Electrochemical Characterization of a Polymer Inclusion Membrane Made of Cellulose Triacetate

12 Pages·2017·1.63 MB·English

separations
Article
Electrochemical Characterization of a Polymer
Inclusion Membrane Made of Cellulose Triacetate
and Aliquat 336 and Its Application to
Sulfonamides Separation
JuanaBenavente1,*,VirginiaRomero1,MaríaIsabelVázquez1,EnriquetaAnticó2and
ClàudiaFontàs2
1 DepartamentodeFísicaAplicadaI,FacultaddeCiencias,UniversidaddeMálaga,E-29071Málaga,Spain;
[email protected](V.R.);[email protected](M.I.V.)
2 DepartmentofChemistry,UniversityofGirona,C/M.AurèliaCapmany69,17003Girona,Spain;
[email protected](E.A.);[email protected](C.F.)
* Correspondence:[email protected];Tel.:[phone]
Received:23November2017;Accepted:20December2017;Published:15January2018
Abstract: Anelectrochemicalcharacterizationofapolymerinclusionmembrane(PIM)fabricated
with the ionic liquid (IL) Aliquat 336 (26%) and the polymer cellulose triacetate (CTA) (76%)
is presented. Considering the use of PIMs in separation systems to remove pollutants from
water,thecharacterizationwasperformedwithNaClsolutionsbymeasuringmembranepotential,
electrochemicalimpedancespectroscopy,andsaltdiffusionandresultswerecomparedwiththose
obtainedfromdrymembranes. Resultsshowedasignificantreductioninthemembranediffusive
permeabilityandelectricalconductivityaswellasthetransportnumberofcationNa+ acrossthe
PIMwhencomparedwithsolutionvalues, whichcouldbemainlyrelatedtothedensecharacter
ofthemembrane. Membraneapplicationintheseparationofdifferentsulfonamides(sulfathiazole,
sulfapyridine,sulfamethazine,andsulfamethoxazole)fromwater,with1MNaClsolutionasstriping
phase, was also considered. These results indicated that the different chemical characteristics of
thecompounds, aswellasthecompactstructureofthePIM,limitedthetransportoftheorganic
moleculesthoughit.
Keywords: polymer inclusion membrane; Aliquat 336; membrane potential; impedance
spectroscopy;sulfonamides
1. Introduction
As it is well known, ionic liquids (ILs) are salts composed of an organic cation and an
inorganic/organicanionwithverylowvaporpressure,lowsolubility,goodelectricalconductivity,
andthermalstability[1],whichallowtheiruseinalargenumberofdifferentapplicationssuchas
solventsfororganicreactionsandcatalysis,energy,electro-deposition,greenchemistry,separation
processes or CO capture, among others [2–9]. Moreover, ILs can be used as extractants in
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the preparation of polymer inclusion membranes (PIMs). PIMs basically consist of a polymeric
matrix, normally made of cellulose triacetate (CTA) or poly(vinyl chloride) (PVC) [10,11] and an
extractant (named carrier), which is responsible for transport across the membrane. Usually the
addition of a plasticizer or some modifiers is needed to obtain adequate elastic characteristics of
the membranes [12]. However, the use of ILs as carriers favors the flexibility and mechanical
strength of PIMs avoiding the need of the addition of other components, and their high viscosity
helpstoretaintheminthepolymergel-likenetwork. DifferentcommercialILssuchasCyphos101
(trihexyl(tetradecyl)phosphoniumchloride),Cyphos102(trihexyl(tetradecyl)phosphoniumbromide),
Separations2018,5,5;doi:10.3390/separations5010005 www.mdpi.com/journal/separations
Separations2018,5,5 2of12
Cyphos104(trihexyl(tetradecyl)phosphoniumbis-2,4,4-(trimethylpentyl)phosphinate),orAliquat336
(trioctylmethylammoniumchloride)havebeenreportedascarriersinPIMsusedintheseparationof
metalspeciessuchasCd,Cu,orIn,organiccompoundssuchasantibioticsorwaterpollutants[13–17].
In previous works, different physicochemical characteristics of PIMs (contact angle,
Youngmodulus,conductivity,ordielectricconstant)wereanalyzedformembranescontainingdifferent
Aliquat336(AlqCl)percentagesinbothCTAorPVCpolymers[18],aswellasforPIMswherethe
counter-anion of Alq was modified (AlqCl, AlqNO or AlqSCN) [19]. However, information on
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other electrochemical characteristic parameters, such as ion transport numbers or ionic diffusion
coefficients [20,21], determined from measurements carried out with PIMs in contact with saline
solutionsseemstobealsoofinterestfortheirapplications.
Thus, in this study an electrochemical characterization of an Aliquat 336/CTA PIM (26%
AlqCl/74% CTA) is presented performed by membrane potential and impedance spectroscopy
measurements using a typical model electrolyte (NaCl) at different solution concentrations.
A comparison of other physicochemical parameters depending on the PIM state (dry or wet) is
alsoindicatedtohaveamoreadequatepictureofthemembranebehaviorinusualworkingconditions
forwatertreatment.
Separation abilities of the characterized PIM are studied using a mixture of four common
antibiotics, belonging to the sulfonamides family, which can be found in natural waters due to
theirwideuseandincompleteeliminationthroughsewagetreatment.
2. ResultsandDiscussion
Taking into account that PIMs are aimed to transport and separate compounds of interest in
aqueoussolution,itisimportanttodeterminetheirphysicochemicalcharacteristicsinawetmediaand
tocomparethemwithrespecttodrymembranes. ItisimportanttobearinmindthatPIMscanlose
partoftheILincorporatedinthepolymericnetworkwhenmembranesareimmersedindistilledwater
oraqueouselectrolytesolutions[22,23]. ThismasslossisassociatedwithboththenatureoftheILand
thepolymer. Inparticular,Kagayaetal.[23]foundarelativemasslossofaround20%foranAliquat
336/PVCmembraneafter48himmersionindistilledwater. However,themasslossdiminishedto2%
whenimmersedinaNaClsolution.
