High pressure CO2 absorption studies on imidazolium-based ionic liquids: Experimental and simulation approaches

ContentslistsavailableatSciVerseScienceDirect

Fluid

Phase

Equilibria

j o ur na l ho me p a g e :w w w . e l s e v i e r . c o m / l o c a t e / f l u i d

High

pressure

CO

2

absorption

studies

on

imidazolium-based

ionic

liquids:

Experimental

and

simulation

approaches

Ferdi

Karadas

a

_,

_Banu

_Köz

b

_,

_Johan

_Jacquemin

c

_,

_Erhan

_Deniz

a

_,

_David

_Rooney

c

_,

_Jillian

_Thompson

c

_,

Cafer

T.

Yavuz

d

_,

_Majeda

_Khraisheh

a

_,

_Santiago

_Aparicio

e,∗

_,

_Mert

_Atihan

a,∗∗

a_{DepartmentofChemicalEngineering,QatarUniversity,Doha,Qatar}

b_{DepartmentofEnergySystemsEngineeringKaramanogluMehmetbeyUniversity,Karaman,Turkey}

c_{SchoolofChemistryandChemicalEngineering,Queen’sUniversityBelfast,Belfast,NorthernIreland,UnitedKingdom} d_{GraduateSchoolofEEWS(WCU),KoreaAdvancedInstituteofScienceandTechnology(KAIST),Daejeon,RepublicofKorea} e_{DepartmentofChemistry,UniversityofBurgos,Burgos,Spain}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received24May2012

Receivedinrevisedform6October2012 Accepted8October2012

Available online 2 November 2012 Keywords: Ionicliquids High-pressure Carbondioxide Solubility Imidazolium Moleculardynamics

a

b

s

t

r

a

c

t

Acombinedexperimental–computationalstudyontheCO2absorptionon1-butyl-3-methylimidazolium

hexafluophosphate, 1-ethyl-3-methylimidazolium bis[trifluoromethylsulfonyl]imide, and 1-butyl-3-methylimidazoliumbis[trifluoromethylsulfonyl]imideionicliquidsis reported. Thereportedresults allowedtoinferadetailednanoscopicvisionoftheabsorptionphenomenaasafunctionofpressure andtemperature.Absorptionisothermsweremeasuredat318and338Kforpressuresupto20MPa forultrapuresamplesusingastate-of-the-artmagneticsuspensiondensimeter,forwhich measure-mentproceduresaredeveloped.AremarkableswellingeffectuponCO2absorptionwasobservedfor

pressureshigherthan10MPa,whichwascorrectedusingamethodbasedonexperimental volumet-ricdata.Theexperimentaldatareportedinthisworkareingoodagreementwithavailableliterature isotherms.Soave–Redlich–KwongandPeng–Robinsonequationsofstatecoupledwithbi-parametric vanderWaalsmixingrulewereusedforsuccessfulcorrelationsofexperimentalhighpressure absorp-tiondata.Moleculardynamicsresultsallowedtoinferstructural,energeticanddynamicpropertiesof thestudiedCO2+ionicliquidsmixedfluids,showingtherelevantroleofthestrengthofanion–cation

interactionsonfluidvolumetricpropertiesandCO2absorption.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Ionicliquids(ILs)aresaltsexistingintheliquidstateatroom temperature,consistingoflargeasymmetricorganiccationsand anions,which areresulted in low lattice energy and moderate coulombicforces[1,2].Thevariabilityofcationsandanionsleading toILsallowstailor-madeILswithdesiredphysicalandchemical propertiesofinterest[3,4].MostmajorcharacteristicsofILsare theirlowvolatilityandhighionicconductivitytogetherwiththeir goodthermalandelectrochemicalstability[5–7],forwhichthey haveattractedacademicandindustrialinterest[8–12]in applica-tionssuchassolventsandcatalystsforchemicalreactions,andflue gasseparationagentsforchemicalandotherindustrialprocesses

[13,14].

Compared to traditional organic solvents, the non-volatility natureofILsmakesthembenignsolventsforgastreatmentand

∗ Correspondingauthor.Tel.:+34947258062;fax:+34947258831. ∗∗ Correspondingauthor.Tel.:+97444034142;fax:+97444034131. E-mailaddresses:[email protected](S.Aparicio),[email protected] (M.Atihan).

separationprocesses[15].ILsarereportedforhighsolubilitiesof waterandCO2comparedtoconventionalorganicsolvents[16–19].

DuetosolubilitydifferencesbetweenCO2andothergasessuchas

N2,O2,andCH4,ILshavestartedtogaingreatacademicand

indus-trialinterestforseparationofCO2fromfluegasornaturalgasatpre

orpostcombustionprocessstreams[14,20,21].Literaturecontains alargenumberofarticlesonthesolubilityofCO2inILs[18,22–30],

whichshowtheinterest,bothinindustryandacademia,onthis ILsapplication. Experimental studieson determiningCO2

solu-bilities in selected ILshave been widely investigated in recent years[27,31–40].Inparticular,classicimidazolium-basedILshave beenstudiedcomprehensivelyfortheirCO2captureperformances

[41–43].Blanchardetal.[44]reportedfortheveryfirsttimethe propertiesofimidazolium-basedionicliquidsasCO2 absorbents,

showingthenullsolubilityofionicliquidsinCO2incomparison

withtheremarkablesolubilityofCO2 in theionic liquidphase.

ZhangandChan[45]reviewedtheavailableliteratureontheuseof imidazolium-basedionicliquidsinsustainablechemistry, includ-ingCO2 capture.Huang etal. [46]analyzedtheCO2 absorption

mechanism in imidazolium-based ionic liquids from molecular dynamicssimulationsresultsthroughthespatialrearrangementof availablefreevolumetoaccommodateCO2moleculesbutwithout

0378-3812/$–seefrontmatter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fluid.2012.10.022

remarkablechangesinanion–cationinteractions.TheCO2

absorp-tionisstronglydependentonaniontype,andinaminorextension inalkylchain lengthsinimidazoliumrings[46].Brennecke and coworkers[47]analyzedtheanioneffectonCO2solubility,

show-ingincreasingabsorptionwithanionfluorination.Nevertheless,the availableresultsshowlowCO2 captureabilityfor

imidazolium-basedILs,i.e.upto3.5mol%atambienttemperatureandpressure

[45], which is clearly insufficientfor industrial purposes, espe-ciallyforfluegasestreatment.Therefore,neweffortshavebeen developedintheliteraturetoimproveCO2capturingabilityusing

imidazolium-basedionic liquids.Severalauthorshaveproposed theuseofamine-functionalizedimidazolium-basedionicliquids, whichleadstochemicalabsorption,andthusshowingremarkable increaseinCO2solubilitybutalsoimportantproblemssuchastheir

veryhighviscosityandhighenergypenaltiesforthedesorption processes[48–51].

