Effect of the Addition of Water-Soluble Polymers on the

EffectoftheAdditionofWater-SolublePolymersontheStructureofAerosolOTWater-in-OilMicroemulsions:A

FourierTransformInfraredSpectroscopyStudy

C.Gonza´lez-Blanco,L.J.Rodrı´guez,andM.M.Vela´zquez*

DepartamentodeQuı´micafı´sica,FacultaddeFarmacia,UniversidaddeSalamanca,

Apdo.449,E-37080,Salamanca,Spain

ReceivedApril16,1996.InFinalForm:October18,1996X

UsingFouriertransforminfrared(FT-IR)spectroscopyandelectricalconductivitymeasurements,thestudyofwater-in-oil(w/o)microemulsionsofAerosolOTdissolvedintoluenewithdifferentwatercontents,intheabsenceandinthepresenceofpoly(vinylpyrrolidone)(MW10000),PVPK15,andpoly(sodium4-styrenesulfonate)(MW75000),PSS,hasbeencarriedout.Conductivitymeasurementsshowattractiveinteractionsbetweendropletsofw/omicroemulsionswithoutpolymerscontaininghighwatercontents,w0g25.Theadditionofpolymerspreventstheseinteractionsevenatthelowestpolymerconcentrationusedinthiswork,2%.FromFT-IRspectradifferentmicellarstructuresweredetectedintheabsenceofpolymers,andvibrationalmodesofthesurfactantmoleculeallowstructuralinformationtobeobtainedofeachkindofaggregate.Thestructureofeachkindofaggregatehasagreatdependenceonthewatercontent.InfraredspectraofthesurfactantmoleculeofmicroemulsionscontainingdifferentconcentrationsofPVPK15andPSSwerealsorecorded.Thecomparativeanalysisofthesespectrawiththoseintheabsenceofpolymerspermits,inafirstplace,localizationofthepolymerintheaqueouscoreand,later,studyofthestructuralchangesinducedbytheadditionofthesepolymers.

Introduction

Thereversemicellesandthew/o(water-in-oil)micro-emulsionsmodifiedbyadditionofwater-solublepolymersareroutinellyusedinsynthesisofmicroparticules,1polymerizationandseparationprocesses,2andoilrecov-ery.3However,thereisnotenoughinformationaboutthepolymer-surfactantinteractionorabouttheeffectsofthepolymeronthedropletstructure,size,andshape.Mostoftheworkshavestudiedthesemicroemulsionsinthedomainofthegelformationusingthegelatinagents,4-6butabetterunderstandingofsuchsystemsrequiresmodifiedconditionsofformationforthesemicroemulsions,i.e.,varyingthekindofpolymer,thepolymercontent,orthewatercomposition.

Therefore,theaimofthisworkistoobtainstructuralinformationofthew/omicroemulsionperturbedbytheadditionoftwodifferentwater-solublepolymers,poly-(vinylpyrrolidone),PVPK15(MW10000),andpoly-(sodium4-styrenesulfonate),PSS(MW75000).Thewater-in-oilmicroemulsionwasmadeusingbis(2-ethyl-hexyl)sodiumsulfosuccinate,AerosolOT,AOT,assurf-actantandtolueneassolvent.Thewatercontent,givenaswater-surfactantratio,w0)[H2O]/[S],wasmodifiedbetweenw0)0.34andw0)35.Thepolymercontent,expressedaspolymer-waterfractioninweight,R)wp/wwater,wasalsomodifiedintherange2-20%(w:w).Particularly,thespecificobjectivesoftheinvestigationaretostudytheeffectoftheadditionofthesepolymersonthestructureofthewaterpool,ontherotationalisomerism,andonthechainpackingoftheAerosolOTmoleculeintoaggregates.

*Authortowhomthecorrespondenceshouldbeaddressed.XAbstractpublishedinAdvanceACSAbstracts,March1,1997.(1)Arriagada,F.J.;Osseo-Asare,K.J.ColloidInterfaceSci.1995,178,8.

(2)Sheu,E.;Goklen,K.E.;Hatton,T.A.;Chem,S.H.Biotechnol.Prog.1987,4,175.

(3)Caponetti,E.;Lizzio,A.;Triolo,R.;Compere,A.L.;Griffith,W.L.;Johnson,J.S.Langmuir19,5,357.

