Assessing the activity coefficients of water in cholinium-based ionic liquids: experimental measurements and COSMO-RS modeling

(1)

ContentslistsavailableatScienceDirect

Fluid

Phase

Equilibria

j o u r n a 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

Assessing

the

activity

coefﬁcients

of

water

in

cholinium-based

ionic

liquids:

Experimental

measurements

and

COSMO-RS

modeling

Imran

Khan

a

_,

_Kiki

_A.

_Kurnia

a

_,

_Tânia

_E.

_Sintra

a

_,

_Jorge

_A.

_Saraiva

a

_,

Simão

P. Pinho

b

_,

_João

_A.P.

_Coutinho

a,∗

a_Departamento_de_Química,_CICECO_and_QOPNA,_Universidade_de_Aveiro,_Campus_{Universitário}_de_Santiago,_3810-193_Aveiro,_Portugal

b_LSRE/LCM,_Departamento_de_Tecnologia_Química_e_Biológica,_Instituto_Politécnico_de_Braganc¸_a,_Campus_de_Santa_Apolónia,_5301-857_Braganc¸_a,_Portugal

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received28July2013 Receivedinrevisedform 20September2013 Accepted16October2013 Availableonline24October2013 Keywords: Wateractivity Choline COSMO-RS Abinitio

a

b

s

t

r

a

c

t

Thevaporliquid-equilibriumofwater+ionicliquidsisrelevantforawiderangeofapplicationsofthese compounds.Itisusuallymeasuredbyebulliometrictechniques,butthesearetimeconsumingand expen-sive.Inthisworkitisshownthattheactivitycoefﬁcientsofwaterinaseriesofcholinium-basedionic liquidscanbereliablyandquicklyestimatedat298.15Kusingahumiditymeterinstrument.The cholin-iumbasedionicliquidswerechosentotestthisexperimentalmethodologysincedataforwateractivities ofquaternaryammoniumsaltsareavailableintheliteratureallowingthevalidationoftheproposed technique.

TheCOSMO-RSmethodprovidesareliabledescriptionofthedataandwasalsousedtounderstand themolecularinteractionsoccurringonthesebinarysystems.Theestimatedexcessenthalpiesindicate thathydrogenbondingbetweenwaterandionicliquidanionisthedominantinteractionthatgoverns thebehaviorofwaterandcholinium-basedionicliquidssystems,whiletheelectrostatic-misﬁtandvan derWallsforceshaveaminorcontributiontothetotalexcessenthalpies.

Theresultsherereportedshowthatwateractivitymeasurementsallowaquickscanforselectingionic liquidsaccordingtotheirbehaviorinmixtureswithwater.

1. Introduction

Ionicliquidshaveuniquepropertiessuchasnegligiblevapor

pressureatroomtemperature,non-ﬂammability,andtheirability

todissolveorganicandinorganiccompounds,makethemattractive

forthescientiﬁc communityandavarietyofindustrial

applica-tions.Chemically,ionicliquidsarecomposedbyabulkyorganic

cationcoupledwithadispersecharge,andanasymmetricorganic

orinorganicanion[1].Limitlesscombinationsofcationandanion

thatform theionicliquidslead todifferentphysical properties

andphasebehavior,thusallowingionicliquidstobelabeledas

“designer”solvents.Althoughmostworksonthissubjectdealwith

imidazolium-basedionicliquidsthesehaveseverallimitationsin

whatconcernstheirthermalandchemicalstability,price,toxicity

andbiodegradability[2–7].Toovercometheseissues,ionsderived

fromnaturalresourceshaveemergedascompletelybio-derived

ionic liquids, suchas cholinium-based ionic liquids. Cholinium

chloride(alsoknownascholinechloride,2-hydroxyethyltrimethyl

ammoniumchlorideorvitaminB4) isacheaporganicsaltwith

highmelting/degradationpoint(298–304◦C).Earlyworksonthe

∗ Correspondingauthor.Tel.:+351234401507;fax:+351234370084. E-mailaddress:[email protected](J.A.P.Coutinho).

useofcholinechloridewerepublishedin2001whenAbbotetal.[8]

reportedfortheﬁrsttimethenovel“deepeutecticsolvents”

com-posedofMCl2(M=ZnorSn)withcholinechlorideina2:1ratio.

Sincethen,thesecholinium-baseddeepeutecticsolventshavebeen

studiedonmanyapplications,especiallyassolventfor

electrode-positionofmetals[9–14].

Recentworkshavereportedthepreparationofnovel

cholinium-basedionicliquidswithdifferentanions[15–17].These

cholinium-basedionicliquidsshowexcellentbiodegradability[18]andlow

toxicity[15,16,19,20].Pernaketal.reportedthesynthesisand

char-acterizationof63choliniumbasedionicliquids[15].Theirresults

showedthattheseionicliquidswereactiveagainstmicrobesand

manifestexcellentantielectrostaticproperties.Besidesbeing

bio-compatibleandbiodegradable,cholinium-basedionicliquidscan

alsobepreparedfromcheapnaturalresources.Notsurprisingly,

theuseofcholinium-basedionicliquidsasalternativestothe

tradi-tionalionicliquidshasattractedthescientiﬁcinterestforthepast

fewyears,withapplicationsasdiverseassolventforabsorption

ofammonia[21,22]andCO2[23],isolationofsuberinfromcork

[24],effectivesolventsforpretreatmentofricestrawbiomass[25]

andlignocellulosicmaterial[26],catalyticreactions[27,28],and

extractionofantibioticsusingaqueousbiphasicsystems[29].

Despitetheirinterest,thephysicalpropertiesofpure

cholinium-basedionicliquids [15,30,31]andtheirmixtures withwater or

(2)

otherorganicsolventsarestillpoorlycharacterized[32–34].The

thermodynamicandtransportpropertiesofionicliquidsandtheir

mixtureswithwaterarecrucialtounderstandthemolecular

inter-actions between the components of the mixture and also for

designing applicationssuchasabsorptive cooling,gas

dehydra-tion,extractive distillation,and liquid–liquid extraction, among

others.Asuitableandwidelyacceptedapproachforthedescription

ofthoseinteractionsisthedeterminationofactivitycoefﬁcients

[35].Theyareafundamentalthermodynamicquantityproviding

aconvenientmeasureofthesolutionnonidealityandcomponent

compatibility.Theirexperimentalassessmentrequires the

mea-surementofphaseequilibriumdata.Therearemultiplepossible

approachesdependingonthephysicalstateofthecompoundsto

carry thesephase equilibrium measurements. For aqueous

sys-temswitha nonvolatileliquid compoundthemeasurement of

vapourliquidequilibriumisthemostused,butin aqueous

sys-temscontainingionicliquidsitisexpensiveandtimeconsuming.