Inthisstudy,chemicalsurfacedifferencesbetweenawetPIM(sample26%AlqCl/74%CTA-w,
w indicating ‘wet state’, that is, after 48 h immersed in distilled water) and a dry PIM (sample
26%Alq336/74%CTA-d,dindicating‘drystate’)wasdeterminedbyanalyzingX-rayphotoelectron
spectroscopy (XPS) spectra. XPS is a well-known technique able to give information on chemical
changes on membrane surfaces since it provides elemental identification, relative composition,
and chemical state of the elements present in a surface region [24]. Consequently, differences in
theatomicconcentrationpercentages(A.C.%)determinedfortheelementsfoundonthesurfacesof
wetanddrymembranesamplesshouldbeanindicationofAliquat336modification/lost. Table1
showstheA.C.%oftheelementspresentonthesurfacesofthePIMs(C,O,N,Cl,andSi)indryand
wetstates(samples26%AlqCl/74%CTA-dand26%AlqCl/74%CTA-w),whichweredetermined
byanalyzingtheXPSspectra. Moreover,A.C.%oftheelementsfoundonthesurfaceofaCTAfilm
in dry and wet states (CTA-d and CTA-w samples, respectively) are also indicated in Table 1 for
comparisonreasons. Asexpected,bothCTA-filmsdonotshowanyClcontentsincethiselementis
associatedtoAliquat336. However,Si,anon-characteristicmembrane/filmelement,isdetectedin
allsamples. Thisfactcanbeattributedtoamanufacturecontamination,anditshigherpercentage
in wet samples might be related to element surface reorganization associated to the hydrophobic
characterofsilica[25,26]. Infact,surfacereorganizationofmorehydrophobicorganicmatterandN
impuritiescouldbeoneofthereasonsfordifferencesbetweendryandwetsamples. Inthiscontext,
itshouldalsobeindicatedthatthetotalN(%)obtainedfordryandwetPIMsamplesalsoseemsto
includecontaminationcontribution,whilethespecificAliquat336-Ncontribution(bindingenergy
at402.2eV[27])isindicatedinbrackets. Inthatcase,theN/Clratioissimilartothetheoreticalone
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((N/Cl)T =1). Consequently,thecomparisonoftheresultsobtainedforwetanddryPIMsamples
sinh otwhes paerrecdeunctatigoens inoft hCel panerdc eNn taasgseoscoiaftCedl atnod ANliqaussaot c3ia3t6e dint othAe lwiqeuta sta3m36pilne (tahreowunetds 2a5m%p)l.e T(ahrios ufancdt
2se5e%m).s Ttoh iisnfdaicctastee eam psartotiainl ldoiscsa toef athpea IrLti afrlolmos sthoef mtheemILbrfaronme stuhrefamcee,m bubrt atnhee esuffrefcatc oef, beluetmthenete sffuercftaocef
ereleomrgeanntizsuatrifoance dreepoerngdaninizga otino nthdeierp heingdhienrg/loownethr ehiyrdhriogphhero/bliocw chearrhaycdterro mphigohbti caclshoa braec itnevromlvigehdt. also
beinvolved.
Table 1. Atomic concentration percentages of the elements found * on the surfaces of wet (X-w) and
Tdarby l(eX1-d.A) PtoIMm iocrc ConTcAe nfitlrmat.i onpercentagesoftheelementsfound*onthesurfacesofwet(X-w)and
dry(X-d)PIMorCTAfilm.
PIM C (%) O (%) N (%) Cl (%) Si (%)
26% AlqCl/74% CTA-w 72.6 23.7 1.6 (0.38) 0.31 1.6
PIM C(%) O(%) N(%) Cl(%) Si(%)
26% AlqCl/74% CTA-d a 66.7 29.6 0.89 (0.50) 0.44 0.3
26%AlqCl/74%CTA-w 72.6 23.7 1.6(0.38) 0.31 1.6
CTA-w 73.7 24.1 0.65 n.d. 1.6
26%AlqCl/74%CTA-da 66.7 29.6 0.89(0.50) 0.44 0.3
CTA-d 66.6 32.3 n.d. n.d. 0.9
CTA-w 73.7 24.1 0.65 n.d. 1.6
* Other elemCeTnAts -wdith A.C. % < 0.3 a6r6e. n6ot indicate3d2;. 3a values fronm.d r.eference [1n8.d].. n.d. = not d0e.9tected.
*OtherelementswithA.C.%<0.3arenotindicated;avaluesfromreference[18].n.d.=notdetected.
Wet PIMs also exhibit different elastic behavior than dry ones, as can clearly be observed in
Figure 1, where a comparison of the strain-stress/elongation curves for wet and dry PIM samples is
Wet PIMs also exhibit different elastic behavior than dry ones, as can clearly be observed in
presented. Wet PIM sample shows a more plastic character, with a significant increase in the
Figure1,whereacomparisonofthestrain-stress/elongationcurvesforwetanddryPIMsamplesis
elongation at break (around eight times higher) and lower Young modulus (40% reduction), which
presented. WetPIMsampleshowsamoreplasticcharacter,withasignificantincreaseintheelongation
is determined from the slope of the linear relationship obtained at low strain-stress values (insert in
atbreak(aroundeighttimeshigher)andlowerYoungmodulus(40%reduction),whichisdetermined
Figure 1). These changes are attributed to the plasticizer character of water, as it was previously
from the slope of the linear relationship obtained at low strain-stress values (insert in Figure 1).
obtained for regenerated cellulose membranes [28]. This fact should be considered in the selection of
Thesechangesareattributedtotheplasticizercharacterofwater,asitwaspreviouslyobtainedfor
the Aliquat 336 content used for PIM manufacture, since it can play an important role on the
regeneratedcellulosemembranes[28]. ThisfactshouldbeconsideredintheselectionoftheAliquat
mechanical behavior (high plasticity or brittle) of the membranes, according to the significant
336contentusedforPIMmanufacture,sinceitcanplayanimportantroleonthemechanicalbehavior
reduction in Young modulus associated to Aliquat 336 content in dry AlqCl/CTA membranes [18].
(highplasticityorbrittle)ofthemembranes,accordingtothesignificantreductioninYoungmodulus
associatedtoAliquat336contentindryAlqCl/CTAmembranes[18].

Figure1. Strain-stressvs. elongationcurvesforwet((cid:3))anddry((cid:4))samplesofthe26%AlqCl/74%
Figure 1. Strain-stress vs. elongation curves for wet (□) and dry (■) samples of the 26% AlqCl/74%
CTAPIM.
CTA PIM.