(4)Rees,G.D.;Robinson,B.H.Adv.Mater.1993,5,608.

(5)Quellet,Ch.;EickeH.F.;Gehrke,R.;Sager,W.Europhys.Lett.19,9,293.

(6)Petit,G.;Zemb,Th.;Pileni,M.P.Langmuir1991,7,223.

Toobtainthisinformation,FT-IRspectroscopyhasbeenused.Thistechniqueisnoninvasive,functionalgroupselective,andparticularlysuitabletodetectthechangesofthemicroenvironmentalproperties,andithasbeenusedbyothersauthorstostudythestructureofsomenonperturbedw/omicroemulsions.7-10Inparticular,wediscussthespectrumofthewaterofthemicellarcore,waterpool,theskeletalhydrocarbonbandsofthesurf-actantmolecule,andthecarbonylandsulfonatevibra-tionalmodesofthesurfactantheadgroup.Thecompara-tiveanalysisofspectracorrespondingtomicroemulsionswithandwithoutpolymers,allowsstructuralinformationtobeobtainedofthesenewmicroemulsionsandoftheeffectsofchangesonthewaterandpolymercontents.Ontheotherhand,innonperturbedw/omicroemulsionsattractiveinteractionsbetweendropletshavebeende-tectedathighvaluesofw0.11Ingeneraltheadditionofwater-solublepolymersdecreasestheseinteractions,12,13whereasoil-solublepolymersincreasethem.12Itisknownthatelectricalconductivitymeasurementscanbeusedtopredictqualitativelytheinteractionbetweendroplets.w/omicroemulsionsexhibitasmallconductivityaswaterdropletsareseparatedbythesurfactantlayers;however,whenthewatercontentincreasesbeyondacertaincriticalvalue,theconductivityincreasessharply.Thesecon-ductivityvalueshavebeenrelatedwithanincreaseofattractiveinteractions,becausetheseinteractionsproduceanincreaseofthelifetimeoftwoormoreassociateddropletswhichfacilitatethemigrationofthesurfactantcounterionsalongconnetedpathsthroughthemicro-emulsions,leadingfinallytotheelectricalpercolationphenomenon.11-13Therefore,inordertoknowtheeffectoftheadditionofPVPK15andPSSontheinteractions

(7)MacDonall,H.;Bedwell,B.;Gulari,E.Langmuir1986,2,704.(8)Jain,T.K.;Varshney,V.;Maitra,A.J.Phys.Chem.19,93,7409.

(9)Onori,G.;Santucci,A.J.Phys.Chem.1993,97,5430.(10)Mora´n,P.D.;Bowmaker,G.A.;Cooeny,R.P.Langmuir1995,11,738.

(11)VanDijk,M.A.;Casteleijn,G.;Joosten,J.G.H.;Levine,Y.K.J.Chem.Phys.1986,85,626.(12)Sua´rez,M.J.;Levy,H.;Lang,J.J.Phys.Chem.1993,97,9808.(13)Sua´rez,M.J.;Lang,J.J.Phys.Chem.1995,99,4626.

S0743-7463(96)00451-9CCC:$14.00©1997AmericanChemicalSociety

MicroemulsionsandPolymersbetweendroplets,electricalconductivitiesofallw/omicroemulsionsweredetermined.

ExperimentalSection

Materials.Bis-(2-ethylhexyl)sodiumsulfosuccinate,AerosolOT,AOT,waspurchasedfromFluka.Itwaspurifiedaccordingtothepublishedmethod.14Afterthesamplewasdried,somewatermoleculesremainboundtothesurfactant.TheanalysisofthewaterconcentrationofthestocksolutionofAOTintoluenewithaKarl-Fishertitratorrevealedthepresenceof0.34molofresidualwater/molofsurfactant.Thiswatercontentwastakenintoaccountforthecalculationofthew0ratio.Toluene,spectroscopicgradefromFluka,wasstoredinsodium.

ThesamplesofPVPK15(Fluka)andPSS(Aldrich)hadmolecularweightof10000and75000g/mol,respectively,providedbythemanufacturers.