ExperimentalstudieshaveshowedthattheCO2solubilityinILs

increaseswithpressureanddecreaseswithtemperature[39,52]. Nevertheless, mostof the data available areat atmospheric or low-pressureconditions(e.g.lowerthan10MPa)[14].Themain reasonforthisscarcityinhighpressureabsorptiondatarisesfrom thetechnicaldifficultiestocarry out thesemeasurementswith acceptableuncertainties.HighpressuredataonCO2captureisalso

importanttotestpossiblesorbentmaterialsonbothpreandpost combustionCO2capturepurposesforindustrialscaleapplications.

Pre-combustionCO2 conditions are setathigher pressures,e.g.

awater–gasshiftreactionproducesamixtureofH2 (61.5%)and

CO2(35.5%)at30bar[53].Pipelinecompressionofcrudenatural

gasreaches[54]uptopressuresinexcessof175barwhileCO2

transportforsequestrationdemands[55]pressuresof150bar. Liq-uefiednaturalgas(LNG) stacktemperatureshave tobelimited toa maximum of 180◦C byregulations[56].Ideally, a sorbent shouldtolerate all theseconditions (0–175barand 40–180◦C), whileretainingitsCO2capacity.Mostgasadsorptionand

absorp-tion(sorption) experiments are measuredwitha volumetric, a gravimetricorcombinedvolumetric/gravimetricequipment.There arealsosomeotherexperimentaltechniques,whichdonotget seri-ousinterestduetolimitations,accuracyandrepeatabilityproblems

[57]. For measurementsat conditions similartoprocess condi-tionsthevolumetricmethodhassomelimitations,becauseonly atmosphericconditionsareexperimented.Recently,Rubotherm®

state-of-the-artmagneticsuspensionsorptionequipmenthasbeen usedforILsand CO2 solubilitymeasurements[24,58–60].With

thisapparatus,measurementscanbeperformedfrom253to523K forpressuresupto35MPa,withsuitableaccuracyinthewhole pressure–temperatureranges.

Animportantproblemrisingin thestudyofthermophysical behaviorofILsstandsonthepurityoftheusedsamples,which hasastrongeffectonILsproperties[13].Manyavailablestudies onCO2 absorptioninILswere carriedout withsamplesof not

clarifiedpurity.Moreover,Freireetal.[61]reportedthe hydrol-ysisof thecommonhexafluorophosphateand tetrafluoroborate anions,whichwereusedinmanyCO2 absorptionstudiespaired

withimidazolium-basedcations.Thereforetheuseofultrapureand properlycharacterizedILssamplesisrequiredtoobtainreliable results[62].

Moleculardynamics simulationsis a powerfultool tostudy nanoscopicbehaviorofcomplexsystemssuchasthosecontaining ILs+CO2.Deschampsetal.[63]studiedtheCO2+[bmim][PF6]

sys-tem,pointingtoCO2moleculesnon-interactingremarkablywith

C2positioninimidazoliumringandplacedpreferentiallynearthe anion.Cadenaetal.[29]analyzedtheCO2+[bmim][PF6]system

showingthesmallvolumeexpansionandthenegligiblechanges inthe ionicliquid structure upon CO2 absorption.Huang et al.

[46]studiedtheCO2+[bmim][PF6]systemanalyzingthe

molec-ularlevelreasonsofthelowvaluesforCO2partialmolarvolume,

andshowingthatCO2moleculesoccupycavitiesrisingfromsmall

angularrearrangementsoftheanionswithoutremarkable expan-sion.Bhargavaet al.[64] studiedtheCO2+[bmim][PF6] system

showingaremarkablevolumeexpansionandspecificinteraction between CO2 molecules and [PF6]− anions; these authors also

reportedthattheCO2 solvation in[bmim][PF6] isprimary

con-trolledbytheanion[65].Kerleetal.[66]simulatedthetemperature dependenceofCO2solubilityin[emim][Tf2N]and[bmim][Tf2N],

showingthatthesolvationfreeenergyofCO2 isalmost

insensi-tivetothealkylchainlengthontheimidazoliumcation.Shimetal.

[67]studiedthesolvationstructureofCO2in[bmim][PF6]

show-ingtheappearanceofpreferentialsolvationforadiatomicprobe. Yueetal.[68]carriedoutmoleculardynamicsstudiesofseveral CO2+imidazolium-basedionicliquidsincluding[bmim][PF6]and

[emim][Tf2N],showingthat CO2 moleculesoverlap with[PF6]−

anions around the imidazoliumcation, whereas in thecase of [Tf2N]−systemsnooverlappingisinferred.Ghobadietal.[69]

car-riedoutNPTMonteCarlosimulationsfortheCO2absorptionin

[bmim][PF6]and[bmim][Tf2N]ILs;theauthorsproposedtheuse

ofanewsolubilityindexobtainedfromcalculated intermolecu-larinteractionenergiestoanalyzegassolubility.Zhangetal.[15]

publishedareviewworkrecentlyontheuseofionicliquidsfor CO2capturingpurposesinwhichavailableresultsfrommolecular

modelingarealsoanalyzed.

Theenormousamountofpossibleanion–cationcombinations leadingtoILs[70]makenecessarytocarryoutsystematicstudies ontheirpropertiesandmolecularlevelstructure,whichis espe-ciallyrelevanttofindILswithsuitableCO2propertiesforindustrial

purposesbetweentheplethoraofpossiblecandidates.This objec-tivecanbefulfilledusingacombinedexperimental–computational approach,whichononesidewouldprovidewiththerequiredCO2

absorptiondata,andontheothersidewouldleadtothenanoscopic visionof theabsorptionprocess, andthus,allowingtoobtaina visionoftherelationshipsbetweensorptionabilitiesandfluids’ structuringandproperties.Therefore,theresultsofthecombined highpressureCO2absorptionandmoleculardynamicsstudieson

threeselectedimidazolium-basedILs:butyl-3-methylimidazolium hexafluophosphate, [bmim][PF6], 1-ethyl-3-methylimidazolium

bis[trifluoromethylsulfonyl]imide, [emim][Tf2N], and

1-butyl-3-methylimidazolium bis[trifluoromethylsulfonyl]imide, [bmim][Tf2N],are reported in this work.The objectives of the

workwere:(i)toextendavailableinformationofCO2absorption

data for imidazolium-based ionic liquids to the high pressure regionusingreliableandaccuratestate-of-the-artapparatus,(ii)to usesamplessynthesizedinourlaboratories,beingultrapureand well-characterized,and(iii)toinfermolecularlevelinformation from simulation results that combined with the experimental measurementswillleadtovaluableinformationontheabsorption process.

2. Materialsandmethods

2.1. Materials

AllstudiedILsweresynthesizedandpurifiedin-houseinthe Queen’sUniversityIonicLiquidLaboratories(QUILL)Research Cen-tre.Priortouse,ILsweredriedanddegassedatpressureslower than1Pafor15hat323.15Kwhilstbeingstirred.Afterthis treat-ment,theirhalide contentsweredeterminedby usingAgilent®

suppressedionchromatography(IC), andlithiumcontentofthe samplespreparedfromthelithiumsaltwasdeterminedbyAgilent®

inductively coupled plasma analysis (ICP). Water content was determinedbeforeand aftereachmeasurement byCoulometric Karl–Fishertitrationusinga GRScientifictitrator. 99.99%purity CO2wasused.RelevantpropertiesaresummarizedinTable1.