Iontransportacrossamembraneisusuallycharacterizedbytheionicdiffusioncoefficients(D)
Ion transport across a membrane is usually characterized by the ionic diffusion coefficients (Dii)
ortheionictransportnumbers(t),whicharerelatedbythefollowingexpression: t =zD/ΣizD,
i i i i i i
or the ionic transport numbers (ti), which are related by the following expression: ti = ziDi/ΣiziDi, or
orti=Di/(D++D−)inthecaseof1:1singleelectrolytesolutions[29], beingzi thevalenceofioni.
Itoi n= tDrai/n(sDp+o r+t Dnu−)m ibne trhser ecparsees eonf t1t:h1 esifnragcltei oenleocftrtohleytteo tsaollcuutriorennst [(2I9]), tbraeninsgp ozrit ethdef ovraolenneceio onf (iton= Ii. /IoIn)
T i i T
transport numbers represent the fraction of the total current (IT) transported for one ion (ti = Ii/IT)
and, consequently, in the case of single 1:1 electrolytes they should fulfil that t+ + t− = 1. Ion transport
numbers are usually determined from membrane potential (∆Φmbr) values by [20,30]
∆Φmbr = (RT/F)(1 − 2t−)ln(a1/a2) = (RT/F)(1 − 2t−)ln(c1/c2) (1)
Separations2018,5,5 4of12
and,consequently,inthecaseofsingle1:1electrolytestheyshouldfulfilthatt++t− =1. Iontransport
nSuepmarbateiornssa 2r0e18u, s5,u 5a llydeterminedfrommembranepotential(∆Φ )valuesby[20,30] 4 of 11
mbr
where R and F are ∆thΦem gbars= a(nRdT F/aFr)a(1da−y 2cto−n)slnta(na1ts/,a T2 )r=ep(rReTse/nFt)s( 1th−e 2tetm−)plne(rca1t/ucr2e) of the system, wh(1il)e
a and a are the activities of the NaCl solutions at both membrane sides, which can be replaced by
1 2
wshoeluretioRna ncodnFceanretrtahteiogna s(ca1 nadndFa cr2a)d iany tchoen sctaasnet so,fT drieluptreedse nsotsluthtieontesm. Fpiegruartue r2e oshfothwess ythstee mva,rwiahtiiolena o1f
amndema2braarneet hpeotaecnttiivailtsi ewsiothf tthhee sNolauCtilosno cluotnicoennstraattiboont hfomr tehme b2r6a%n eAslqidCels/7,4w%h CicThAc amnebmebrreapnlea,c aesd wbeyll
saoslu tthioen ccaolcnucleantetrda tsioonlu(tcio1na nddiffcu2)siionnt hpeotceansteiaolf vdailluuetesd (∆soΦludiftoi o=n s(.RFTi/gFu)(r2et+2o s−h o1)wlns(cth1/ec2v))a, rwiahtieorne otf+o
membranepotentialswiththesolutionconcentrationforthe26%AlqCl/74%CTAmembrane,aswell
represents the Na+ transport number in solution, that is, when there is not any membrane separating
asthecalculatedsolutiondiffusionpotentialvalues(∆Φ o=(RT/F)(2t o −1)ln(c /c )),wheret o
the electrolyte solutions (dot-dashed line) [29]. For comdpifarison reasons+, membran1e p2otentials fo+r a
representstheNa+transportnumberinsolution,thatis,whenthereisnotanymembraneseparating
commercial positively charged ion-exchange membrane (AR204-SZRA-412 by Ionics) are also drawn
theelectrolytesolutions(dot-dashedline)[29]. Forcomparisonreasons,membranepotentialsfora
in Figure 2.
commercialpositivelychargedion-exchangemembrane(AR204-SZRA-412byIonics)arealsodrawn
inFigure2.

Figure2.MembranepotentialasafunctionofNaClsolutionconcentration:(♦)26%AlqCl/74%CTA
mFeigmubrrea n2.e ,M((cid:78)em)iborna-nexec phoatnegnetriaAl Ras2 0a4 f-uSZncRtAio-n4 1o2f mNeamClb sraonluet.iSoonl ucotinocnednitfrfautsioionn: (p◊o)t 2e6n%tia Al:ldqaCslh/7d4o%t lCinTeA.
membrane, (▲) ion-exchanger AR204-SZRA-412 membrane. Solution diffusion potential: dash dot
Alisneit. canbeobservedinFigure2,∆Φ valuesfor26%AlqCl/74%CTAandAR204-SZRA-412
mbr
membranesarerathersimilar,beingthosecorrespondingtotheion-exchangesampleslightlymore
negatiAvse .itM caonre boev eorb,sienrbvoedth inca Fsiegsu,r∆eΦ 2, ∆Φvmalbure vsadluifefesr fosirg 2n6ifi%c aAnltqlCylf/r7o4m% tChTosAe acnordr eAsRpo20n4d-iSnZgRtAo-t4h1e2
mbr
NmaeCmlsborlauntieosn a,rwe hriacthheisr asniminildairc,a btieoinngo fththoesec ocnotrrroelsopfobnodtihngm teom tbhrea nioens-oenxcihonansgtrea nsasmpoprlte. Isolingthrtalnys mpoorrte
nnuemgabteirvsea. cMroosrseothveerP,I iMn bwoethre cdaesteesr, m∆Φinmedbrb yvamlueeasn dsiofffeErq suigantiiofinca(1n)t,lya nfrdotmhe thfoolsloe wcoinrrgesapvoernadgiengv atolu tehse
wNeareCol bstoaliunteido:nt,− w=h0ic.9h1 i±s 0a.n0 3inanddicat+tio=n0 .0o9f 0th±e 0c.0o0n3t.roClo nofs ebqoutehn tmly,etmhebrdainffeuss iovne tiroannss ptorartnospfocartti. oInosn
atcrraonsssptohret sntuudmibederPs IaMcrsoesesm thset oPbIMep wraecrteic daleltyeremxcilnuedde db.yI nmfeaacnt,s toakf eEnqiunattoioanc c(o1u),n atnthde trheel aftoiollnoswhiinpg