Methods.Reversemicellesandw/omicroemulsionswerepreparedbyaddingtheappropiateamountofwatertoastocksolutionof0.08moldm-3ofAOTintolueneandstirringuntilthesolutionbecametransparent.Inthepresenceofpolymers,aggregateswerepreparedbyaddingtheappropiatevolumeofaconcentratedaqueouspolymersolutiontoasteadyw/omicroemulsionandstirringuntilallthepolymerwasincorporatedtothemicellarcore,andthesolutionbecametransparent.Allmicroemulsionswerestableatleastforseveralmonths.

Tostudytheeffectofthecompositiononthestructureofthemicroemulsion,thewaterandthepolymercontentshavebeensystematicallymodified.Allthespectrawererecordedat25.0°C.

Spectroscopy.InfraredabsorptionspectrawererecordedwithaPerkin-Elmer1730FT-IRspectrophotometer.Aspectralrangeof4000-1000cm-1andaneffectiveresolutionof2cm-1wasused.Inordertoobtainagoodqualityofthespectra,aminimumof20scanswereaccumulated.Duetothehighqualityofthespectra,nosmoothinghasbeennecessary.AcellofCaF2windowsofvariablepathlengthhasbeenused.Thepathlengthwasaround0.0675mmandwasdeterminedwiththeinterferencefringesmethod.15Inallcasesthebandsoftoluenehavebeensubtracted.

Inordertoquantifythecomponentsofthehydroxylandcarbonylstretchingbandsandtheirchangeswithwaterandpolymercontents,spectrahavebeensubjectedtoadeconvolutionprocessinGaussianbands.ThedeconvolutionprocedureusedconsidersthespectralcontourascomposedofanumberNofGaussianbands,allowinganexpressionoftheabsorbanceA(ν)asafunctionofthefrequency,ν,inthefollowingway:

A(ν))

∑A(ν)exp{-ln2[(ν-ν)/δi

]2

i

i

i

}

whereAiisthemaximumabsorbanceateachbandcentralfrequency,νi,andδi,isthehalf-width.Thefitprocesshasbeencarriedoutwithacomputerprogramdevelopedinthislabora-tory.16

ConductivityMeasurements.TheelectricalconductivititywasmeasuredusingaCDM83(Radiometer,Copenhagen)conductometerandalow-conductivitycell.ThecellconstantwascalibratedwithsolutionsofKClofaknownconcentration.17

ResultsandDiscussion

ElectricalConductivityMeasurements.Figure1showsthevariationofthespecificelectricalconductivity,κ,asafunctionofthewaterconcentration,fornonper-turbedw/omicroemulsionsandformicroemulsionscom-posedofAOTintoluenecontainingtwodifferentpolymercontents,givenaspolymer-waterfractioninweight:3%ofPVPK15and2%ofPSS.

(14)Menger,F.M.;Yamada,K.J.Am.Chem.Soc.1979,101,6731.(15)Seeforexample:Skoog,D.A.;LearyJ.J.InPrinciplesofInstrumentalAnalysis,4thed.;SaunderColl.Pub.;NewYork,1992;p274.

(16)Gonza´lez-Hierro,P.;Vela´zquez,M.M.;Cachaza,J.M.;Rodrı´guez,L.J.J.Pharm.Biomed.Anal.1987,5,395.

(17)Lind,J.E.;Zwolenik,J.J.;Fuoss,R.M.J.Am.Chem.Soc.1959,81,1557.

Langmuir,Vol.13,No.7,19971939

Figure1.Variationoftheelectricalconductivitywiththewatercontentofthefollowingw/omicroemulsions:0,AOT/toluene0.08M;2,AOT/toluene0.08M2%PSS;9,AOT/toluene0.08M3%PVPK15.

Intheabsenceofpolymers,thespecificconductivityincreasesuntilaw0valueof25.Atagreaterwatercon-tentasharpincreaseoftheconductivityisobserved.Theseresultsmayshowthepresenceofattractiveinteractionsbetweendropletsofw/omicroemulsionswithwatercontenthigherthan25.