Table1

Molarmass(M),halidemassfractioncontent(wh−),lithiummassfractioncontent(wLi+),andwatercontentinmassfractions(ww)ofthesamplesusedinthiswork.

M(gmol−1_{) ww×10}−3 _w h−×10−6 wLi+×10−6 Purity(mass%) [bmim][PF6] 284.18 0.135 <15a – [bmim][Tf2N] 419.37 0.055 <5a 0.8 [emim][Tf2N] 391.31 0.012 <5a 2.7 CO2 – – – – 99.995

a_{Bromidemassfractioncontent.}

2.2. Absorptionmeasurements

High-pressuresorptionisothermdataofCO2werecollectedin

amagneticsuspensionbalancemanufacturedbyRubotherm®_that

canbeoperatedupto35MPa.Detailsoftheequipmentaregiven elsewhere[71,72].Pressuretransducers(Paroscientific,US)were usedinarangefromvacuumupto35MPawithanaccuracyof0.01% infullscale.Thetemperaturewaskeptconstantwithanaccuracy of±0.5Kforeachmeasurement(MincoPRT,US).Insitudensity valuesforCO2aremeasuredduringsorptionmeasurementsasitis

necessarytocalculatetheabsorbedCO2amount,anddensityvalues

arecross-checkedwithREFPROP9.0[73]forconsistencypurposes. Absorptionmeasurementswerecarriedoutusing2–3mL sam-ples for the corresponding IL. First the system is taken under vacuumfor24hat60◦C.Carbondioxideisthenpressurizedvia TeledyneIsco260Dfullyautomatedgasboosterandchargedinto thehigh-pressurecell,thenCO2absorptiononthesamplebegins.

Onceequilibriumisreached(_∼45min),4differentsetof measure-mentsaretakenforaperiodof10min;eachdatapointiscollected atevery30s.Attheendofeachpressurepoint,systemgoes, auto-matically,tothenextpressuremeasurementpoint.Inthiswork, pressureupto20MPaisusedformaximumpressure,toprovide fullscaleperformancescan,andattheendofeachisotherm, hys-teresischeckisconductedateachisothermbycollectingdesorption dataasthesystemisdepressurized.

Absorptiondataisanalyzedandtheamountofadsorbedcarbon dioxideonthesampleiscalculatedbyusingEq.(1):

W+Wbuoy,sample+Wbuoy,sink=mads+msample+msink (1)

where W is the signal read by the instrument,

Wbuoy,sample=Vsample×dgas is the buoyancy correction due to

sample,Vsample isthevolumeof thesample, dgas isthedensity

of the gas, Wbuoy,sink=Vsinker×dgas is the buoyancy correction

due to sinker, Vsinker is the volume of the sinker, mads is the

absorptionamount,msampleisthemassofthesample,andmsinkis

themassofthesinker.Themassofemptysinkerwasdetermined atseveralpressuresusingheliumtodetermineWbuoy,sink.Vsinker

wascalculatedfromtheslopeofweightvs.densityplotobtained fromthis measurement. A blankmeasurement atvacuum was, firstly, performedtodeterminemsink.msample is determinedby

performingameasurementatvacuum. 2.3. Moleculardynamicsmethods

ClassicalmoleculardynamicssimulationsforCO2+ILsmixed

fluids were carried out using the MDynaMix v. 5.0 molecular modelingpackage[74].SimulationswereperformedintheNPT ensembleusingtheNose–Hoovermethodtocontrolthe temper-atureandpressureofthesimulationsystem[75].Theequations of motion were solved by Tuckerman–Berne double time step algorithm [76] withlong and short time stepsof 1 and 0.1fs, respectively.TheEwaldsummationmethod[77]wasimplemented fortheCoulombicinteractionswithradiuscut-offof1.5nm.The simulatedsystemsconsistofcubicboxesofpureionicliquidsor ionicliquidswithabsorbedCO2,withthecompositionsreportedin

TableS1(Supplementarydata).Simulationboxeswerebuiltusing

125ionicpairsandanumberofCO2 moleculesfrom14 to125,

tostudycompositioneffectonmixturepropertiesupto equimo-larsystems.Initialboxesweregenerated placingrandomlyions andCO2 moleculesinaFCClatticeatlowdensity(∼0.2gcm−3),

thenNPTsimulationswereperformedattheselectedpressureand temperature,equilibrationwascheckedthroughconstant poten-tialenergy.Afterequilibration,10nsruns(timestep1fs)inthe NPTensembleatthestudiedpressure(2.5MPa)andtemperature (298K)were performedfor theanalysisof systems’properties. Equimolarcompositionforthestudiedpressureandtemperature aresupersaturatedconditions,aswemayinferfromexperimental measurements,Table2;nevertheless,theobjectiveofthe computa-tionalworkistoanalyzemolecularlevelinteractionfeatures,notto predictphaseequilibria.Forcefieldparameterizationforthe stud-iedionicliquidswereobtainedfromtheliterature[78,79],aswell asthoseforCO2molecules[80].

3. Resultsanddiscussion

3.1. CO2absorptionmeasurements

Sorptiondatafrommagneticsuspensionapparatusisgivenin

Fig.1.Reportedresultsshowthatforacommon[bmim]+_cation,

CO2 sorption increases ongoingfrom [PF6]− to[TF2N]− anion,

andforcommon[TF2N]− anionsorptionincreaseswith

increas-ingalkylchainlengthinimidazoliumcation[46,47].Nevertheless, theanioneffectonsorptionabilityismoreremarkablethanthe cationeffect.Temperatureincreaseleadtolowersorption abili-ties,asitmaybeexpected,whereasthepressureeffectseemsto bemorecomplexfromFig.1.Althoughincreasingpressureleadto increasingCO2sorption,itappearsthatforpressureshigherthan

0 40 80 120 160 200

P

/ MPa

0 0.2 0.4 0.6 0.8

x(CO

2

)

[BMIM][PF6] - 318 K [BMIM][PF6] - 338 K [BMIM][TF2N] - 318 K [BMIM][TF2N] - 338 K [EMIM][TF2N] - 318 K [EMIM][TF2N] - 338 K

Fig.1.SwellinguncorrectedhighpressureCO2absorptiondataon[bmim][PF6],

Table2

ExperimentaldataofsolubilityofCO2inthestudiedionicliquids,swellingcorrectedaccordingtomethodproposedinprevioussection.a

[bmim][PF6] [bmim][Tf2N] [emim][Tf2N]

318K 338K 318K 338K 318K 338K

P(MPa) x(CO2) P(MPa) x(CO2) P(MPa) x(CO2) P(MPa) x(CO2) P(MPa) x(CO2) P(MPa) x(CO2)