baevtwereaegne ivoanluteras nwspeorer tonbutaminbeedrs: ta−n =d 0d.9if1f u±s i0o.n03c oaenfdfi cti+e n= ts0.[02990] ,±w 0e.0o0b3.t aCino:nts−eq/ut+en=tlDy,− t/hDe +di=ffu10si.v1;e
thtraatnmspeoarnts ,oCf lc−atsieoenms satcoromsso vthe1e0 sttiumdeisedh igPhIMer tsheaenmNs ato+ ibnet hperaPcItMic.aIlolyn etrxacnluspdoedrt. nIunm fabcetr, vtaaklueens ianrteo
raatchcoerusnitm thilea rrteolatthioonseshriepp obrettewdebeyn oiothne rtraauntshpoorrst fnourmPIbMersso abntadi ndeidffufosriodni fcfeoreefnfitciIeLnst[s2 [02]9.], we obtain:
t−/t+ E= lDec−t/rDo+c h=e 1m0.i1c;a lthimatp medeaanncse, Cspl−e scetreomscso tpoy m(EoIvSe)1m0 etiamsuerse hmigehnetsr athlsaonp Nroav+ iidne tvhael uPaIMbl.e Iionnfo trrmanastpioonrt
onnuemlebcterri cvaallcuheasr aacrete rraistthicesr osfimmielamr btora tnheoss.eF irgeuproert3esdh boyw sotthheerN ayuqthuoisrts pfloort P(−IMZsim ogbvtasi.nZerdea lf,oFr idguifrfeer3ean)t
aInLds t[h2e0]B. odeplot(−Zimgvs. frequency,Figure3b)obtainedforthe26%Alq336/74%CTAmembrane
separaEtliencgtrtowchoem0.0ic3alM imNpaeCdalnscoelu stpioecntsr.osTcwopoys (eEpIaSr)a mteedascuornetmriebnuttsi oanlssoa psrsoovciidatee vdatluoatbhlee imnfeomrmbraatnioen
(forneq euleecntcriycarla cnhgairnagctbeeritswtieces nof1 m0ekmHbzraanneds. 1F0i0g0urkeH 3z s)haonwds tthhee Nelyeqcturioslty ptelosto (l−uZtiimogn vsp.l aZcreeadl, Fbiegtuwree e3na)
thanedm theme Bboradnee plaontd (−tZhiemg evlse.c ftrroeqdueesn(cfyre, qFuigeunrcey 3>b) 1o0b0t0ainkeHdz f)ocr atnheb 2e6%ob Aselrqv3e3d6/7in4%F iCgTuAre m3,emwbhriacnhe
csoerpreasrpaotinndg ttowaod 0e.p0r3e sMse dNsaeCml iscoirlculteio(mnse. mTbwraon see)paanrdataedse cmoinctirricbleut(ieolencst raoslsyotecisaoteludt itoon )t.hTe hmisekminbdraonfe
im(frpeeqduaennccey cruarnvgeinisg tbyeptiwcaeleonf 1c0o mkHpzo sainteds 1y0s0te0m ksH,zin) apnadrt itchuel aerleicttirsolsyimtei lsaorlutotiothna ptleaxchedib ibteetdwbeyenlo twhe
membrane and the electrodes (frequency > 1000 kHz) can be observed in Figure 3, which correspond
to a depressed semicircle (membrane) and a semicircle (electrolyte solution). This kind of impedance
curve is typical of composite systems, in particular it is similar to that exhibited by low permeability
systems such as dense non-swollen membranes [31] or nanofiltration/reverse osmosis membranes
[32–34], and it is related with the significant difference in the electrical characteristics of the two parts
Separations2018,5,5 5of12
Separations 2018, 5, 5 5 of 11
fSoerpmaraitniogn s 2th01e8 , 5c,o 5m posite system: the membrane and the electrolyte solution, respectively. 5 Tofh 1e1
permeabilitysystemssuchasdensenon-swollenmembranes[31]ornanofiltration/reverseosmosis
equivalent circuit for the whole electrolyte(e)/membrane(m) system consists in the addition (series
membranes[32–34],anditisrelatedwiththesignificantdifferenceintheelectricalcharacteristicsof
afossromciinatgi onth) eo fc towmop sousibt-ec irscyustitesm: a: pthaer almleel masbsroacniaet ioannd o ft ha er eesliestcatrnocley t(eR )s aonludt iao nca, praecsiptoerc t(iCve) lfyo. r Tthhee
thetwopartsformingthecompositesystem: themembraneandtheelectrolytesolution,respectively.
eelqeuctirvoallyetnet csiorlcuutiito nfo rp tlhaece wd hboeletw eeleecnt rothlyet ee(ele)/cmtreomdebsr aanned(m t)h sey smteemm bcorannsies ts(R ienC teh),e paldudsi taio np a(srearllieels
The equivalent circuit for the whole electrolyte(e)/membrane(m) system consists in the addition
aassssoocciiaattiioonn )o of fa trwesoi sstuanbc-cei racnudit as: gae npearraallilzeel da scsaopcaiactiitoonr (oRfm aQ rme)s fiostra tnhcee m(Re)m abnrda nae c caopnatcriitbourt (ioCn) dfoure t htoe
(seriesassociation)oftwosub-circuits: aparallelassociationofaresistance(R)andacapacitor(C)
ae ldeicsttrroiblyuttei osno oluf trieolna xpatliaocne dti mbeestw [3e5e]n ( stehee M ealteecrtiraolds easn da nMde tthhoed sm seecmtiborna)n. e (ReCe), plus a parallel
fortheelectrolytesolutionplacedbetweentheelectrodesandthemembrane(R C ),plusaparallel
e e
association of a resistance and a generalized capacitor (RmQm) for the membrane contribution due to
associationofaresistanceandageneralizedcapacitor(R Q )forthemembranecontributionduetoa
m m
a distribution of relaxation times [35] (see Materials and Methods section).
distributionofrelaxationtimes[35](seeMaterialsandMethodssection).