InthepresenceofPVPK15andPSSthespecificconductivityalsoincreasesasw0;however,itdoesnotincreasesharplyinthewholerangeofw0studied.Thisbehaviorisobservedatallpolymercontentsusedinthiswork.Theresultsmayindicatethatthepresenceofpolymerspreventsattractiveinteractionsbetweendrop-letsandareconsistentwithdatainliterature.12,13Theoriginofthisbehaviorwasexplainedinothermicroemul-sionsassumingthattheadditionofwater-solublepolymersdecreasesthedropletsize,decreasingtheinterpenetrationoftheinterfaciallayeroftwocollidingdropletsand,asconsequence,theattractiveinteractions.12

InfraredSpectroscopicStudyoftheStructureofw/oMicroemulsionsintheAbsenceofPolymers.ThefollowingsectiondealswiththeinfraredspectraofAerosolOTaggregatescontainingdifferentpolymercompositionsincomparisonwiththoseofnonperturbedaggregates.Therefore,infirstplaceitisnecessarytodiscussthespectraofmicroemulsionswithoutpolymer.Thesespectraarediscussedinthissection.

ThefollowingregionsofinfraredspectraofAOTaggregatesintoluenewithdifferentwatercontentshavebeenexamined:(a)theOHstretchband,3100-3700cm-1;(b)thecarbonylstretchingregion,1700-1750cm-1;(c)thesulfonatestretchingregion,1210-1245cm-1;(d)theCHwaggingandscissoringbands,1100-1500cm-1.TheWaterBands.TheOHstretchingbandisanasymmetricalbandthatcanbefittedtoaminimiumoffourGaussianfunctionscenteredaround3321,3450,3550,and3635cm-1,withhalf-widthsof65,80,60,and45cm-1respectively.ResultsfortheresolvedspectraareshownalongwiththeexperimentaldatainFigure2.Thelow-frequencypeakisduetotheOHstretchinahydrogen-bondedpolymericchain.18The3450cm-1peakisassignedtothehydrogen-bondeddimersboundattheinterface.18The3550cm-1bandhasbeenassignedtothestretchofnon-hydrogen-bondedwatermoleculeswhichhavepen-etratedintothesurfactantlayer,7andthehigh-frequency

(18)D’Aprano,A.;Lizzio,A.;TurcoLivery,V.;Aliota,F.;Vasi,C.;Migliardo,P.J.Phys.Chem.1988,92,4436.

1940Langmuir,Vol.13,No.7,1997Gonza´lez-Blancoetal.

Figure2.DeconvolutionofOHstretchingvibrationbandofwatercontainedinw/omicroemulsionsatselectedwaterandpolymer

contents.DottedlinesareGaussiancomponentbandsandthefulllineisthecalculatedspectrumcontour.Upperlineistheabsolutedeviationfromexperimentalline.

peakat3635cm-1isascribedtomonomericwaterobservedinpurewater.19

TheseresultsdifferfromthoseobtainedforAOTmicroemulsionsdissolvedinheptane,carbontetrachloride,andisooctanebyotherauthors,7-9inthatfourGaussianpeaksareobtainedinthedeconvolutionprocessoftheOHstretchingband,whereasonlythreeGaussianbandshavebeendetectedinotherw/omicroemulsions.Thepresenceoffourkindsofwatermoleculeshasnotbeendetectedbyinfraredspectroscopybyotherauthors;however,byusingavarietyofphysicaltechniques,suchasNMR,electronspinresonancespinlabeling,ordif-ferentialscanningcalorimetry,severaltypesofboundwaterspecieshavebeendetected.20ItseemsthatIRspectroscopyrevealsonlysomeofdifferentkindsofthemicroemulsionwatermolecules.

Inourcase,thefourthGaussianpeakiscenteredaround3550cm-1.ThisbandwasobservedinAOTmicroemul-sions7andhasbeenascribedtotheOHstretchingofmonomerwatermoleculesattheinterface.Thepresenceoftwotypesofwatermoleculesattheinterface,monomersanddimers,maybeattributedtothedifferenthydratationshellscorrespondingtothesulfonatesurfactantgroupandsodiumcounterion.20

Theseresultsareconsistentwiththethree-statemodelproposedbyTamura21andGoto.22Inthismodel,waterexistsinthreephases:thephases1,isthewateratthe

(19)Tso,T.L.;Lee,E.K.C.J.Phys.Chem.1985,,1612.

(20)Hauser,H.;Haering,G.;Pande,A.;Luisi,P.L.J.Phys.Chem.19,93,7869.

(21)Tamura,T.;Schelly,Z.J.Am.Chem.Soc.1981,103,1018.(22)Goto,A.;Yoshiota,H.;Kishimoto,H.;Fujita,T.Langmuir1992,8,441.

interface;thephaseb,isthewaterexistinginthecore,andthethirdphase,existingbetweenthebandthes1phases,iscalledphases2.