0.0120 0.0000 0.0120 0.0000 0.0130 0.0000 0.0120 0.0000 0.0120 0.0000 0.0120 0.0000 0.0970 0.0000 0.0980 0.0006 0.1060 0.0000 0.0990 0.0000 0.0963 0.0001 0.0990 0.0000 0.4950 0.0282 0.4951 0.0346 0.5000 0.0589 0.4990 0.0550 0.4957 0.0759 0.4960 0.0326 0.9951 0.0835 0.9953 0.0761 0.9980 0.1461 0.9960 0.1211 0.9989 0.1603 1.0015 0.0990 1.4952 0.1358 1.4951 0.1153 1.4950 0.2211 1.4953 0.1800 1.4950 0.2297 1.4978 0.1597 1.9952 0.1842 1.9951 0.1509 1.9951 0.2845 1.9970 0.2322 1.9960 0.2911 1.9950 0.2128 2.4951 0.2267 2.4949 0.1839 2.4951 0.3409 2.4960 0.2810 2.4956 0.3424 2.4960 0.2588 2.9952 0.2647 2.9950 0.2137 2.9970 0.3902 2.9960 0.3213 2.9974 0.3888 2.9960 0.3015 3.4954 0.2990 3.4947 0.2416 3.4960 0.4324 3.4961 0.3583 3.4951 0.4285 3.4951 0.3400 3.9958 0.3308 3.9952 0.2677 3.9952 0.4715 3.9958 0.3941 3.9959 0.4658 3.9961 0.3741 4.4954 0.3586 4.4951 0.2916 4.4960 0.5042 4.4958 0.4243 4.4959 0.4974 4.4953 0.4044 4.9950 0.3853 4.9950 0.3137 4.9955 0.5344 4.9963 0.4522 4.9960 0.5262 4.9955 0.4333 7.4956 0.4741 7.4949 0.3976 7.4958 0.6428 7.4955 0.5583 7.4957 0.6304 7.4957 0.5403 9.9955 0.5225 9.9949 0.4518 9.9952 0.7095 9.9966 0.6288 9.9955 0.7003 9.9958 0.6132 12.4955 0.5260 12.4947 0.4812 12.4960 0.7282 12.4960 0.6768 12.4961 0.7348 12.4953 0.6670 14.9955 0.5317 14.9942 0.4924 14.9964 0.7399 14.9960 0.7008 14.9962 0.7455 14.9961 0.6977 17.4952 0.5375 17.4916 0.4966 17.4959 0.7479 17.5016 0.7119 17.4958 0.7553 17.4959 0.7139 19.9948 0.5428 19.9909 0.4997 19.9974 0.7562 19.9973 0.7149 19.9965 0.7638 19.9977 0.7226

a_{u(T)=±0.1K;u(P)=±0.01%(fullscale);u(x)=±0.0001.}

10MPa,absorbedamountsofCO2onILsshowdecreasingtrends,

whichisduetotheswellingeffectbeingpreviouslyreportedfor

high-pressureCO2solubilityexperimentsinILs,andinpolymers,

[81–84].Therefore,theswelling-uncorrecteddatareportedinFig.1

shouldbecorrectedconsideringtheswellingeffectrisinguponCO2

absorption,forwhichseveralproceduresarepossible. 3.2. Correctionofswellingeffectonsorptiondata

Swellingphenomenaneedscorrectionfordeterminingabsolute CO2captureperformanceatelevatedpressures[22–24,81].Most

reportedexperimentalsorptiondatahavebeenobtainedatlowor moderatepressures,wheretheCO2solubilityisrelativelylowand

theILswellingcanbeneglected.UncorrectedCO2 highpressure

sorptiondataatelevatedpressures,Fig.1,areanalyzedconsidering Eq.(2).

m=mA+ms+mSK−WSK−WS-A (2)

wheremstandsfortheexperimentalreadings,mAforthemass

ofabsorbedgas(CO2),mSfortheILmass,mSKforthesinkermass,

WSKforthebuoyancycorrectionduetosinker,andWS-A forthe

combinedbuoyancycorrectionduetoionicliquidandabsorbed gas.BuoyancycorrectionsarecalculatedaccordingtoEqs.(3)and (4): WSK=VSKdG (3) WS-A=



mS+mA dS-A



dG=VS-AdG (4)

whereVSKstandsforthesinkervolume,dS-AandVS-Afordensity

andvolume,respectively,ofILsamplewithabsorbedgas.The buoy-ancyeffectduetoILsampleandabsorbedgas,WS-A,changeswith

theamountofabsorbedgasduetotheswellingeffect,with increas-ingamountsofabsorbedgasthefluidexpandsleadingtolarger VS-AvaluesandtolargerWS-Acontributions.AsmAincreaseswith

pressureforisothermalconditions,WS-Awillbemoreimportant

forthehigh-pressureregionwhereasitisalmostnegligibleinthe low-pressureregion.RearrangingEqs.(2)and(5)isobtained:

mA=m−mS−mSK+WSK+WS-A (5)

ThethirdfirsttermsinEq.(5)areexperimentalamounts:

C=m−mS−mSK (6)

The problem rises into the calculation of WS-A through the

determinationofexactvaluesofVS-A.Inthiswork,experimental

swellingdataforthestudiedsystemscomingfromtheliterature is usedto calculatetheVS-A term.Aki et al.[47] reported CO2

absorptionandvolumetricdatafor[bmim][PF6]and[bmim][Tf2N]

ionicliquids.Renetal.[85]reporteddatafor[emim][Tf2N].Aki

etal.[47]reporteddataat313.15and333.15Kfor[bmim][PF6]

and[bmim][Tf2N],andRenetal.[85]at323.15and343.15Kfor

[emim][Tf2N],whereasexperimentalmeasurementswerecarried

outat318and338Kinthiswork.Nevertheless,molarvolumeis reportedinFig.2,Vm,asafunctionofCO2molefractionforthe

twotemperaturesclosetomeasurementsreportedinthis work, showingthatVmdoesnotchangeforthestudiedtemperaturerange

forthethreeconsideredILs.Therefore,experimentalVmdata

com-ingfromtheliterature[47,85]isused,fromwhich,experimental mSwererecalculated,andthus,VS-Aisobtainedasafunctionof

mA,Fig.3.These valuesdo notchangewithinthestudied

tem-peraturerange,andthus,theywerefittedtogether(temperature independent)accordingtoalinearmodelforeachIL,Eq.(7):

VS-A=a1mA+a0 (7)

CombiningEqs.(5)–(7),Eq.(8)isobtained:

mA=C+WSK+(a1mA+a0)dG (8)

Eq.(8)wassolvedforeachexperimentalpointleadingtoswelling correctedCO2 absorptiondata,Table2andFig.4.Swelling

cor-recteddatawerecomparedwiththoseavailableintotheliterature,

Figs.S1–S5(Supplementary data). Themaindifferences risefor [bmim][PF6], Fig.S1(Supplementary data);deviations withAki

etal.[47] andLiuet al.[86]aremore thanthecomparisonsto those reportedby Zhanget al. [87], Shiflettand Yokozeki [88], Kumelanetal.[89],andKampsetal.[90].Akietal.[47]reported thestrong effectof decomposition productsfrom the degrada-tionof[PF6]−anionontheCO2 solubilitymeasurements,which

couldjustifythedifferencesbetweenthedatareportedinthiswork andthosereportedinsomeliteraturesources,althoughitshould beremarkedtheexcellentagreementbetweendatareportedin thisworkandotherliteraturesources.Datafor[bmim][Tf2N]and

[emim][Tf2N]are in excellent agreementwithliterature values

(a)[bmim][PF6] (b)[bmim][Tf2N] (c)[emim][Tf2N] 0 0.2 0.4 0.6 0.8 x(CO2) 80 120 160 200 Vm / cm 3 mo l -1 313.15 K 333.15 K 0 0.2 0.4 0.6 0.8 x(CO2) 80 120 160 200 240 280 Vm / cm 3 mo l -1 313.15 K 333.15 K 0.2 0.4 0.6 0.8 x(CO2) 80 120 160 200 240 Vm / cm 3 mo l -1 323.15 K 343.15 K

Fig.2.Variationofmolarvolume,Vm,withthemolefractionofabsorbedCO2.