Notonlytheavailableequipments,developedforconventional

sol-vents,requirea largesample volumerangingfromthe250cm3

ofWangetal.[36]downtotherecentlyreportedusedofaglass

FischerLabodestequilibriumstillthatrequiresasamplevolumeof

circa35cm3_[37–43]_._Moreover_the_high_viscosity_of_the_mixtures

richinionicliquiddifﬁcultaregularboilingofthemixture,what

causeslongequilibrationtimes,easilyreaching6h.Toovercome

theselimitationswehaverecentlyreportedamicroebulliometer

ofjust 8cm3 _of _capacity_to_study_these_systems_[44]_._The

ebul-liometermeasurementsremain,however,adifﬁcultandverytime

consumingtasksowithaimatdevelopinganexperimental

tech-niquethatwouldprovideafast,reliableandcheapscanofionic

liquids,andtheirinteractionwithwater,themeasurementofwater

activitiesusingahygrometerisproposed.Thisisafastandreliable

techniquetoassesstheactivitycoefﬁcientsofwater insystems

withnonvolatilesolutes[45,46],anditishereused,fortheﬁrst

time,formeasurementsofionicliquidsystems.Theavailabilityof

wateractivitydataforsomequaternariumammoniumsalts[47]

thatallowsthevalidationofthetechniqueforsimilarcompounds

wasoneofthereasonstostudythecholinium-basedionicliquids.

Thisworkisaimedatproposinganewexperimentaltechnique

forstudyingthemolecularinteractionsofwaterand

cholinium-basedionicliquids.Forthispurpose,itisinitiallyshownthatthe

methodhereadoptedforthemeasurementofwateractivitiesis

reliableforaqueoussolutionsofquaternaryammoniumsaltsand

choliniumchloride,forwhichpreviouslydataintheliteratureare

availablefrom isopiesticmeasurements. The water activitiesin

nineaqueoussolutionsofcholinium-basedionicliquidsarethen

measuredandthewateractivitycoefﬁcientscalculatedfromthese

values.Inaddition,COSMO-RS,aquantumchemical-based

thermo-dynamicpredictionmodel,wasusedtoinvestigatethemolecular

interactionsofwaterandionicliquids.Theadvantageof

COSMO-RS[48,49],comparedtothewidelyusedNon-RandomTwoLiquid

(NRTL)thermodynamicmodelused[50,51]isthatnotonlyitcan

predicttheactivitycoefﬁcientsofwaterinionicliquids,butitalso

canprovidethesigmaproﬁleandpotentialofpurecompounds,and

predictionsofthermodynamicpropertiessuchasexcessenthalpy

andentropies.The-proﬁlecanbeusedtounderstandthe

behav-iorofthemoleculeintermsoftheirpolarity,whilethe-potential

describesthelikelinessofthemoleculesstudiedtointeractwith

othermoleculesthathavea chargedensity[px()]andpolarity

[48,49].Whereastheexcesspropertiescanbeusedto

under-standtheinteractionbetweenionicliquidandwaterinthemixture

state.TheCOSMO-RSmodelhasbeensuccessfullyusedinthepast

toestimate thethermodynamic properties of systems

contain-ingionicliquids,includingliquid–liquidequilibriumandactivity

coefﬁcientsofitsmixturewithwaterororganicsolutes[52–59].

AfterevaluatingthecapacityoftheCOSMO-RSmodeltodescribe

theexperimentalactivitycoefﬁcients,thisisusedtoanalyzeand

discussthemolecularinteractionsthataffectthemixingproperties

ofthecholinium-ionicliquidsandwater,whicharealsosupported

byabinitiocalculations[60–63].

2. Experimental

2.1. Materials

Cholinium chloride, [Ch]Cl; cholinium dihydrogencitrate,

[Ch]DHCit;choliniumbicarbonate,[Ch]Bic;andcholinium

bitar-trate, [Ch]Bit, were acquired from Sigma Aldrich. Cholinium

acetate,[Ch]Ac;choliniumdihydrogenphosphate,[Ch]DHphand

cholinium salicylate, [Ch]Sal were obtained from Iolitec and

choliniumglycolate,[Ch]Glyandcholiniumlactate[Ch]Lacwere

synthesizedinourlabaccordingtowellestablishedprocedures

[15,64]. Fig. 1 depicts the chemical structures of the studied

cholinium-based ionic liquids. To remove traces of water and

volatilecompounds,individualsamplesofeachionicliquidwere

dried at moderate temperature (≈323K) and at high vacuum

(≈10−5_Pa),_under_constant_stirring,_and_for_a_minimum_period_of

48h.Afterthispuriﬁcation,thepurityofallionicliquidsamples

wasfurthercheckedby1_H,_and13_C_NMR,_and_shown_to_be_≥98_wt%.

Thewater contentof each ionicliquid wasdeterminedbyKarl

Fischer titration (Mettler Toledo DL32 Karl Fischer coulometer

usingtheHydranal–CoulomatEfromRiedel-deHaenasanalyte)

andfoundtobelessthan30×10−6massfraction.Doubledistilled

water, passed through a reverse osmosis system and further

treatedwithaMilli-Qplus185waterpuriﬁcationequipment,was

usedinallexperiments.

2.2. Measurementofwateractivities

The measurements of water activities (aw) were performed

usingaNovasinahygrometerLabMaster-aw(Lucerne,Switzerland).

Themeasuringprincipleoftheinstrumentisbasedon

resistive-electrolytic method.Theaccuracy oftheinstrumentis0.001aw,

enabling measurements undercontrolled chambertemperature

conditions(±0.20K),andwasinitiallycalibratedwithsixsaturated

puresaltstandardsolutions(wateractivityrangingfrom0.113to

0.973),whichwereincludedintheinstrument.However,toachieve

thegivenaccuracyacalibrationcurvewasbuiltusingatleastsix

aqueoussolutionsofKClorCaCl2atdifferentsaltmolalities.These

werechosenbasedontheexpectedvaluesforthewateractivityto

bemeasuredintheaqueoussolutionscontainingtheionicliquids.