ThespectroscopicdataobtainedinthisworkforAOTintoluenemayindicatethatinthephases1,interfacialwater,twodifferentwatermolecules,dimersandmono-mers,areincorporated;thephasebisformedbymonomerwatermoleculesinsidetheaqueouscore,andthes2phaseisformedbyastructuredhydrogen-bondedpolymericchainwatershellbetweenbands1phases.Theseresultsarealsoconsistentwiththeoreticalmoleculardynamicsstudiesofthestuctureanddynamicsofwaterinthevicinityofachargedbiomembrane.23

ThetotalpeakareaoftheOHstretchingincreaseswithw0,butthisincreaseisdifferentforeachwaterspecies.Tobetterdetectthesedifferences,fractionsofeachwaterspecies,Pi,havebeencalculatedfromtheareaofeachGaussianwithrespecttothetotalpeakarea,assumingthatthetotalareaisthesumoftheareaofthedistinctstatesofwater.8Fractionsofwaterspeciescannotbepreciselycalculatedfromtheintegratedareaofthevibrationalband,asnocouplingeffectshavebeenconsidered.24However,weusethesevaluesforcompara-tivepurposesasameasureoftherelativeabundanceofthedifferentspecies.

PlotsofPivsw0inFigure3showthatpolymericchainbondedwatermoleculesincreasewiththeincreaseofthe

(23)Bopp,P.InIntermolecularForces:AnIntroductiontoModernMethodsandResults;Huyskens,P.L.,Luck,W.A.P.,Zeegers,T.,Eds.;Springer-Verlag:Berlin,1991;Chapter14.

(24)Zeegers-Huyskens,T.InIntermolecularForces:AnIntroductiontoModernMethodsandResults;Huyskens,P.L.,Luck,W.A.P.,Zeegers,T.,Eds.;Springer-Verlag:Berlin,1991;Chapter6.

MicroemulsionsandPolymersFigure3.Variationoffractionsofthedifferentkindsofwatermoleculeswiththewater-surfactantratio,w0:9,polymericchainbonded;2,dimersattheinterface;*,monomersattheinterface;0,freemonomers.

watercontentuntilw0)7.Athigherw0valuesthisfractionreachesaconstantvalue.Fordimerandmonomermoleculesattheinterfaceandforthemonomermoleculesinsidethewaterpool,thebehaviorisreversed.Atawatercontentlowerthan7,thefractionofthesekindsofwatermoleculesdecreaseswiththewatercontent,butforvalueshigherthan7,thefractionremainsconstant.

Thisisconsistentwithmicrostructuralchangesinmicellaraggregates.Atthelowestwatercontents,w03,aggregateshavealowaggregationnumber,25andwateremoleculesarehighlyimmobilizedduetoastrongelec-trostaticinteractionbetweenthecounterionandthesurfactantheadgroup,20sothefractionofwaterdimersattheinterfaceishigher,≈0.6.Atw0dimersandmonomersattheinterfacegdecrease.3,fractionsThisoffactisassociatedtothegrowthofmicellaraggregates,probablyduetothedecreaseoftheelectrostaticinteractionbetweentheionsattheinterface.Simultaneously,thesizeofthestructuredshellincreaseswiththetotalwatercontentuntilreachingaw0valueof7,whereswollenreverseaggregatesarepredominant,andthefractionofallkindsofwatermoleculesremainsconstant.

Itmustbenotedthatwhenattractiveinteractionsbetweendropletsarepredominant(w0>25),nosignificantchangesaredetectedonwatermoleculebandsascomparedtothecorrespondingonesobservedinswollenreversemicelles.Thisfactmayindicatethatinteractionsbetweendropletsdonotmodifythestructureofthewaterpoolofthesemicroemulsions.