Figure 3. Impedance plots for the 26% AlqCl/74% CTA membrane separating two NaCl solutions at

0F.i0g3u Mre c3o.nImcepnetrdaatniocne. p(alo) tNsyfoqrutihste p2l6o%t (A−ZlqimCg lv/s7.4 Z%reaCl);T (Ab)m Beomdeb rpalnoet (s−eZpiamrga vtisn. gfrtewquoeNncayC)l. solutionsat
0F.i0g3uMre c3o. nImcepnetrdaatniocne. p(lao)tNs fyoqru tihset p2l6o%t( A−lZqCl/7v4s%.Z CTA); m(be)mBobdraenpel osetp(−arZatingv tsw.for eNqauCenl csyo)lu.tions at
img real img
T0.h0e3 Mfi ct onocfe ntthraet ioinm. (pa)e dNaynqcueis t cpulortv (e−sZ imogb vtsa. iZnreeadl) ; (fbo)r B otdhee p lNot a(C−Zl imgc ovsn.c ferenqtruaetnicoyn)s. studied (see
electrochemical impedance spectroscopy (EIS) measurements in the Experimental section) allows us
ThefitoftheimpedancecurvesobtainedfortheNaClconcentrationsstudied(seeelectrochemical
the dTetheer mfiint atoiof n tohfe thiem mpeedmabnrcaen ec uanrvde es leoctbrtoaliynteed e lefoctrr ictahle r eNsisatCanl cec o(nRcme natnrda tRiosn, sr essptuecdtiievde ly()s aeet
impedancespectroscopy(EIS)measurementsintheExperimentalsection)allowsusthedetermination
eealeccht rNocahCelm soiclualt iiomnp, eadnadn Fcieg supreec 4trao sshcoopwys (tEhIeS )d mepeeansduerenmcee nRtms ainn dth Re sE wxpitehr iemleecntrtoally steec tcioonnc)e anlltorawtiso uns.
ofthemembraneandelectrolyteelectricalresistance(R andR ,respectively)ateachNaClsolution,
m s
Mthoer deoevteerrm, tinakaitniogn ionft ot haec cmoeumntb rthane er ealnadti oenle cbterotwlyeteen e lmecetmricbarla rnees ieslteacntcreic (aRl mr easnisdta Rnsc, er easnpde cetilvecetlryi)c aalt
andFigure4ashowsthedependenceR andR withelectrolyteconcentration. Moreover,takinginto
m s
caeocanccodhu uNncttaiCtvhilet ysro e(lσluamtti)io o[n3n,6 b]a entdw Feeignumree m4ab srhaonweesl ethcter idceaplerensdisetnacnec eRamn adnedl eRcst rwiciathlc eolnecdturoctliyvtiet yco(σnce)n[t3r6a]tion.
Moreover, taking into account the relation between membrane electrical resistance andm electrical
σm = Δxm/RmSm (2)
conductivity (σm) [36] σ =∆x /R S (2)
m m m m
where ∆xm and Sm represent the membrane σthm i=ck Δnxems/sR amnSdm area respectively (assuming the transp(o2r) t
twakheesr ep∆laxce tahnrdouSghr tehper ewsehnotleth meemmebmrabnraen aerethai)c, kvnaersiastaionnd oafr emaermesbpreacntiev eelleyc(tariscsaulm coinngduthcetivtriatyn swpiotrht
m m
NtwaakhCeeslr epc ∆loaxncmce eatnhntrdroa Sutimgo hnre tpchareenws eahnlots olteh emb eme moembbtrbaairnnaeendea. r tehFaii)cg,kuvnraeers isa4 tabino ndsh oaorfwemas ermtehsbep reaσcntmie-vσeesll yerc e(talrasictsiauolmncsoihnnigdp u th(cmeti vtermiatynbsrwpaointrhet
cNtoaaknCedsul pcctloiavncicetey tn hvtrsro.a sutoigolhun ttichoaenn wcoahnlsodolue cbmteiveomitbybt)ar, aiwnnehed ea.rree tFahi)ge, uvsiargernia4iftbiiocasnnh oto fbw masrertimhereb erσfafneec-tσ e olfe rctehtlreai ct2ia6ol% ncso AhnildpquC(clmt/i7ve4im%tyb CwraTinAthe
m s
PcNoIManCd olu ncc ttoihvneict ieyonvntsr t.arstaionolsnup tiocorantn cc aoannls dboue cobtbiev sieotrybv)t,eawdin,h ewedrh.e icFthhige cuosriugeln d4i fibbce a srnhetloabwtaesrdr itteohr ete hfσfee mdc-teσnos fserteh sletart2ui6oc%ntusArheil pqa Cn(dlm/ l7oe4mw%b ioCranTniAce
dcoifnfudsuioctniv aictyro vsss. tshoilsu tkiionnd c oofn mduecmtibvriatyn)e, sw [h20e,r3e1 t]h. e significant barrier effect of the 26% AlqCl/74% CTA
PIMontheiontransportcanbeobserved,whichcouldberelatedtothedensestructureandlowionic
PIM on the ion transport can be observed, which could be related to the dense structure and low ionic
diffusionacrossthiskindofmembranes[20,31].
diffusion across this kind of membranes [20,31].