After,themeasuredvalueswerecomparedtothoserecommended

intheextensivereviewsbyArcher[65]forKCl,orRardandClegg

[66]forCaCl2.Foreachmeasurement,samplesofapproximately

2–3cm3 _prepared_in_the_entire_solubility_range_of_the_ILs,_were

chargedinproperdishesandplacedintheair-tightequilibrium

chamber.Theexchangeoffreewatertakesplaceuntilthepartial

pressure ofwater vaporreachestheequilibrium,which is

con-ﬁrmedfollowingtheawvariationwithtime.Whenaconstantvalue

isreached,thewateractivityisrecorded.Dilutedsolutionsreach

equilibriuminlessthan1h,butsolutionswithhighconcentration

ofionicliquidscouldtakeupto8h.

2.2.1. Computationaldetails

ThestandardprocedureforCOSMO-RScalculationsconsistsof

twosteps.First,thecontinuumsolvationCOSMOcalculationsof

electronicdensityandmoleculargeometrywereperformedwith

theTURBOMOLE6.1 programpackage onthe densityfunctional

theorylevel,utilizing theBPfunctional B88-P86witha triple-␨

valencepolarizedbasis set(TZVP)andtheresolution ofidentity

standard(RI)approximation[67].Second,theactivitycoefﬁcient

ofwater inionicliquidwascalculatedusingCOSMOthermX2.1

(3)

Fig.1. Chemicalstructuresofthestudiedcholinium-basedionicliquids:(i)choliniumchloride,[Ch]Cl;(ii)choliniumacetate,[Ch]Ac;(iii)choliniumbicarbonate,[Ch]Bic;(iv) choliniumsalicylate,[Ch]Sal;(v)choliniumdihydrogenphosphate,[Ch]DHph;(vi)choliniumbitartrate,[Ch]Bit;(vii)choliniumdihydrogencitrate,[Ch]DHCit;(viii)cholinium glycolate,[Ch]Glyand(ix)choliniumlactate,[Ch]Lac.

GmbH&Co KG,Leverkusen, Germany) [68]. The detailsof the

calculationandprocedureofestimatingactivitycoefﬁcientusing

COSMO-RScan be found elsewhere [54]. The ab initio

calcula-tionswerealsoperformedusingTURBOMOLE6.1withthesame

parametersdescribedabove.Itshouldbenotedthattheabinitio

calculationsweredonebyotherauthorsusingasimilarapproach

andnopackingeffectswithintheliquidswereassumed[62].

3. Resultsanddiscussion

3.1. Experimentalresults

Avalidationofthewateractivitymeasurementtechniquehere

used,on systems similarto those under study,was carried by

themeasurementofwater activitiesofsixtetraalkylammonium

halides[47]and[Ch]Cl[69,70],previouslyreportedinliterature.

Agoodagreementbetweentheexperimentalvaluesmeasuredin

thisworkandtheliteraturevalueswasobtainedasshowninFigs.

S1-S3intheSupportingInformation.

Thewateractivitycoefﬁcient(w)canbecalculatedfromthe

wateractivitiesas

w=aw

xw

(1)

whereawisthewateractivity,andxwisthemolefractionofwater.

Table1presentstheexperimentalresultsforthewateractivities

andactivitycoefﬁcientsofwaterinthestudiedcholinium-based

ionicliquidsintheregionofcompletemiscibility.Excepting[Ch]Cl,

tothebestofourknowledgenodataonactivitycoefﬁcientofwater

inallotherstudiedcholinium-basedionicliquidswaspreviously

reportedintheliterature.Allsystemsstudiedpresentwateractivity

coefﬁcientslowerthan1,indicatingfavorableinteractionsbetween

waterandcholinium-basedionicliquidsasdepictedinFigs.2and3

(detailedresultsarepresentedinFigs.S4-S12).Itisremarkableto

seethatthesesystemspresentdifferenttrendsinthewater

activi-tiesandactivitycoefﬁcientswiththeionicliquidconcentration,e.g.

signiﬁcanteffectofconcentrationontheactivitycoefﬁcientwas

observedfor[Ch]Ac,meanwhile[Ch]Bicand[Ch]Clpresentedan

almostlineardependency.Thewateractivitycoefﬁcients follow

thetrend[Ch]Ac<[Ch]Lac≈[Ch]Gly<[Ch]DHph≈[Ch]Bit≈[Ch]Cl

<[Ch]DHCit<[Ch]Bic<[Ch]Sal.Theseresultsshowthatwater

pre-sentedthestrongestinteractionwith[Ch]Acascanbeobserved

fromitslowestactivitycoefﬁcient.Ontheotherhand,[Ch]Salhas

theweakerinteractionwithwater.

0.2 0.4 0.6 0.8 1.0 1.0 0.9 0.8 0.7

γ

H 2O X_H2O 0.90 0.92 0.94 0.96 0.98 1.00 1.00 0.98 0.96 0.94 0.92 0.90

Fig.2.Experimentalactivitycoefﬁcientsofwater,H2O,asfunctionofthe

concen-trationofwaterat298.2Kfor[Ch]DHCit( ,longdashedline),[Ch]Bit( ,dashed andtwodottedline),[Ch]Cl( ,line),[Ch]DHph( ,dottedline),and[Ch]Bic( , dashed-dottedline).Thesymbolsandlinesrepresenttheexperimentaland COSMO-RSpredictions,respectively.(Forinterpretationofthereferencestocolourinthis ﬁgurelegend,thereaderisreferredtothewebversionofthisarticle.)

0.0 0.2 0.4 0.6 0.8 1.0 1.0 0.9 0.8 0.7 0.6 0.5 γH 2O XH2O 0.80 0.85 0.90 0.95 1.00 1.00 0.96 0.92 0.88

Fig.3.Effectofstructuralvariationofcholinium-basedionicliquidswith carboxyl-ateanionsonwateractivitycoefﬁcient,[Ch]Ac( ,fullline),[Ch]Gly( ,longdashed line),[Ch]Lac( ,dashed-dottedline),and[Ch]Sal( ,dottedline).Thesymbols andlinesrepresenttheexperimentalandCOSMO-RSpredictions,respectively.(For interpretationofthereferencestocolourinthisﬁgurelegend,thereaderisreferred tothewebversionofthisarticle.)