TheCarbonylBands.ThecarbonylgroupoftheAerosolOTmoleculegivesrisetoastrongasymmetricinfraredband.ThisbandwasfittedtoasumoftwoGaussianfunctionscenteredaround1737and1720cm-1,withhalf-widthsof8.2and9.3cm-1,respectively.Figure4showssomeoftheseresolvedspectra.Thesebandscorrespondtodifferentconformationsabouttheacylbondofthesuccinatebackbone.Thebandcenteredaround1737cm-1hasbeenassignedtothegaucheconformationandthebandcenteredaround1720cm-1tothetransconformation.8,10,26Thetransconformationisrelatedwithaweakinteractionbetweenthesodiumcationandthecarbonylgroup.10Fractionsofeachconformer,corre-spondingtomicroemulsionswithdifferentwatercontents,havebeencalculatedfromtheareaofeachGaussianwithrespecttothetotalarea.Resultsshowthatbothfractionsofgaucheandtransconformersremainconstantinthe

(25)Ueda,M.;Schelly,Z.A.Langmuir1988,4,653.

(26)Martin,C.A.;Magid,L.J.J.Phys.Chem.1981,85,3938.

Langmuir,Vol.13,No.7,19971941

wholerangeofw0studied.Thisfactmayindicatethattheadditionofwaterdoesnotmodifytheinteractionbetweenthesodiumcationandthecarbonylgroup,evenwhentheinteractionbetweendropletstakesplace.Thefractionofthegaucheconformer(63%)isalwaysgreaterthanthefractionofthetransconformer(27%).

TheSulfonateBands.Theinfraredspectrumofthesurfactantsulfonategroupiscomposedofabandcenteredaround1050cm-1attributedtoasymmetricstretchingmodeandofadoubletcenteredaround1212and1242cm-1,ascribedtotheantisymmetricstretchingmode.Thepositionofthebandcorrespondingtothesymmetricstretchingmodeandthesplittingofbandscorrespondingtothedoublethavebeenrelatedtotheseparationbetweenthesodiumionandthesulfonategroupandbetweentheadjacentsulfonategroups.10Theincreaseinthesepara-tionbetweentheseionsisaccompaniedbyashiftlowingthewavenumberofthesymmetricstretchingmodebandandsimultaneouslybythedecreaseofthesplittingofbandsascribedtotheantisymmetricmode.10

ThevariationofthemaximumofthesymmetricsulfonatestretchingbandofAOTintoluenewithrespecttow0isplottedinFigure5.Resultspresentedinthisfigureshowthatthisbandisshiftedfrom1051to1047cm-1asthewatercontentincreasesuntilreachingaw0valuearound3-4.Athigherw0valuesthebandpositionremainsconstant.Asimilartrendisobservedwithrespecttotheantisymmetricmodebands.Thesplittingofthesebandsisaround40cm-1atlowwatercontent,w0)0.34,anddecreasesuntilavalueof20cm-1,whenthewatercontentincreasesandswollenreversemicellesarepre-dominant.

Allresultsmayindicatethattheseparationbetweenionsincreaseswhenswollenreversemicellesareformed.Thisbehaviorwasobservedinotherw/omicroemulsionsofAOTdissolvedincyclohexaneorinisooctane10andwasassociatedtothehydratationofmicelles,whichresultsintheremovalofsodiumcounterionfromthesulfonategroupofthesurfactant.

Attractiveinteractionsbetweendropletsdonotaffecttheseparationofthesulfonateandsodiumions.

TheCH2WaggingBandsProgressionandtheCH2ScissoringVibrations.Themethylenewaggingbandprogressionisaseriesofweakandregularlyspacedbandsintheregionof1180-1370cm-1whichresultsfromthewaggingvibrationofthehydrocarbonchain.27Resultsobtainedinthisworkshowthatthesurfactantspectrumofmicroemulsionswithlowwatercontent,w0)0.34,doesnotcontainthewaggingbandprogression.Thisfactischaracteristicofthefullyextendedhydrocarbonchainsofliquidn-paraffins.Atw0valuesbetween1and15,abroadbandcenteredaround1360cm-1andtwonewbandscenteredat1338and1100cm-1appear.Thesebandsareduetothepresenceofsometransconformationinthehydrocarbonchain.27Atw0>20thebandcenteredaround1360cm-1isshiftedto1356cm-1.Thisbandhasbeenassignedtogauche(GG)conformations.27ThepresenceofsomeGGconfigurationsisalwaysrelatedtoanincreaseofthedegreeofdisorderonthehydrocarbonchain.Takingintoaccountthatattractiveinteractionsbetweendropletswereobservedinthesemicroemulsions,theincreaseoftheconformationaldisordercouldbeduetotheinter-penetrationofinterfaciallayersoftwocollidingdroplets.Allspectrapresenttwobandscenteredat1313and1375cm-1.ThesebandsareascribedtoaCH3rockingvibrationandaCH3symmetricdeformation,respec-tively.27,28

(27)Snyder,R.G.J.Chem.Phys.1967,47,1316.

