebar(50nm)isidenticalforallfourimages(samemagnification).

Table1

ZetapotentialofDWCNTsuspensionsforthethreeionicliquids(1mM). DWCNTsuspensionswithionicliquid Zetapotential(mV)

[M-E-C17-Im]I 60.6

[M-E-C15-Im]I 66.0

[M-E-C13-Im]I 45.0

[M-E-C₁₇-Im]I.Thesevaluesarehoweversignificantlyhigherthan thezetapotentialmeasuredfor[M-E-C13-Im]Iinthesame

exper-imentalconditions.

Wewereabletostabilize50mg/LofDWCNTsusinga50mg/L ofimidazolium-basedionicliquidscomparedtousing1000mg/L ofionicliquid-typeGeminiimidazoliumforsuspending200mg/L ofMWCNTs[25].

Thevariationof theviscosityversus shearrateforthethree ionicliquidsandtheDWCNTsuspensionswiththethreeionic liq-uidsisshowninFig.9.Wehaveobservedalowviscosityofthe DWCNTsuspensionswiththethreeionicliquids;thislowviscosity canbeexplainedbybetterchargerepulsionbetweenhydrophobic groups(thehydrocarbonchains)orientedalongCNTs,whenthe hydrophilicheadsaredirectedtowardstheaqueousphase,which preventstheagglomerationoftheCNT[17,56,57].

InordertodemonstratethestabilitythroughtimeoftheDWCNT suspensions,sizemeasurementsofdispersedcolloidswere per-formedbythetechniqueofstaticlightdiffusion.Forthispurpose, suspensionsof carbonnanotubeswerepreparedwiththethree

ionicliquidsat1mMasstabilizers.Thesizesweremeasuredfor 48hatregulartimeintervals(Fig.10).

Measurementsofparticlesizeforthethreesuspensionsof DWC-NTswitheachionicliquidclearlyshowthatCNTsareintheform ofverysmallagglomerateswithanaveragediameterbetween150 and250nm.Wemeasuredmonomodalparticlesizedistributions andapolydispersityindexof0.068(thevalueofthisindexgives information about monodisperse systems, for which the index shouldbebetween0.05 and0.08).Thesizeoftheagglomerates inthesuspensions ofDWCNTswaspracticallystableduringthe 48hofmeasurementandespeciallyafter20h.

Transmissionelectronmicroscope(JEOL1400)imagesof DWC-NTssuspensionspreparedwiththreeionicliquids,aswellasthe onespreparedwithdeionizedwateralone,areshowninFig.11. These images showthat thethree DWCNTs samplesstabilized byionicliquidsformedsmallerbundlescomparedtoDWCNTsin deionizedwateralone.Therefore,theuseofionicliquidslimited theagglomerationofDWCNTs.However,it isimportanttonote thatTEMobservationswereperformedafterdryingthesamples onTEMgrids,sotheimagesareprobablynotshowingthereal sit-uationinthesuspensionsinthepresenceofliquid(agglomeration upondryingduringsamplepreparation).

Finally,DWCNTshaveamorphologyveryclosetoSWCNTsand sharewiththemtheabilitytofromlongandflexiblebundles.In ourDWCNTssamples,thebundles are notvery large(typically 20–30nmmaximum)andthesamplescontainmanyindividual(or atleastindividualpartoftheirlength).However,theyarethefirst

multi-walledcarbonnanotubesoftheseriesandthusalsoshare withlargerMWCNTsthefactthattheouterwallprotectsallthe innerones.Astheinteractionwithasurfactantmoleculeistaking placeonlyattheinterface,theouterwallistheonlytobeconcerned. DWCNTsarethusatthefrontierbetweenSWCNTsandMWCNTs ingeneralandforthisreasonwecanexpectthattheresultsofthis studycanextrapolatedtoallotherkindsofCNTs,aslongasthey frombundles(whichisstillthecaseformanykindsofMWCNTs). However,itisclearthatresultsmaynotbeextrapolatedtomuch shorterandindividualCNTs,whichcouldoccureitherintentionally ifcuttingproceduresareapplied(SWCNTs)orevenunintentionally inthecaseoftoostrongsonicationconditionsformostMWCNTs. 4. Conclusion

Imidazolium-basedionicliquids 1-methyl-(1-ethanol)-2-alkyl-imidazoline([M-E-Cn-Im]I,n=13,15and 17)weresynthesized

fromthecorrespondingnaturalfattyacidsandshowntobeable toeffectively suspend carbonnanotubes inwater, even atvery low concentration (1mM), for a period ofmore than a month. Quality of suspension was evidenced by optical density mea-surement,ofzetapotential,particlesizeandTEM.Theresultsof these analyses evidenced that we obtained stable suspensions, andalsomanagedtopartiallyde-bundletheDWCNTs. Compar-ingthedispersing strengthof thethreeionicliquids,we found thatthedispersibilityincreasedwithincreasingthelengthofthe hydrocarbonchain,whichisexplainedbytheincreased hydropho-bicinteractionsbetweenthehydrocarbonchainandDWCNTs.In addition,thesuspensionsofDWCNTswithionicliquidshada vis-cositysimilartothatof water,which is suitablefor interaction withbiologicalenvironments.Moreover,theseresultsencourage the suspension of DWCNTs using imidazolium-basedionic liq-uids,forpossibleapplicationinthestudyof toxicity/ecotoxicity ofcarbonnanotubes,thankstotheirexpectedlowintrinsic toxic-ity(whichishoweverstilltobemeasuredforourseriesofnew compounds).Acknowledgments

TheauthorsexpresstheirsincerethankstotheMinistryofHigh EducationandScientificResearch(Algeria)andtheFrenchMinistry ofForeignandEuropeanAffairsforfinancialsupportofPHC-TASSILI 10MDU797project.

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