(4)

Table1

Experimentalvaluesofwateractivity(aw)andactivitycoefﬁcientofwater(w)in

cholinium-basedionicliquidsat298.2K.

H2O+[Ch]Cl H2O+[Ch]Ac H2O+[Ch]Bic xw aw w xw aw w xw aw w 0.985 0.972 0.987 0.988 0.972 0.984 0.988 0.980 0.992 0.969 0.944 0.975 0.970 0.933 0.961 0.974 0.962 0.988 0.948 0.906 0.955 0.955 0.888 0.930 0.958 0.941 0.982 0.920 0.844 0.917 0.931 0.808 0.867 0.939 0.910 0.970 0.887 0.762 0.859 0.903 0.701 0.776 0.917 0.870 0.949 0.840 0.648 0.772 0.858 0.530 0.618 0.887 0.805 0.908 0.772 0.488 0.632 0.797 0.356 0.446 0.854 0.736 0.862 0.668 0.299 0.448 0.697 0.179 0.257 0.742 0.512 0.690 0.647 0.369 0.570 H2O+[Ch]Sal H2O+[Ch]DHph H2O+[Ch]Bit xw aw w xw aw w xw aw w 0.982 0.972 0.990 0.990 0.978 0.988 0.992 0.980 0.988 0.968 0.958 0.989 0.978 0.959 0.981 0.983 0.967 0.984 0.952 0.939 0.986 0.963 0.931 0.967 0.970 0.943 0.972 0.930 0.912 0.981 0.942 0.889 0.943 0.955 0.916 0.959 0.900 0.867 0.963 0.918 0.832 0.906 0.935 0.872 0.933 0.854 0.792 0.928 0.882 0.742 0.841 0.908 0.810 0.892 0.831 0.609 0.733 0.892 0.771 0.865

H2O+[Ch]DHCit H2O+[Ch]Gly H2O+[Ch]Lac

xw aw w xw aw w xw aw w 0.993 0.984 0.991 0.989 0.978 0.989 0.990 0.974 0.984 0.985 0.972 0.987 0.975 0.946 0.970 0.976 0.947 0.970 0.974 0.954 0.979 0.959 0.904 0.943 0.961 0.901 0.937 0.961 0.933 0.971 0.937 0.833 0.889 0.916 0.734 0.802 0.943 0.902 0.957 0.869 0.595 0.685 0.825 0.431 0.523 0.917 0.852 0.929 0.718 0.252 0.351 0.771 0.229 0.441 0.876 0.770 0.879 0.808 0.692 0.856

3.2. COSMO-RS:descriptionofpurecompounds

Fig.4presentsthesigmaproﬁleandpotentialforwater,[Ch]Ac,

and [Ch]Sal. Thesigma proﬁle and potential for theremaining

cholinium-basedionicliquidsstudiedaregiveninFigs.S13andS14

intheSupportingInformation.Thesigmaproﬁleofwaterpresents

peakscorrespondingtostrongH-bonddonorat−1.6enm−2 and

acceptor at 1.8enm−2. The sigma potential of this compound

presentsstronglyattractiveinteractionswithbothH-bonddonor

andacceptorinthemixture,althoughitshowsahighercapacity

asH-bonddonor,asdepictedontheirsigmapotential(Fig.4b).

Therefore,it isexpectedthatwater willhave highattractionto

compoundsthathaveH-bondacceptorgroup.Regardingthe

stud-iedcholinium-basedionicliquids,thesigmapotentialalsopresents

attractiontoboth H-bonddonor andH-bondacceptorgroupin

themixture.TheH-bonddonorcharacterarisesfromthehydroxyl

groupof thecholinium cationthat is indicated bya peak with

lowintensityat−1.8enm−2 ontheirsigmaproﬁle(Fig.4a).All

thestudiedanionsofthecholinium-based ionicliquidspresent

peakswithintheelectronegativeareaindicating theircapability

ashydrogen-bondacceptor.Forexamplepeaks,at2enm−2

cor-respondingtoAc− andSal− anions,withtheformerhasslightly

higherintensityindicatinggreaterabilitytoactasH-bondacceptor

(Fig.4a).ThepresenceofaromaticringofSal−anionisindicatedby

signiﬁcanthighintensityofpeaksat−0.5and0.6enm−2withinthe

non-polarregionthatmake[Ch]Salmorehydrophobicthan[Ch]Ac.

Inaddition,thehydroxylgroupattachedtothearomaticring,which

isknownaselectronwithdrawing,causetheCOO−ofSal−anion

becomeslessnegative,thusreducingitsabilityasH-bond

accep-tor(Fig.4a). Therefore, withhigher hydrophobiccharacter and

lesserabilityasH-bondacceptor,itisexpectedthatwaterwillhave

lessattractivecharacterto[Ch]Salthan[Ch]Ac.Amongthestudied

Fig.4.Sigmaproﬁle(a)andpotential(b)ofH2O,[Ch][Ac],and[Ch][Sal](fullline,

dashedline,dashed-dottedline,respectively).

cholinium-basedionicliquids,[Ch]Acshowsthegreatestcapacity

asH-bondacceptor,whichisinagreementtostrongerinteractions

shownbythewateractivitycoefﬁcientsmeasured.

3.3. COSMO-RS:descriptionofbinarymixtures

The activity coefﬁcient of water in cholinium-based ionic

liquidsispresentedinFigS4-S12andtheaverageabsolute

devi-ations are reported in Table S1in theSupporting Information.