Figure4.(a)Variationof26%AlqCl/74%CTAmembrane(♦)andNaClsolution(×)electricalresistance
Fwigituhreel e4c.t r(oal)y tVeacroinactieonnt raotfi o2n6.%(b )AMlqeCml/b7r4a%n eCcoTnAd umcteimvibtyravneer s(u◊s) saonludt ioNna(CNl asColl)uctoionnd u(c×t)i veilt eyc.trical
resistance with electrolyte concentration. (b) Membrane conductivity versus solution (NaCl)
cDFoinigfdufuuresc itvi4v.e it(ypa.)e rVmareiaabtiiolnit yofa c2r6o%s sAalqmCle/m74b%r aCnTeA, Pm,eimsbarnanoeth (e◊r) panadra mNaeCtel rsuolsuutaiolnly (d×)e teelermctriincaeld for
s
memrbersaisnteancchea rwacitthe riezlaectitoronl,yoteb tcaoinnceednftrraotmions.a l(tbd) ifMfuesmiobnramnee acsounredmucetinvtisty. Avcecrosruds insgoltuotioFnic k(’NsafiCrls)t law,
Dcoinffduuscivtive itpye. rmeability across a membrane, Ps, is another parameter usually determined for
membrane characterization, obtained from salt diffusion measurements. According to Fick’s first law,
Diffusive permeability across a membrane, Ps, is another parameter usually determined for
membrane characterization, obtained from salt diffusion measurements. According to Fick’s first law,
Separations2018,5,5 6of12
Separations 2018, 5, 5 6 of 11
ththee ssoolluuttee flfluuxx ccrroossssininggt htheem memembrbarnaenaet astte satdeya-dsyta-tset,aJtse,, aJns,d atnhde ctohnec ceonntrcaetniotnradtiioffne rdenifcfeerwenhciceh wcahuicshes
csauucshesfl suuxc,h∆ fclu=x(, c∆dc− = (ccrd) ,−a crer),r ealraet eredlabtyed[3 b7y] [37]
JJs= =d dnn//ddtt == PPs ∆∆cc == PPs (c(dc − −cr)c ) (3)( 3)
s s s d r
Equation (3) can also be expressed as
Equation(3)canalsobeexpressedas
dcr/(cd − cr) = (Sm/Vo)Ps dt (4)
dc /(c −c )=(S /Vo)P dt (4)
r d r m s
Vo and Sm are the volume of the receiving cell and the membrane area, respectively. Taking into
aVccooaunndt tShme amreastsh ecovnotliunmuietyo: fctho e+r eccoe i=v icngt +c ecllt a=n dcotnhsetamnte, mwbhrearnee caroe aa,ndre scpoe cintidvieclayt.e Ttahkei ningitiinatlo
d r d r d r
accountthemasscontinuity: c o +c o =c t +c t =constant, wherec o andc o indicatetheinitial
concentrations and c t and ct at ad time rt, we dobtainr the expression d r
d r
concentrationsandc tandc tatatimet,weobtaintheexpression
d r
ln([1 − (2ct/c t)] = −2[S /(Vo ∆x )] P t (5)
r d m m s
ln([1−(2c t/c t)]=−2[S /(Vo∆x )]P t (5)
r d m m s
where Vo represents the cell volume (that means at t = 0). Figure 5 shows time evolution of solution
concentration separated by the PIM at a NaCl donor solution concentration of 0.01 M. Diffusive NaCl
whereVorepresentsthecellvolume(thatmeansatt=0). Figure5showstimeevolutionofsolution
permeability across the PIM was determined from the slope of the straight line shown in Figure 5,
concentrationseparatedbythePIMataNaCldonorsolutionconcentrationof0.01M.DiffusiveNaCl
apcceormrdeinabgi tliot yEqacuraotsiosnt h(5e)P, aIMndw thaes vdaeltueer mPsi n=e 3d.8f r×o 1m0−t8 hme/ssl owpaes oofbttahiensetdr.a Tighhist lvianleuseh ios wsinmiinlarF itgou threat5 ,
oabctcaoinrdeidn gftoorE qluowati onpe(5rm),eaanbdiltihtye vaolru erPev=er3s.e8 ×os1m0−o8sims//nsanwoafsilotrbattaioinne d.mTehmisbvraalnuees is[s3im1,3il2a]r toanthda, t
s
consequently, it seems to be associated to the dense structure of the membrane, which controls the
obtainedforlowpermeabilityorreverseosmosis/nanofiltrationmembranes[31,32]and,consequently,
transport when no carried mediated effect is involved.
itseemstobeassociatedtothedensestructureofthemembrane,whichcontrolsthetransportwhen
nocarriedmediatedeffectisinvolved.

Figure5.Timeevolutionofsolutionconcentrationsdifferenceratioatbothsidesofthe26%AlqCl/74%
Figure 5. Time evolution of solution concentrations difference ratio at both sides of the 26% AlqCl/74%
CTAPIMforNaCldonorconcentrationc o=0.01M.∆ct:solutionconcentrationdifferenceattimet;
CTA PIM for NaCl donor concentration cddo = 0.01 M. ∆ct: solution concentration difference at time t;
∆Co:initialsolutionconcentrationdifference(t=0).
∆Co: initial solution concentration difference (t = 0).
TThhee loloww ppeermrmeeaabbiliiltiyty aanndd bbaarrrireier reefffefecct ttoto NNaaCCl ltrtarannssppoortr teexxhhibibitietedd bbyy ththee PPIMIM ddeetetermrminineedd bbyy
ddififfeferreenntt kkiinnddss ooff mmeeaassuureremmeenntstsi sios foinf tienrteesrtefsotr fiotsr aiptsp laipcaptliiocnatiinonn aitnu rnaaltwuaratel rwsyastteerm ssys(wtehmicsh (uwshuiaclhly
uhsauvaellya lshoavNea Callsion NsoalCutli oinn asnoldu/tioorno tahnedr/soarl to)tshinerc esiatlts)u spipnocert sitt hseupelpimoritns atthioen eolifmthineairticoonn torfib tuhteioirn
cwonitthribreustpioenct wtoitcha rrerisepde-cmt teod icaatrerdietdr-amnsepdoiartt.ed transport.
ApplicationofthePIMtotheSeparationofSulfonamides
Application of the PIM to the Separation of Sulfonamides
TThhee trtarannspspoortr tkkinineetitcisc soof ffofouur rssuulflofonnaammididees saaccrorosss sththee PPIMIM isis aa ssimimuultlatanneeoouuss ccoommbbininaatitoionn oof f
eexxtrtaracctitoionn aanndd bbaacckk-e-exxtrtaractcitoionn rereaactcitoionns soocccucurrrirningg inin nnoonn-e-qequuiliilbirbiruiumm cocnondditiitoinosn sata tththe esasmame etitmime.e .
The fundamentals of sulfonamides (S) extraction at basic pH by Aliquat 336 incorporated in a PIM
can be described by the generic stoichiometric equation (Equation (6)) [15]
[AlqCl−]mem + Ant−aq  [AlqAnt−]mem + Cl−aq (6)
Separations2018,5,5 7of12
Thefundamentalsofsulfonamides(S)extractionatbasicpHbyAliquat336incorporatedinaPIMcan
bedescribedbythegenericstoichiometricequation(Equation(6))[15]
Separations 2018, 5, 5 7 of 11
− − (cid:28) − −
[AlqCl ] +Ant [AlqAnt ] +Cl (6)
where mem and aq refer to the membmraenme and aqauqeous phases, remsepmectivelya.q
As it is indicated in Equation (6), sulfonamides are extracted via formation of an ionic pair with
where and refertothemembraneandaqueousphases,respectively.
mem aq
the triocylmethyl ammonium cation while chloride is released in the aqueous solution. Thus, the
AsitisindicatedinEquation(6),sulfonamidesareextractedviaformationofanionicpairwiththe
release of the extracted sulfonamides is possible in a stripping phase containing high amounts of
triocylmethylammoniumcationwhilechlorideisreleasedintheaqueoussolution. Thus,thereleaseof
chloride anion.
theextractedsulfonamidesispossibleinastrippingphasecontaininghighamountsofchlorideanion.