ExperimentaldatafromAkietal.[47]andRenetal.[85].

0 0.01 0.02 0.03 0.04 mA / g 0.075 0.08 0.085 0.09 0.095 0.1 VS-A /c m 3 323.15 K 343.15 K 0 0.01 0.02 0.03 mA / g 0.07 0.08 0.09 0.1 VS-A / cm 3 313.15 K 333.15 K 0 0.02 0.04 0.06 0.08 mA / g 0.18 0.2 0.22 0.24 0.26 VS-A /c m 3 313.15 K 333.15 K

(a)[bmim][PF6] (b)[bmim][Tf2N] (c)[emim][Tf2N]

V_S-A= 0.7429m_A+0.1967 V_S-A= 0.8692m_A+0.0711 VS-A= 0.7617mA+0.0732

m_S= 0.101960 g

m_S= 0.101106g

mS= 0.265319 g

Fig.3.Variationofsamplevolume(swelling),VS-A,withtheamountofabsorbedCO2(mA).

3.3. Correlationofabsorptiondatausingcubicequationsofstate Experimental CO2 absorption data (vapor–liquid

equilib-rium data) were correlated using Peng–Robinson (PR) and

0 4 8 12 16 20

P / MPa

0 0.2 0.4 0.6 0.8

x(C

O

2

)

[BMIM][PF6] - 318 K [BMIM][PF6] - 338 K [BMIM][TF2N] - 318 K [BMIM][TF2N] - 338 K [EMIM][TF2N] - 318 K [EMIM][TF2N] - 338 K

Fig.4.IsothermalexperimentalsolubilitydataofCO2inthestudiedionicliquids.

x(CO2)standsforCO2molefraction.

Table3

CriticalpropertiesandacentricfactorsusedforEOScalculations.

Tc(K) Pc(MPa) Vc(cm3mol−1) w

[bmim][PF6]a 708.9 1.73 779.5 0.7553

[bmim][Tf2N]a 1265.0 2.76 1007.1 0.2656

[emim][Tf2N]a 1244.9 3.26 892.9 0.1818

CO2b 304.13 7.38 94.1 0.2239

a_{DatafromValderramaandRobles[95].} b_{DatafromREFPROP9.0[73].}

Soave–Redlich–Kwong(SRK)equations ofstate(EOS),combined withvanderWaals2-parametermixingrule[94].Critical proper-tiesandacentricfactorsrequiredforEOScalculations,Table3,were obtainedfromValderramaandRobles[95]whousedagroup con-tributionmethodtopredicttheseproperties.Correlationresults arereportedinTable4 andFig.5,both EOSleadtosatisfactory

Table4

Binaryinteractionparameters(k12andl12)andequationsofstatecorrelationresults

forionicliquid+CO2vaporliquidequilibria.%AARDstandsforpercentageabsolute

averagerelativedeviation.

T(K) PR SRK k12 l12 %AARD k12 l12 %AARD [bmim][PF6] 318 0.2096 0.0400 1.57 0.2134 0.0394 1.49 338 0.2491 0.0536 1.00 0.2528 0.0462 0.91 [emim][Tf2N] 318 0.0547 0.0267 1.73 0.0555 0.0270 1.77 338 0.0508 0.0294 1.43 0.0555 0.0317 1.49 [emim][Tf2N] 318 0.0536 0.0330 1.11 0.0523 0.0332 1.15 338 0.0440 0.0251 1.56 0.0446 0.0281 1.62

Fig.5.ResultsofCO2absorptiondatacorrelationusingPeng–RobinsonandSoave–Redlich–KwongequationsofstatewithbinaryinteractionparametersreportedinTable2.

correlation,withpercentage absoluteaveragerelativedeviation lowerthan1.77%forbothfluidsandEOS.

Renetal.[85]reportedtheappearanceofvapor–liquid–liquid equilibriumandliquid–liquidequilibriumfor[emim][Tf2N]+CO2

system at 298.15K. High-pressure phase diagrams for [emim][Tf2N]+CO2 system were extrapolated using the binary

interactionparameters reportedin Table4,for 318and 338K, which are reportedin Fig. 6 in comparison withthat reported byRenetal.[85]at298.15K.TheresultsreportedinFig.6show mixturecritical points,which someauthors have discardedfor CO2+ionicliquidsystems[26,85]byconsideringitasanartifact

oftheEOSpredictionsinthehighpressureregions.Theresults reportedinFig.6for318and 338Kdo notshowtheexistence neitherofthreephaselines(vapor–liquid–liquidequilibria),nor ofliquid–liquidequilibriabecausetheyareabovethecriticalpoint ofpureCO2.

3.4. Moleculardynamics

Simulationswere carriedout as a function ofabsorbed CO2

mole fraction (from pure ionic liquids to CO2 equimolar

sys-tems,TableS1,Supplementarydata)forthethreestudiedionic liquids([emim][Tf2N],[bmim][Tf2N]and[bmim][PF6]).The

anal-ysisofmoleculardynamicsresultsreportedinthis workwillbe doneconsidering:(i)structuralmicroscopicfeatures,usingradial distribution functions (RDFs)and spatial distribution functions (SDFs), together with volume expansion upon CO2 absorption

(swellingeffect)(ii)energeticfeatures,usingintermolecular inter-actionenergies,and(iii)dynamicproperties,usingself-diffusion

coefficients.DetailedcenterofmassRDFs,andtheircorresponding coordination numbers,arereported in Figs.S4–S9, Supplemen-tary data. The main conclusion that may be inferred from the evolution of RDFs with increasing CO2 mole fraction is

Fig.6. Phaseequilibriafor[emim][Tf2N]+CO2.LinesshowPeng–Robinson

cor-relationresultswithparametersreportedinTable2,circlesshowexperimental data.Dataat298KobtainedfromRenetal.[85].Dashedpinklinesshowthe Peng–Robinsonpredictionsofmixturecriticalpoints.(Forinterpretationofthe ref-erencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthe article.)