Figs. 2 and 3 present the comparison between experimental

values and COSMO-RS prediction. AAD lesser than 4% were

obtainedforthebinarymixturesofH2O+[Ch]Sal,H2O+[Ch]DHph,

H2O+[Ch]Bit,H2O+[Ch]DHCit,andH2O+[Ch]Gly,indicating

abil-ityofCOSMO-RStoquantitativelypredicttheactivitycoefﬁcient

ofwaterinthebinarymixtures.Eventhoughthebinarymixtures

H2O+[Ch]Cl,H2O+[Ch]Ac,H2O+[Ch]Bic,andH2O+[Ch]Lachave

AAD between10 and 16%, COSMO-RSstill abletoqualitatively

predictthechangeof activitycoefﬁcientof water inthese

sys-tems,evenifinsomecasestherelativemagnitudeoftheactivity

coefﬁcients do notfollow theexperimentaltrend. Fig.3allows

thestudy ofthe impactof structuralvariations of

carboxylate-based ionic liquids. The addition of OH group to the acetate

anion,as inthecaseof glycolateanion, increasesthehydrogen

(5)

for[Ch][Ac]=−81.25,[Ch][Gly]=−111). Thusthewater requires

moreenergytodestroytheH-bondbetweenthe[Ch]+_cation_and

[Gly]−anionleadingtothelessinteractionofwaterwith[Ch][Gly]

observed.Thesubstitutionofthe CH3groupofAc−withan

aro-maticring,asinthecaseofSal− alsosigniﬁcantlyreducestheir

interactionwithwater,asobservedfromCOSMO-RSpredictions

aswellasexperimentally.In thiscase, thepresenceof the

aro-maticring,asindicatedbythebigpeakat0.6enm−2inthenon

polarregion(Fig.4a),leadstoahigherhydrophobiccharacterofthe

respectiveionicliquids,makingthenthuslessattractivetowater.

Althoughasuperiorquantitativeagreementwasobtainedforthe

activitycoefﬁcientofwaterin[Ch]Acand[Ch]Gly,deviationswere

observedfor[Ch]Lacand[Ch]Sal.Itseemsthehigherdeviationsare

relatedtotheincreaseofthehydrophobiccharacteroftheionic

liquids.Thesedeviations,alsofoundfor[Ch]Cland[Ch]Bic,might

alsobecausedfromtheIL-watermicrostructureformationthatitis

nottakenintoaccountbytheCOSMO-RScalculations[52,71].

Nev-ertheless,theresultsobtainedfromCOSMO-RScalculationsshow

anacceptableagreementwiththeexperimentaldata,depictinga

goodpredictingcapacityofthismethod.Therefore,theresultswill

befurtherdiscussedbasedontheinteractions describedbythe

COSMO-RSmodel.

COSMO-RScanbeappliedforastraightforwardestimationof

interactionenergy between ionic liquid and water in the

mix-turestate. The HE

m is thechange in enthalpy upon mixture of

the two components. In the case of water and ionic liquid, it

involvesthedisruptionofinteractionbetweenwater–waterand

ionicliquid–ionicliquids,andtheestablishmentofinteractionin

themixtureofwater–ionicliquid[59].Inapreviousworkwehave

shownthecapabilityofCOSMO-RStopredicttheexcessenthalpies

ofbinarysystemscomposedofwaterandionicliquids[59].In

gen-eral,COSMO-RSproducesagoodqualitativeagreementwiththe

reportedexperimentaldata,anditcanthusbeusedasapriori

pre-dictivemethod.TheestimationofHE

mintheCOSMO-RSmethod

resultsfromsummingthespeciﬁcmolecularinteractions,namely

electrostatic/misﬁt,HE

m,MF;hydrogenbonds,HEm,HB; andvander

Waalsforces,HE

m,VdW.Thecontributionofeachspeciﬁcmolecular

interactiontotheHE

mcanbewrittenas,

HE

m=Hm,MFE +HEm,HB+HEm,VdW (2)

Therefore,theCOSMO-RSmethodcanbeusedtoanalyzetheHE

m

valuesintermsofspeciﬁcmolecularinteractionbetween

compo-nentswaterandcholinium-basedionicliquids.

TheestimatedHE

m,HEm,MF,Hm,HBE ,andHm,VdWE throughoutthe

wholecompositionforthestudiedsystemsaregiveninFigures

S15-S22intheSupportingInformation.Theestimatedexcessenthalpies

werenegativethroughoutthewholecomposition,asalsoobserved

experimentallyfor(H2O+[Ch]Gly)and(H2O+[Ch]Lac)reportedby

Constantinescuetal.[32].Thereportedexperimentaldata,along

withitsCOSMO-RSprediction,aregiveninFigureS23.Itis

remark-abletoseethatminimaarepresentonallsystemsatxH2O∼0.66

thatseem toindicate theformation ofa complexbetweentwo

molecules of water and one molecule of the cholinium-based

ionicliquids.Fig.5presentstheestimatedHE

m,HEm,MF,Hm,HBE ,and

HE

m,VdWforwaterandcholinium-basedionicliquidsat298.2Kat

xH2O=0.66. Thenegative H

E

m valueindicatesfavorable

interac-tionsbetweenwater andthecholinium-based ionicliquids.For

all binarymixtures, thedominant interaction is the

hydrogen-bonding,whichcontributestotheexothermicityofthemixtures.

Theseresultsarise fromtheruptureof hydrogenbondsamong

watermoleculesandthecholinium-basedionicliquidsitself,and

theestablishmentofnewhydrogenbondsbetweenwaterandthe

ionicliquidsanion.Forexample,inthecaseofwaterand[Ch]Ac,it

canbedescribedasanexothermicmixtureduetostrongH-bond

acceptorofacetateanion,whichpromotesstronginteractionsof

water and[Ch]Ac, ascan bededucedfromtheCOSMO-RS.The

electrostaticinteractionsareattractive,asindicatedbyits

nega-tivevalues,whichslightlycontributetothetotalexcessenthalpy

ofthemixture.ThevanderWaalsforcesmakethesmallest

contri-butiontotheexcessenthalpyvaluesofthesemixtures.Therefore,

accordingtotheCOSMO-RSmodel,theinteractionofwaterand

cholinium-basedionicliquidscanbeenhancedbyproperselection

oftheanioncounterpart.

TheexcessGibbsfreeenergywasestimatedusingCOSMO-RS

usingthefollowingequation

GE

RT =(xw·lnw)+(xIL·lnIL) (3)

wherexand aremolefractionandactivitycoefﬁcient,

respec-tively,andthesubscriptwreferstowaterandILtotheionicliquid.