The transient concentration profiles of sulfathiazole (STZ), sulfapyridine (SPY), sulfamethazine
Thetransientconcentrationprofilesofsulfathiazole(STZ),sulfapyridine(SPY),sulfamethazine
(SMZ), and sulfamethoxazole (SMX) in the feed and stripping solutions as a function of time are
(SMZ), and sulfamethoxazole (SMX) in the feed and stripping solutions as a function of time are
presented in Figure 6. The results show an important decrease of the concentration of both STZ (with
presentedinFigure6. TheresultsshowanimportantdecreaseoftheconcentrationofbothSTZ(with
a thiazole functional group) and SMX (which is the most acidic antibiotic in this study) in the feed
athiazolefunctionalgroup)andSMX(whichisthemostacidicantibioticinthisstudy)inthefeed
solution the first 8 h of experiment. However, the transport of these species to the stripping phase is
solutionthefirst8hofexperiment. However,thetransportofthesespeciestothestrippingphaseis
much slower. Hence, it can be observed that 60% of the initial STZ and 50% of SMX are remain in the
muchslower. Hence,itcanbeobservedthat60%oftheinitialSTZand50%ofSMXareremaininthe
membrane after 24 h of experiment, pointing out that the transport of these species through the dense
membraneafter24hofexperiment,pointingoutthatthetransportofthesespeciesthroughthedense
membrane is the limiting step for their separation. SPY and SMZ exhibit less affinity towards Aliquat
membraneisthelimitingstepfortheirseparation. SPYandSMZexhibitlessaffinitytowardsAliquat
336 due to their less acidic characteristics and their transport is not significant.
336duetotheirlessacidiccharacteristicsandtheirtransportisnotsignificant.
(a) (b)
(c) (d)

Figure 6. Transient concentration profiles in sulfonamides transport experiments using the 26%
Figure 6. Transient concentration profiles in sulfonamides transport experiments using the 26%
AlqCl/74% CTA PIM. (a) STZ; (b) SMX; (c) SPY; (d) SMZ. Triangles: feed solution; squares:
sAtrlqipCpli/n74g%so CluTtAio nP.IM(F.e e(ad) sSoTlZut; i(obn): S5MmXg; (Lc−) 1SPoYfs; u(dlf)o SnMamZi.d TersiaantgplHes:9 f,esetrdi pspoliuntgiosno;l ustqiuonar:e1s:M strNipapCiln).g
solution. (Feed solution: 5 mg L−1 of sulfonamides at pH 9, stripping solution: 1 M NaCl).
3. Materials and Methods
3.1. Membrane Preparation
PIMs were prepared dissolving 200 mg of cellulose triacetate (CTA) in 20 mL of chloroform and
stirred during 5 h. Afterwards, 1.3 mL of a 0.5 M Aliquat 336 IL (trioctyl methylammonium chloride
Separations2018,5,5 8of12
3. MaterialsandMethods
3.1. MembranePreparation
PIMswereprepareddissolving200mgofcellulosetriacetate(CTA)in20mLofchloroformand
stirredduring5h. Afterwards,1.3mLofa0.5MAliquat336IL(trioctylmethylammoniumchlorideor
AliquatCl)solutioninchloroformwasaddedtothepolymersolutionandletstirred2hmore. Then,
thesolutionwaspouredintoaflatbottomglassdishandthesolventwasallowedtoevaporatefor
24hatroomtemperature. Theresultingfilmwascarefullypeeledofffromthebottomoftheglassdish.
PreparedPIMshadacompositionof26%ofAliquatCland74%ofCTAandthicknessof30±3µm.
TheILAliquat336andthepolymerCTAwerepurchasedfromFlukaChemie(Steinheim,Germany),
whileotherchemicalsarefromPanreac(Barcelona,Spain).
XPSMeasurements
MembranesurfacechemicalcharacterizationwasperformedbyanalyzingXPSspectraobtained
withaPhysicalElectronicsspectrometerPHI5700(PhysicalElectronics,Chanhanssen,MN,USA),
usingX-rayMgKαradiation(300W,15kV,1253.6eV)astheexcitationsource. High-resolutionspectra
wererecordedatagiventake-offangleof45◦ byaconcentrichemispherical(±0.1eVbindingenergy
accuracy)andanalyzedfollowingtheprocedureandexperimentalconditionsalreadyexplainedin
previouspapers[24,38].Measurementswereperformedwithamembraneinwetstate(26%AlqCl/74%
CTA-wsample),whichmeans,aftertheirimmersionindistilledwaterfor48h,sampleweregently
surfacedriedwithpaper,andstoredforoneweektoensurethelossoftheadsorbedwater;XPSanalysis
ofdryPIMsample(26%AlqCl/74%CTA-d)wasalreadyreported[18]. Moreover,acellulosetriacetate
film(withoutIL)indryandwetstates(CTA-dandCTA-w,respectively)wasalsoanalyzedbyXPS
forcomparison.
3.2. ElasticMeasurements
Elastic characterization was carried out using a force digital gauge (ES20 model, Mark-10
Corporation,NewYork,NY,USA),withamaximumtensionof100N,lengthaccuracyof±0.01mm,
andatastrengthrateof10mm/s,connectedtoacomputer. Elasticmeasurementswereperformed
withthemembraneinair(sample26%AlqCl/74%CTA-d)andplacedinadistilledwatertank(sample
26%AlqCl/74%CTA-w).
3.3. ElectrochemicalCharacterization: CellPotentials,ElectrochemicalImpedanceSpectroscopy,andSalt
DiffusionMeasurements
Electrochemicalcharacterizationofthe26%AlqCl/74%CTAPIMwascarriedoutinadead-end
testcellsimilartothatdescribedin[39], withthesolutionstirredat540rpmtominimizesolution
concentration-polarizationatthemembranesurfacesandatroomtemperature(25±2)◦C.