0 4 8 12 16 20 r / Å 0 1 2 3 4 5 g( r) 0 20 40 60 80 N 0 1 2 3 4 5 g( r) 0 20 40 60 80 N 0 1 2 3 4 5 g( r) 0 20 40 60 80 N

(a) [emim][Tf

2

N]

(b) [bmim][Tf

2

N]

(c)

[bmim][PF

6

]

Fig.7. Anion–cationcenter-of-massradialdistributionfunctions,g(r),andthe cor-respondingcoordinationnumbers,N,forxCO2+(1−x){(a)[emim][Tf2N]or(b)

[bmim][Tf2N]or(c)[bmim][PF6]}systems,at298Kand2.5MPaobtainedfrom

moleculardynamicssimulations.Continuouslinescorrespondtox=0(pureionic liquids)anddashedlinestox=0.5systems.

theabsenceof remarkablechanges in theion–ion interactions,

Figs.S4,S6,andS8(panelsa–c,Supplementarydata).Onlyvery sub-tlechangesareinferredforanion–cationRDFs,asshowninFig.7, leadingtoadecreaseinthecoordinationnumber.Neverthelessthis decreaseisalmostnegligibleinthefirstsolvationshell(∼8 ˚A)for thethreestudiedionicliquids,andthusshortandmediumrange anion–cationinteractionsarenotaffectedbythepresenceofCO2

moleculeswhen consideringconcentrationsashighas equimo-larsolutions.CO2–ionRDFsarereportedinFig.8,thestructuring

of CO2 is different around anion and cation, and also

remark-abledifferencesareinferredwhenconsidering[PF6]−and[Tf2N]−

anions.Kanakuboetal.[96]reportedaX-raydiffractionstudyof [bmim][PF6]+CO2mixturesshowingCO2moleculespreferentially

solvatingthe[PF6]−anion.Thispreferentialsolvationisconfirmed

bytheRDFsreportedinFig.8[29,64,65].Nevertheless,for[Tf2N]

containingsystems, thedifferencesof CO2 distributionsaround

anionandcationaremoresubtle.Amoredetailedpictureofthe spatialdistributionoftheinvolvedmoleculesmaybeobtainedfrom

1 2 3 4 5 g( r) 20 40 60 80 N [emim][Tf2N]+CO2 (x=0.5) [bmim][Tf2N]+CO2 (x=0.5) [bmim][PF6]+CO2 (x=0.5) 0 4 8 12 16 20 r / Å 0 1 2 3 4 g( r) 0 20 40 60 N (a)

CO

₂

- A

(b)

CO

₂

- C

Fig.8. (a)CO2–anionand(b)CO2–cationcenter-of-massradialdistribution

func-tions,g(r),andthecorrespondingcoordinationnumbers,N,forx(=0.5)CO2+(1−x)

{[emim][Tf2N]or[bmim][Tf2N]or[bmim][PF6]}systems,at298Kand2.5MPa

obtainedfrommoleculardynamicssimulations.Astandsforanion,andCforcation.

SDFsreportedinFigs.9and10andFig.S10(Supplementarydata), whichshowSDFsfor{[emim][Tf2N]or[bmim][Tf2N]}+CO2

sys-temsbeingclearlydifferenttothosefor[bmim][PF6]+CO2.SDFs

aroundimidazoliumcationsshowthattheyinteractwith[Tf2N]−

anionpreferentially throughtheregionopposite tothefluorine atoms,withoutremarkablechangesbetweenpureionicliquidsand equimolarCO2solutions,Figs.9and10andFig.S10

(Supplemen-tarydata,panelsaandb).For[bmim][PF6]systems,imidazolium

cationssolvate[PF6]−anionsinthewell-knownoctahedral

distri-butionbecauseofitssphericalsymmetryandchargedistribution

[65].Nevertheless,aniondistributionaroundimidazoliumcations is very similarboth for [Tf2N]− and [PF6]−,Figs. 9 and 10 and

Fig.S10(Supplementarydata,panelsdande).Thedistributionof CO2aroundanionsisremarkabledifferentfor[Tf2N]−and[PF6]−

ions,Figs.9and10andFig.S10(Supplementarydata,panelc),and alsodifferentforthedistributionaroundcations(Figs.9and10and

Fig.S10,Supplementarydata,panelf).Thereforethepresenceof spherical[PF6]−anionleadstoremarkablechangesinthemolecular

structuringandCO2distributioninthestudiedionicliquids.

Thestructural differencesrisingfromthepresence of[PF6]−

anionincomparisonwith[Tf2N]−shouldjustifythedifferentCO2

solubility(lowerfor[PF6]−basedsystems).Severalstudieshave

analyzedtherelationshipofavailablefree spacein ionicliquids withgassolubility,andtheionicliquidvolumeexpansionupon CO2 absorption.Huangetal.[46]carriedoutsimulation studies

toanalyzethesmallpartialmolarvolumeofCO2in[bmim][PF6],

whichwasjustifiedthroughthespatialrearrangementofcavities intheionicliquid.Themostrelevantvolumetricpropertiesforthe threestudiedsystemsarereportedinFig.11.Molarvolumefollows alinearincrease withincreasingCO2 molefraction,withslopes

increasingwithincreasingalkylchainlengthonthecationfora fixedanion([Tf2N]−),andwithlowerslopesfor[PF6]−containing

Fig.9. SpatialdistributionfunctionsinxCO2+(1−x)[emim][Tf2N]systemsasafunctionofcomposition,at298Kand2.5MPaobtainedfrommoleculardynamicssimulations.

Inpanelsa–dblueandredsurfacesshow7timesaveragebulkdensity,andinpanelscandfyellowsurfacesshow6timesaveragebulkdensity.AstandsforanionandCfor cation.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthearticle.)

systems, Fig. 11a. Percentage volume expansion, %Vexpansion, is

definedasthechangeinabsolutevolumeoftheionicliquid[47], eq.(9):

%Vexpansion=100×Vmix(T,P,x)−VIL(T,P0

) VIL(T,P0)

(9) where subindices mix and IL stand for the volume of IL+CO2

and IL systems, respectively. %Vexpansion reported in Fig. 11c

is larger than values obtained from experimental data [47]. Aki et al. [47] reported a percentage volume expansion of

16.8% and 14.2% at 298.15K and equimolar composition, for [bmim][PF6] and [bmim][Tf2N], respectively, whereas data

cal-culatedin this workare31% and 17.9%,respectively. Themain differences appear for [bmim][PF6], which couldrise from the

used samples for experimental measurements, as previously mentioned, and in theforcefield parameterization used in this work. Bhargava et al. [64] alsoobtained slightly larger expan-sions for CO2+[bmim][PF6] system using molecular dynamics

simulations. Nevertheless, the trends are in agreement with experimentaldata: (i)lower expansions forlarger imidazolium

Fig.10. SpatialdistributionfunctionsinxCO2+(1−x)[bmim][PF6]systemsasafunctionofcomposition,at298Kand2.5MPaobtainedfrommoleculardynamicssimulations.

Inpanelsa–dblueandredsurfacesshow7timesaveragebulkdensity,andinpanelscandfyellowsurfacesshow6timesaveragebulkdensity.AstandsforanionandCfor cation.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthearticle.)