Thestudiedmixturesofwaterandcholinium-basedionicliquids

presentnegativevaluesofexcessfreeGibbsenergy(Figs.S24-S25),

whichindicatesspontaneousdissolutionoftheionicliquidsinto

waterandviceversa,asobservedexperimentally.Thus,inthiswork

COSMO-RSprovedtobeatoolofgreatvaluetoselectanddesign

ionicliquidsforagivenapplication,sinceitgivesarationalizationof

theselectionoftheionicliquidstoobtainthetypeofsolvent–solute

interactionrequired.

3.4. Abinitiocalculation

To gather more information on the possibility of the

com-plexformationbetweenwaterandcholinium-basedionicliquids

-12 -10 -8 -6 -4 -2 0

[Ch]Ac [Ch]Gly [Ch]Lac [Ch]Sal [Ch]Cl [Ch]DHph[Ch]DHCit [Ch]Bic [Ch]Bit

H

E mor

G

E/kJ·mol-1

Fig.5.EstimatedexcessfreeGibbsenergy,GE

m(bluebars)andenthalpyof(water+choline-basedionicliquids)binarymixtureat298.2KusingCOSMO-RSatxH2O=0.66in

termsofcontributionofHE

m,MF(orangebars),HEm,HB(greenbars),andHm,VdWE (redbars)tothetotalexcessenthalpy,HmE(yellowbars).(Forinterpretationofthereferencesto

(6)

Fig.6. TheHUMO–LUMOgapforthecholinium-basedionicliquidsandits com-plexeswithwaterSymbols:[Ch]Cl( ),[Ch]DHph( ),[Ch]Ac( ),[Ch]DHCit( ), [Ch]Bic( ),[Ch]Bit( ),[Ch]Sal( ),[Ch]Gly( ),and[Ch]Lac(+).(For interpre-tationofthereferencestocolourinthisﬁgurelegend,thereaderisreferredtothe webversionofthisarticle.)

discussedabove,abinitiocalculationswereperformed.Inquantum

chemistrymethods,thereareseveralbasicparametersof

impor-tancesuchas,HOMO(HighOccupiedMolecularOrbital)energy,

LUMO(LowUnoccupiedMolecularOrbital) energy,and

HOMO-LUMOenergygap.Detailsonthestudyoftheelectronicstructure

principleinstaticanddynamicsituationsaregiveninthe

refer-ence[62].TheHOMOandLUMOenergiesarethemostimportant

featuresfortheinteractionofwaterandcholinium-basedILs.The

interactioniscausedbyelectronﬂowingfromtheHOMO,which

thehighestenergy,totheLUMO.Therefore,thehighertheHOMO

energy,themorereactivethemoleculeis.Ontheotherhand,LUMO

islikelyalocationforabondtooccurasincomingelectronsfrom

othermoleculeswillﬁllfromHOMOintotheLUMOorbital.The

lowerLUMOenergyindicatesalessstablemolecule.The

HOMO-LUMO energy gap is used as quantum chemical descriptor in

establishingcorrelationsforchemicalandbiochemicalsystems.A

largeHOMO–LUMOgapimplieshighstabilityforthemoleculein

thesenseofitslowerchargetransferinthesystem.Forsystems

involvingwater and cholinium-based ionicliquids, the

HOMO-LUMOgapismoreimportantsinceitindicatesthestabilityofthe

complexesformed,asgiveninTableS2ofSupportingInformation

forseveralpossiblecomplexes.WaterpresentsthelowestHOMO,

highestLUMO,andhighestHOMO–LUMOgapenergythat

trans-latesthehighstabilityof thismolecule. Ontheotherhand,the

studiedcholinium-basedionicliquidspresentlowHOMO–LUMO

energygap.ThisfactisrelatedwiththeirlowLUMOenergy,

mak-ingthemmorereactive,lessstable,andhavinghighertendencyto

attractwater.Thecomplex{[Ch]Ac-H2O}hashigherHOMO–LUMO

gapcomparedtothepure[Ch]Acitself,asdepictedinFig.6,

indicat-inghigherstabilityofthecomplexmolecule.Increasingthenumber

ofwatermoleculestotwo,alsoincreasestheHUMO–LUMOgap.

However,theHUMO–LUMOgapdecreasedforthreemoleculesof

waterinthecluster.Itindicatesthattwomoleculesofwaterand

onemoleculeof[Ch]Acformedthemoststablecomplexandthis

ﬁndingis ingood agreementwiththeexcess enthalpyat

min-imaxH2O∼0.66,inwhich theratioof watertoionicliquidsis2

to1. The other studied cholinium-based IL also showed

maxi-mumHOMO–LUMOgapenergywhentheratioofwatertoionic

liquids is 2 to1. The exception was observedfor thecomplex

of {[Ch]Bic-x·H2O}and {[Ch]Lac-x·H2O},in which theyshowed

insteadminimumHOMO–LUMOgapenergyatthesameratio.In

anycaseallthestudiedsystemsshowedtheformationof

com-plexesbetweentwomoleculesofwaterandonemoleculeofionic

liquids.Thus,theabinitiocalculationsfosteredthe

understand-ingoftheinteractionsoccurringinthebinarysystemofwaterand

cholinium-basedionicliquidsintermofmolecularorbitallevel.

4. Conclusions

Cholinium-basedionicliquidsareregardedasanalternativeto

thetraditionalionicliquidsduetotheirhigherbiocompatibility,

andsincetheycanbeobtainedfromcheaperrawmaterials.The

behaviorofcholinium-basedionicliquidswithwaterisvery

impor-tanttodesignspeciﬁcapplicationsinvolvingthesemixtures.This

workproposesanew,fastandreliableexperimentaltechniquefor

themeasurementoftheactivitycoefﬁcientofwaterinionic

liq-uidsbythemeasurementofwateractivities.Theprocedurewas

validatedonthewateractivitiesofquaternariumammoniumsalts

and[Ch]Cl,andthewateractivitiesandactivitycoefﬁcientsinnine

choliniumbasedaqueoussystemsarereported.

Satisfactoryagreementbetweenexperimentaldataand

predic-tionsusingCOSMO-RSwasobserved.Themodelshowsthatthe

interactionofwater inthesemixturesis stronglyinﬂuencedby

thetype oftheanion present inthecholinium-based ionic

liq-uids.Indeed,itwasobservedH-bondinteractionbetweenwater,

asH-bonddonor,withtheanion,asH-bondacceptor.Theabinitio

calculationsshowedtheformationofstablecomplexesbetween

twomoleculesofwaterandonemoleculeofcholinium-basedILs.