Concentrationorcellpotential(∆E)measurementswereperformedusingtworeversibleAg/AgCl
electrodesconnectedtoadigitalvoltmeter(Yokohama7552,1GΩinputresistance)andchangingthe
concentrationsoftheNaClsolutionsatbothmembranesidesbykeepingconstanttheconcentration
ratio c /c = 2, from c ranging between 0.005 M and 0.2 M. The membranes were maintained
1 2 2
overnight in contact with the lowest concentration solution, but it was renovated previously to
startthemeasurements. Membranepotential(∆Φ )orequilibriumelectricalpotentialdifference
mbr
betweentwosolutionsofthesameelectrolytebutdifferentconcentrations(c andc )separatedbythe
1 2
membranewereobtainedfrommeasuredcellpotentialvaluesbysubtractingtheelectrodepotential
(∆Φ =(RT/F)ln(c /c ),whereRandFrepresentthegasandFaradayconstants,andTisthesystem
elect 1 2
temperature[37],thatis,∆Φ =∆E−∆Φ .
mbr elect
Electrochemicalimpedancespectroscopy(EIS)measurementswereperformedwiththemembrane
placedinthedead-endtestcellbetweentwoNaClsolutionsofthesameconcentration(concentration
rangingbetween0.01Mand0.05M),withPtelectrodesconnectedtoafrequencyresponseanalyzer
Separations2018,5,5 9of12
(FRA,Solartron1260,England),thatis: NaClsolution(c)/electrode//membrane//electrode/NaCl
solution(c)system[4]. Measurementswererecordedfor100datapointsforfrequency(f)ranging
between1Hzand107 Hz,atamaximumvoltageof0.01V[4,31,33]. Theimpedanceofasystem,Z,
isacomplexnumber,Z=Z +jZ ,derivedfromexperimentaldataobtainedfromabroadrange
real img
offrequencies,whichcanbeseparatedintoreal(Z )andimaginary(Z )partsbyalgebrarules.
real img
Forsimpleandhomogeneoussystems,Z andZ arerelatedtotheelectricalresistance(R)andthe
real img
capacitance(C)bythefollowingexpressions
Z =(R/[1+(ωRC)2]);Z =−(ωR2C/[1+(ωRC)2]) (7)
real img
whereωrepresentstheangularfrequency(ω=2πf). Theanalysisoftheimpedancedataisusually
carriedoutbyplotting−ZimgversusZreal(Nyquistplot),whereasemi-circlecorrespondstothe
parallelassociationofaresistanceandacapacitanceor(RC)circuit[35]. Thisdescriptionisonlyvalid
forhomogeneoussystems,whichpresentauniquerelaxationtimeτ(withτ =2πRC),butinthe
max
caseofnon-homogeneoussystemstheNyquistplotisadepressedsemi-circleduetoadistribution
ofrelaxationtimes,thenanon-generalizedcapacitor(usuallyrepresentedasQ)isconsidered[35].
Moreover,forcompositesystemsconsistingoflayerswithdifferentelectricalcharacteristics,aseries
associationofsemi-circles/depressedsemi-circles(oneforeachlayer)iscommonlyobtained[31–34].
For NaCl diffusion, the membrane separated a donor solution (c = 0.01 M) from a receiving
d
dilutedsolution(c ),whichinitiallywasdistilledwater(thatmeans,c o =0)[39]. Timevariationof
r r
donor and receiving solution concentrations was determined by measuring conductivity changes
by means of two conductivity cells, each one submerged in a half-cell, connected to two digital
conductivitymeters(CrisonGLP31,Barcelona,Spain).
3.4. TransportExperiments
TransportacrossthePIMoffourdifferentantibiotics—sulfathiazole(STZ),sulfapyridine(SPY),
sulfamethazine(SMZ),andsulfamethoxazole(SMX)—allpurchasedfromSigma-Aldrich(Steinheim,
Germany) was analyzed. Antibiotics stock solutions (250 mg L−1) were prepared in methanol.
Experiments were carried out in a permeation cell similar to the one described in [15] in which
themechanicallystirred(800rpm)feedandstrippingsolutions(150mLeach)wereseparatedbythe
PIM with an exposed membrane surface area of 11.5 cm2. All experiments were conducted at an
ambienttemperatureof22±1◦C.
Workingsolutioncontainingamixtureof5mgL−1eachcompoundwaspreparedbyappropriate
dilution of the stock solutions with deionized water (Milli-Q Plus system, Millipore Iberica S.A.,
Madrid,Spain)andthepHwasadjustedto9withNaOH.Sodiumchloride(CarloErba,Cornaredo,
Italy)wasusedtopreparethe1MNaClstrippingsolution. Samplesweretakenfromthefeedand
stripping solutions at regular time intervals during the 24 h period of the experiments and were
analyzedbyanHPLCsystem(1200SeriesAgilentTechnologies,SantaClara,CA,USA)withadiode
arraydetector(1290InfinityDADAgilentTechnologies,SantaClara,CA,USA)followingtheprocedure
describedin[15].
4. Conclusions
Electrochemicalcharacterizationofapolymerinclusionmembranepreparedwiththeionicliquid
Aliquat336(26%)andthepolymercellulosetriacetate(74%)wasperformedwithamodelelectrolyte
(NaCl)bymeasuringmembranepotential,electrochemicalimpedancespectroscopy,andsaltdiffusion.
Theseresultsenabletheestimationofcharacteristicelectrochemicalmembraneparameterssuchus
the membrane conductivity, and its dependence with NaCl solution concentration, the diffusive
permeability, and the ion (Na+ and Cl−) transport numbers, which clearly show the barrier effect
of this membrane to the transport of NaCl. Moreover, membrane contact with water or aqueous
solutionsseemstoaffectbothchemicalsurfacecharacteristicsandelasticparameters. Theseparation
Separations2018,5,5 10of12
ofsulfonamidesusingthismembraneisaffectedbyitsdensecharacter,whichlimitsthetransportof
theorganicmoleculesthroughit.
Acknowledgments: The financial support of the research project CTM2016-78798-C2-2-P (AEI/FEDER/UE)
isacknowledged.
Author Contributions: Juana Benavente performed impedance spectroscopy measurements, interpreted
electrochemicalcharacterizationandXPSdata,co-wrotethemanuscriptanditsfinalrevision.MaríaIsabelVázquez
carried out elastic and diffusion experiments. Virginia Romero was responsible for membrane potential
measurementsanddataanalysis.EnriquetaAnticóandClàudiaFontàscontributedtothepreparationofPIMs,
doing the transport experiments of antibiotics, interpretation of data, co-writing of the manuscript, and the
finalrevision.
ConflictsofInterest:Theauthorsdeclarenoconflictofinterest.
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