(a)

(b)

(c)

0 0.1 0.2 0.3 0.4 0.5 x(CO2) 120 160 200 240 280 320 Vm /c m 3mo l -1 CO +[emim]Tf N CO+[bmim]TfN CO+[bmim]PF 0 0.1 0.2 0.3 0.4 0.5 x(CO2) -12 -8 -4 0 V E/ cm 3mo l -1 V CO2_{= 65.4} ± 0.5_{c m}3 mol-1 V CO 2= 3 6.4 ± 1.₇ c m3 mo_l-1 V CO_{2= 4} 9.3 ± 1.3 cm3 mol-1 0 0.1 0.2 0.3 0.4 0.5 x(CO2) 0 10 20 30 40 %Ve xp an sio n

Fig.11.(a)Molarvolume,Vm,(b)excessmolarvolume,VE,and(c)liquidphasevolumeexpansionuponCO2absorption,%Vexpansion,forxCO2+(1−x){[emim][Tf2N]or

[bmim][Tf2N]or[bmim][PF6]}systems,at298Kand2.5MPaobtainedfrommoleculardynamicssimulations.Inpanel(a)linesshowlinearfits,andinpanels(bandc)lines

showpolynomialfits,forguidingpurposes.Inpanel(a)wereportcalculatedCO2partialmolarvolume.

alkyl chain for a fixed anion and (ii) larger expansion for [PF6]−containingsystems.Excessmolarvolume,calculatedfrom

simulateddatausingthemethodpreviouslyproposed[97],shows largenegativevalues (upto_−10.7cm3_mol−1 _{for [bmim][Tf2N],}

Fig.11b),whichisinagreementwiththelowexpansionuponCO2

absorptionandwiththeabsorptionmechanismproposedbyHuang etal.[46].

Intermolecular interaction energies are analyzed in

Figs.12and13andFig.S11(Supplementarydata).Theenergyof thestudiedsystemsiscalculatedinthisworkusingtheforcefield

expressionreportedinEq.(10):

E=



bonds kr(r−req)2+



angles k(−eq)2+Etor +



i



j



4εij



ij rij



12 −



ij rij



6

+ qiqje2 4ε0rij

(10)

where the first three terms correspond to bonds, angles and dihedralcontribution,and thelasttwo termstointermolecular

0 0.1 0.2 0.3 0.4 0.5 x(CO2) -20 -16 -12-2 -1.5 -1 Einte r / kJ mo l -1 0.1 0.2 0.3 0.4 0.5 x(CO2) -18 -15 -12 -4 -2 0 Einte r / kJ mo l -1 0 0.1 0.2 0.3 0.4 0.5 x(CO2) -2000 -1800 -1600 600 800 1000 Einte r / kJ mo l -1 0.1 0.2 0.3 0.4 0.5 x(CO2) -12 -8 -4 0 4 8 12 Einte r / kJ mo l -1 (a) (b) (c) (d) A-A A-C C-C A-CO2 CO2-CO2 C-CO2

coulombic

Lennard-Jones

ion-CO 2 ion-ion

0 0.1 0.2 0.3 0.4 0.5 x(CO2) -2800 -2600 -2400 -2200 600 800 1000 1200 Einte r / kJ mo l -1 0.1 0.2 0.3 0.4 0.5 x(CO2) -16 -12 -8 -4 0 4 Einte r / kJ mo l -1

(a)

(b)

(c)

(d)

A-A A-C C-C A-CO2 CO2-CO2 C-CO2

coulombic

Lennard-Jones

ion-CO 2 ion-ion 0 0.1 0.2 0.3 0.4 0.5 x(CO2) -8 -4 0 4 8 Einte r / kJ mo l -1 0.1 0.2 0.3 0.4 0.5 x(CO2) -10 -8 -6 -4 -2 0 Einte r / kJ mo l -1

Fig.13. SplitofintermolecularinteractionenergiesintheliquidphaseofxCO2+[bmim][PF6]at298Kand2.5MPaobtainedfrommoleculardynamicssimulations.

energies, which are split in Lennard–Jones and coulombic contributions. For [emim][Tf2N] and [bmim][Tf2N] systems,

intermolecularenergiesareverysimilar,Fig.12andFig.S11 (Sup-plementarydata,panelsaandb),althoughbyincreasingthealkyl chainlengthonimidazoliumcation,energyforion–ioninteractions decreases(inabsolutevalue).Forexample,forpureionicliquids coulombicanion–cationenergydecreaseina15%causedthe pres-enceofalargeralkylchain,whichleadstoacertaindegreeofsteric hindranceweakeninganion–cation interaction. Withincreasing CO2 concentration anion–cation coulombic interaction energy

interactiondecreases(inabsolutevalue)inanalmostlinearway (15%and12%for[emim][Tf2N]and[bmim][Tf2N],respectively).

VariationsinLennard–Jonescontributionsforion–ioninteractions arealmostnegligiblewithincreasingCO2molefraction.CO2–anion

andCO2–cationcoulombiccontributionsshowalmostthesame

valuesbutwithoppositesigns,whereastheLennard–Jones con-tributionsarelargerfortheinteractionswiththe[Tf2N]−anion,but

CO2–cationinteractions arealsoimportant.Forthe[bmim][PF6]

ionicliquid,Fig.13,ion–ioninteractionenergiesarelargerthanfor [bmim][Tf2N],forexampleinpureionicliquidsanion–cation

inter-actionenergiesare53%larger(inabsolutevalue)in[bmim][PF6],

ancation–cationinteractionsarealso54% larger.Ion–ion inter-actionenergies decreaseslinearlywithincreasingCO2 amounts

for[bmim][PF6],i.e.anion–cationenergydecreasesa8%ongoing

frompureionicliquidtoequimolarCO2solutions,Fig.13a,which

ishalfofthedecreasingfor[emim][Tf2N]and[bmim][Tf2N]

sys-tems.Therefore,thelargeranion–cationinteractionenergiesfor [bmim][PF6]systemsarerelatedwiththelowerCO2solubilityin

thisionicliquid,theabsorbedCO2isnotabletoweakenitas

effec-tivelyasfor[Tf2N]−containingsystems.Therefore,thisbehavior

togetherwiththelargerexpansionvolumein[bmim][PF6](fitting

CO2 moleculesrequiresmorecavitieschanges)wouldjustifythe

effectof[PF6]− aniononCO2 solubilityin [bmim][PF6]in

com-parisonwith[Tf2N]−-based ILs.Furthermore,from anenergetic

viewpointtheinteractionofCO2withionsin[bmim][PF6]isalso

differenttothosecalculatedin[Tf2N]−systems,showingstronger

ion–CO2coulombicandLennard–Jonesinteractionsinthecaseof

the[bmim][PF6],Fig.13(panelscandd).Nevertheless,although

ion–CO2interactionsarestrongerfor[bmim][PF6],whichshould

beafactortoincreaseCO2solubilityincomparisonwith[Tf2N]−

systems,thedifferencesarenotveryremarkable,andthusthemain effectscontrollingabsorptionrisefromtheabilityofionicliquidto fitCO2molecules(poorerin[PF6]−containingsystems,thereforea

factorloweringsolubility)andtheabilityofCO2moleculesto

dis-ruptionicliquidstructuring(alsopoorerin[PF6]−basedsystems).