Theexperimentalprocedurehereproposedcanbeeasilyappliedto

othermixturescontainingionicliquidswithwatertounderstand

theinteractionsinthemixture.

Acknowledgment

This workwasﬁnanced by national fundingfrom Fundac¸ão

para a Ciência e a Tecnologia (FCT, Portugal),European Union,

QREN,FEDERandCOMPETEforfundingtheCICECO(project

Pest-C/CTM/LA0011/2013), QOPNA (project Pest-C/QUI/UI0062/2011)

and LSRE/LCM (project Pest-C/EQB/LA0020/2013). Imran Khan

and Kiki A. Kurnia acknowledge FCT for the postdoctoral

grantsSFRH/BPD/76850/2011andSFRH/BPD/88101/2012,

respec-tively and also Tânia E.Sintra acknowledgeFCT doctoral grant

SFRH/BD/85871/2012.

AppendixA. Supplementarydata

Supplementarymaterialrelatedtothisarticlecanbefound,in

theonlineversion,athttp://dx.doi.org/10.1016/j.ﬂuid.2013.10.032.

References

[1]P.Wasserscheid,T.Welton(Eds.),IonicLiquidsinSynthesis,2ndedition, WILEY-VCHVerlagGmbH&Co.KGaA,Darmstadt,FederalRepublicofGermany, 2009.

[2]M.Yu,S.M.Li,X.Y.Li,B.J.Zhang,J.J.Wang,Ecotox.Environ.Safe.71(2008) 903–908.

[3]X.Y.Li,J.Zhou,M.Yu,J.J.Wang,Y.C.Pei,Ecotox.Environ.Safe.72(2009) 552–556.

[4]R.F.M.Frade,C.A.M.Alonso,Hum.Exp.Toxicol.29(2010)1038–1054. [5]M.Besnard,M.I.Cabac¸o,F.VacaChávez,N.Pinaud,P.J.Sebastião,J.A.P.Coutinho,

J.Mascetti,Y.Danten,J.Phys.Chem.A116(2012)4890–4901. [6]D.Coleman,N.Gathergood,Chem.Soc.Rev.39(2010)600–637.

[7]A.Romero,A.Santos,J.Tojo,A.Rodríguez,J.Hazard.Mater.151(2008)268–273. [8]A.P.Abbott,G.Capper,D.L.Davies,H.L.Munro,R.K.Rasheed,V.Tambyrajah,

Chem.Commun.(2001)2010–2011.

[9]A.P.Abbott,D.Boothby,G.Capper,D.L.Davies,R.K.Rasheed,J.Am.Chem.Soc. 126(2004)9142–9147.

[10]A.P.Abbott,G.Capper,K.J.McKenzie,K.S.Ryder,J.Electroanal.Chem.599(2007) 288–294.

[11]A.Bakkar,V.Neubert,Electrochem.Commun.9(2007)2428–2435. [12]A.P.Abbott,K.ElTtaib,K.S.Ryder,E.L.Smith,TIMetFinish86(2008)234–240. [13]R.Böck,S.E.Wulf,TIMetFinish87(2009)28–32.

(7)

[15]J.Pernak,A.Syguda,I.Mirska,A.Pernak,J.Nawrot,A.Pradzynska,S.T.Grifﬁn, R.D.Rogers,Chem.Eur.J.13(2007)6817–6827.

[16]P.Nockemann,B.Thijs,K.Driesen,C.R.Janssen,K.VanHecke,L.VanMeervelt,S. Kossmann,B.Kirchner,K.Binnemans,J.Phys.Chem.B111(2007)5254–5263. [17]Z.Li,X.Liu,Y.Pei,J.Wang,M.He,GreenChem.14(2012)2941–2950. [18]M.Petkovic,J.L.Ferguson,H.Q.N.Gunaratne,R.Ferreira,M.C.Leitao,K.R.

Sed-don,L.P.N.Rebelo,C.S.Pereira,GreenChem.12(2010)643–649.

[19]X.Wang,C.A.Ohlin,Q.Lu,Z.Fei,J.Hu,P.J.Dyson,GreenChem.9(2007) 1191–1197.

[20]S.P.M.Ventura,F.A.Silva,A.M.M.Gonc¸alves,J.L.Pereira,F.Gonc¸alves,J.A.P. Coutinho,Ecotox.Environ.Safe.(2013),Submitted.

[21]J.Palomar,M.Gonzalez-Miquel,J.Bedia,F.Rodriguez,J.J.Rodriguez,Sep.Purif. Technol.82(2011)43–52.

[22]J.Bedia,J.Palomar,M.Gonzalez-Miquel,F.Rodriguez,J.J.Rodriguez,Sep.Purif. Technol.95(2012)188–195.

[23]S.Aparicio,M.Atilhan,J.Phys.Chem.B116(2012)9171–9185.

[24]R.Ferreira,H.Garcia,A.F.Sousa,M.Petkovic,P.Lamosa,C.S.R.Freire,A.J.D. Silvestre,L.P.N.Rebelo,C.S.Pereira,NewJ.Chem.36(2012)2014–2024. [25]X.D.Hou,T.J.Smith,N.Li,M.H.Zong,Biotechnol.Bioeng.109(2012)2484–2493. [26]K.Ninomiya,A.Ohta,S.Omote,C.Ogino,K.Takahashi,N.Shimizu,Chem.Eng.

J.215(2012)811–818.

[27]S.Abello,F.Medina,X.Rodriguez,Y.Cesteros,P.Salagre,J.E.Sueiras,D.Tichit, B.Coq,Chem.Commun.(2004)1096–1097.

[28]S.Hu,T.Jiang,Z.Zhang,A.Zhu,B.Han,J.Song,Y.Xie,W.Li,TetrahedronLett. 48(2007)5613–5617.

[29]J.F.B.Pereira,F.Vicente,V.C.Santos-Ebinuma,J.M.Araújo,A.Pessoa,M.G.Freire, J.A.P.Coutinho,ProcessBiochem.48(2013)716–722.

[30]A.J.Costa,M.R.Soromenho,K.Shimizu,I.M.Marrucho,J.M.Esperanca,J.N. Lopes,L.P.Rebelo,ChemPhysChem13(2012)1902–1909.