Self-diffusioncoefficients,D,werecalculatedusingtheEinstein relationship.The reliability of calculated D values wasinferred from the ˇ parameter [98,99], which should be ˇ=1 for fully diffusiveregime.[emim][Tf2N]and[bmim][Tf2N]aremoderately

viscous fluids(34.1 and 51.0mPas, for [emim][Tf2N] [100]and

[bmim][Tf2N][102],respectivelyat298.15Kand0.1MPa),whereas

viscosity for [bmim][PF6] is remarkably larger (270.9mPas at

298.15K and 0.1MPa) [101]. Therefore, fully diffusive regime wouldbereachedfor shortersimulation timesin [emim][Tf2N]

and[bmim][Tf2N]thanin[bmim][PF6].Nevertheless,thelength

of the simulations allows reaching fully diffusive regime, and thus, for [emim][Tf2N] and [bmim][Tf2N] systems a value of

ˇ>0.99 wasused forDcalculations and ˇ>0.95 in thecase of [bmim][PF6]systems.TheresultsreportedinFig.14showlarger

(a)

(b)

(c)

0.1 0.2 0.3 0.4 0.5 x(CO2) 0 1 2 3 4 1 0 9D / m 2s -1 0 0.1 0.2 0.3 0.4 0.5 x(CO2) 0 0.1 0.2 0.3 10 9D /m 2s -1 [EMIM][TfN] [BMIM][TfN] [BMIM][PF ] 0 0.1 0.2 0.3 0.4 0.5 x(CO2) 0 0.1 0.2 0.3 10 9D /m 2s -1

anion

cation

CO

₂

Fig.14.Self-diffusioncoefficients,D,for(a)anion,(b)cationand(c)CO2inxCO2+(1−x){[emim][Tf2N]or[bmim][Tf2N]or[bmim][PF6]}systems,at298Kand2.5MPa

obtainedfrommoleculardynamicssimulations.Linesshowpolynomialfitsforguidingpurposes.

experimentalmeasurementsreportedintotheliterature[102,103], andDvaluesforCO2moleculesareanorderofmagnitudelarger

than those for thecorrespondingions. Ion Dvalues follow the ordering[emim][Tf2N]>[bmim][Tf2N]>[bmim][PF6],which isin

agreementwiththeviscosityvaluesandalsowiththeinteraction energies reportedin Figs. 12 and 13 and Fig.S11 (Supplemen-tarydata).CO2 absorptionleadstoa non-linear increaseof ion

D values with increasing mole fraction: (i) up to x(CO2)∼0.3

weakvariations are inferred and (ii) for x(CO2)>0.3 a steepest

increaseisobtained;Fig.14(panelsaandb).Therefore,thefaster the ionic diffusion, the larger the CO2 solubility, although the

effectofabsorbedCO2 onionicdiffusionisonlyremarkablefor

largerCO2 molefractions. Dvalues for CO2 molecules increase

withincreasingmolefractionwiththelargervaluesobtainedfor [bmim][Tf2N].

4. Conclusions

High-pressure absorption measurements of CO2 in selected

classicalimidazolium-basedionicliquidswerecarriedoutusing new state-of-the-art apparatus up to 20MPa. A remarkable swellingeffectuponCO2absorptionwasobservedfrompressures

higherthan10MPa,whichleadedtoanapparentdecreaseofthe CO2 absorbed amount.Amethodbased ontheuseofavailable

experimentalvolumetricdatafortheCO2+ionicliquidsystemsis

usedtocorrectswellingeffect.Thecorrectedresultsareingood agreementwithmostoftheavailableliteratureexperimentaldata. High-pressure absorption data were successfully correlated using Soave–Redlich–Kwong and Peng–Robinson equations of statecoupledwithbi-parametricvanderWaalsmixingrule.The obtainedbinaryinteractionparametersleadtotheappearanceof mixturecriticalpoints,whichisnotinagreementwiththeavailable literatureinformation.

Classicalmoleculardynamicssimulationswereusedtoanalyze thenanoscopicstructuralbehaviorofCO2+ionicliquidsmixed

sys-temsasa functionofmixturecomposition.Thereportedresults showthedifferentstructuringofCO2aroundanionsandcations,

withthemostimportanteffectofaniontypeonCO2absorption.The

anioneffectisexplainedconsideringthelargerexpansionuponCO2

absorptionfor[PF6]−containingsystems.Fromtheviewpointof

intermolecularinteractionenergies,anion–cationinteraction ener-giesareremarkablyfor[bmim][PF6]incomparisonwith[Tf2N]−

containingsystems,whichleadedtolowerCO2absorptionabilities

for[PF6]−containingsystems.Theanalysisofmixtures’dynamic

propertiesshowedadirectrelationshipbetweenionicdiffusionand CO2solubility.

Theresultsreportedinthisworkshowtheremarkable infor-mation,bothformmacroandmicroscopicviewpoints,thatmay inferred from high pressure absorption measurements coupled withmolecularsimulations

Listofsymbols

[bmim][PF6] 1-butyl-3-methylimidazoliumhexafluophosphate

[emim][Tf2N] 1-ethyl-3-methylimidazolium

bis[trifluoromethylsulfonyl]imide [bmim][Tf2N] 1-butyl-3-methylimidazolium

bis[trifluoromethylsulfonyl]imide EOS equationofstate

IL ionicliquid

PR Peng–Robinsonequationofstate RDF radialdistributionfunction SDF spatialdistributionfunction

SRK Soave–Redlich–Kwongequationofstate %AARD percentageabsoluteaveragerelativedeviation D self-diffusioncoefficient

dgas bensityofthegas,Eq.(1)

dS-A densityofILsampleplusabsorbedgas,Eq.(2)

m experimentalreadings,Eq.(2)

k12 andl12binaryinteractionparameters

mads adsorptionamount,Eq.(1)

mA massofabsorbedgas,Eq.(2)

mS massofionicliquid,Eq.(2)

msample massofthesample,Eq.(1)

msink massofthesinker,Eq.(1)

mSK massofsinker,Eq.(2)

N coordinationnumbersfromRDFs

Vm molarvolume,Vsamplevolumeofthesample,Eq.(1)

VS-A volumeofILsampleplusabsorbedgas,Eq.(2)

Vsinker volumeofthesinker,Eq.(1)

Wbuoy,sample buoyancycorrectionduetosample,Eq.(1)

Wbuoy,sink buoyancycorrectionduetosinker,Eq.(1)

WSK buoyancycorrectionduetosinker,Eq.(2)

WS-A combinedbuoyancy correctionduetoionicliquid and

absorbedgas,Eq.(2) Acknowledgements

ThispublicationwasmadepossiblebyNPRPgrant# [09-739-2-284]fromtheQatarNationalResearchFund(amemberofQatar

Foundation).Thestatementsmadehereinaresolelythe responsi-bilityoftheauthors.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,athttp://dx.doi.org/10.1016/j.fluid.2012.10.022.

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