[31]N.Muhammad,M.I.Hossain,Z.Man,M.El-Harbawi,M.A.Bustam,Y.A.Noaman, N.B.MohamedAlitheen,M.K.Ng,G.Hefter,C.Y.Yin,J.Chem.Eng.Data57(2012) 2191–2196.

[32]D.Constantinescu,K.Schaber,F.Agel,M.H.Klingele,T.J.S.Schubert,J.Chem. Eng.Data52(2007)1280–1285.

[33]Q.Zhou,Y.Song,Y.Yu,H.He,S.Zhang,J.Chem.Eng.Data55(2010)1105–1108. [34]A.J.L.Costa,M.R.C.Soromenho,K.Shimizu,I.M.Marrucho,J.M.S.S.Esperanc¸a,

J.N.C.Lopes,L.P.N.Rebelo,J.Phys.Chem.B116(2012)9186–9195.

[35]J.M.Prausnitz,R.J.Lichtnethal,E.G.Azevedo,MolecularThermodynamicof FluidPhaseEquilibria,3rdedition,PrenticeHall,Inc,NewJersey,1999. [36]J.Wang,D.Zheng,L.Fan,L.Dong,J.Chem.Eng.Data55(2010)2128–2132. [37]A.E.Andreatta,M.Francisco,E.Rodil,A.Soto,A.Arce,FluidPhaseEquilib.300

(2011)162–171.

[38]N.Calvar,B.González,E.Go_mez,_A.N._Domi_nguez,_J._Chem._Eng._Data₅₄₍₂₀₀₉₎ 1004–1008.

[39]V.H.Alvarez,S.Mattedi,M.Aznar,J.Chem.Thermodyn.43(2011)895–900. [40]R.Chen,L.Zhong,C.Xu,J.Chem.Eng.Data57(2012)155–165.

[41]A.Arce,J.Martínez-Ageitos,A.Soto,FluidPhaseEquilib.122(1996)117–129. [42]N.Calvar,B.González,E.Gómez,Á.Domínguez,J.Chem.Eng.Data51(2006)

2178–2181.

[43]N.Calvar,B.González,E.Gómez,A.Domínguez,FluidPhaseEquilib.259(2007) 51–56.

[44]P.J.Carvalho,I.Khan,A.Morais,J.F.O.Granjo,N.M.C.Oliveira,L.M.N.B.F.Santos, J.A.P.Coutinho,FluidPhaseEquilib.354(2013)156–165.

[45]L.Ninni,A.J.A.Meirelles,Biotechnol.Progr.17(2001)703–711. [46]S.P.Pinho,J.Chem.Eng.Data53(2008)180–184.

[47]S.Lindenbaum,G.E.Boyd,J.Phys.Chem.68(1964)911–917.

[48]A.Klamt,COSMO-RSfromQuantumChemistrytoFluidPhaseThermodynamics andDrugDesign,Elsevier,Amsterdam,TheNetherlands,2005.

[49]A.Klamt,WiresComput.Mol.Sci.1(2011)699–709.

[50]G.Vakili-Nezhaad,M.Vatani,M.Asghari,Chem.Eng.Commun.200(2013) 1102–1120.

[51]S.Martinho,J.M.M.Araújo,L.P.N.Rebelo,A.B.Pereiro,I.M.Marrucho,J.Chem. Thermodyn.64(2013)71–79.

[52]M.G.Freire,L.M.N.B.F.Santos,I.M.Marrucho,J.A.P.Coutinho,FluidPhase Equi-lib.255(2007)167–178.

[53]M.G.Freire,S.P.M.Ventura,L.M.N.B.F.Santos,I.M.Marrucho,J.A.P.Coutinho, FluidPhaseEquilib.268(2008)74–84.

[54]M.Diedenhofen,F.Eckert,A.Klamt,J.Chem.Eng.Data48(2003)475–479. [55]K.N.Marsh,J.A.Boxall,R.Lichtenthaler,FluidPhaseEquilib.219(2004)93–98. [56]U.Domanska,A.Pobudkowska,F.Eckert,GreenChem.8(2006)268–276. [57]P.Reddy,M.AslamSiddiqi,B.Atakan,M.Diedenhofen,D.Ramjugernath,J.

Chem.Thermodyn.58(2013)322–329.

[58]A.R.Ferreira,M.G.Freire,J.C.Ribeiro,F.M.Lopes,J.G.Crespo,J.A.P.Coutinho, Ind.Eng.Chem.Res.50(2011)5279–5294.

[59]K.A.Kurnia,J.A.P.Coutinho,Ind.Eng.Chem.Res.52(2013)13862–13874. [60]R.Lü,H.Tangbo,Z.Cao,J.NatGasChem.16(2007)70–77.

[61]E.A.Turner,C.C.Pye,R.D.Singer,J.Phys.Chem.A107(2003)2277–2288. [62]R.Anantharaj,T.Banerjee,FluidPhaseEquilib.293(2010)22–31. [63]S.Aparicio,R.Alcalde,M.Atilhan,J.Phys.Chem.B114(2010)5795–5809. [64]N.Muhammad,M.I.Hossain,Z.Man,M.El-Harbawi,M.A.Bustam,Y.A.

Noa-man,N.B.M.Alitheen,M.K.Ng,G.Hefter,C.Y.Yin,J.Chem.Eng.Data57(2012) 2191–2196.

[65]D.G.Archer,J.Phys.Chem.Ref.Data28(1999)1–17. [66]J.A.Rard,S.L.Clegg,J.Chem.Eng.Data42(1997)819–849.

[67]a.d.o.U.o.K.a.F.K.G.TURBOMOLEV6.12009,1989-2007,25GmbH,since2007; availablefromhttp://www.turbomole.com,inpress.

[68]A.K.F.Eckert,COSMOthermVersionC2.1Release01.08,COSMOlogicGmbH& Co.KG,Leverkusen,Germany,2006,inpress.

[69]R.Fleming,J.Chem.Soc.0(1961)3100–3102.

[70]J.B.Macaskill,M.S.Mohan,R.G.Bates,Anal.Chem.49(1977)209–212. [71]L.M.N.B.F.Santos,J.N.CanongiaLopes,J.A.P.Coutinho,J.M.S.S.Esperanc¸a,L.R.