(28)Susi,H.;Pazner,S.Spectrochim.Acta1962,18,499.

1942Langmuir,Vol.13,No.7,1997Gonza´lez-Blancoetal.

Figure4.Deconvolutionofthecarbonylstretchingvibrationbandofsurfactantinwater/AOT/toluenemicroemulsionsatselected

waterandpolymercontents.DottedlinesareGaussiancomponentbandsandthefulllineisthecalculatedspectrumcontour.Upperlineistheabsolutedeviationfromexperimentalline.

Figure5.Variationofthebandpositioncorrespondingtothesymmetricsulfonatestretchingmodewithw0:2,withoutpolymer;×,2%ofPVPK15;0,2%ofPSS.

Ontheotherhand,CH2scissoringvibrations,locatedintheregionof1460-1480cm-1,arecharacteristicofthenatureofthepackingofthehydrocarbonchain.29Infraredspectraofthesurfactantmoleculepresenttwobandscenteredat1462and1466cm-1andashoulderat1455cm-1.Thesebandswereobservedonspectracorrespond-ingtosystemswithacertaindegreeofpacking,i.e.,phospholipidsinorthorombicormonocliniccrystallat-tice.30

TheinformationobtainedfromIRspectrashowsagreatdependenceofthestructureofaggregateswiththewatercontentw0.Atw0e3aggregateshavelowaggregationnumbersandresultsobtainedinthisworkmayindicate

(29)Umemura,J.;Cameron,D.G.;Mantsch,H.H.Biochim.Biophys.Acta1980,602,32.

(30)Snyder,R.G.J.Mol.Spectrosc.1961,7,116.

that(i)thereisarelativelygreatinteractionbetweenNa+and-SO3-ions,(ii)thatthewatercontainedintheaqueouspoolishighlyinmovilized,and,(iii)thatthesurfactantchainhaveacertaindegreeofconformationalorder,withsomealltranssegmentsinthehydrocarbonchain.

Asecondtypeofaggregateisthatwithw0valuesrangedbetween3and7.Intheseaggregatesdimerwatermoleculesattheinterfacedecreasewithrespecttoaggregatescontaininglowaggregationnumber,andthewaterinsidethemicelleisreorganizedbyformingastructuredshellofhydrogen-bondedpolymericchainwatermolecule.Theseparationbetweenthesodiumcounterionandthesulfonatesurfactantionincreasesandthehydrocarbonchainconformationissimilartothatcor-respondingtomicellesoflowaggregationnumber.

Finally,atwatercontentgreaterthan7,reversemicellesormicroemulsionsarepredominant.Resultsobtainedinthisinvestigationindicatethattheseaggregateshavethefollowingproperties:arelativelyweakerinteractionbetweensodiumandsulfonateions,andasteadywaterpoolorganization,wherethefractionofeachwaterstatethatconstitutestheaqueouscoreremainsconstant.Twodifferenthydrocarbonchainconformationshavebeenobserved.Atw0<15thehydrocarbonchainconforma-tionhasacertaindegreeoforderduetothepresenceofsomealltransbondsegments,whereasmicroemulsionswithwatercontentsw0>20presentsomeGGconfigura-tionsonthesurfactanthydrocarbonchain.Thisindicatesanincreaseofthehydrocarbonchainconformationaldisorderprobablyduetothepresenceofattractiveinteractionsbetweenthesemicroemulsions.

InfraredSpectroscopicStudyoftheStructureofw/oMicroemulsionsPerturbedbytheAdditionofWater-SolublePolymers.TheWaterBands.Theadditionofwater-solublepolymerstow/omicroemulsionsproduceschangesontheshapeoftheOHstretching

MicroemulsionsandPolymersLangmuir,Vol.13,No.7,19971943

Figure6.Variationoffractionsofsomedifferentkindsofwatermoleculeswiththewatercontent:(a)hydrogen-bondedpolymericchainwatermolecules,(b)waterdimersattheinterface;9,withoutpolymer;0,12%PSS,2,2%PVPK15.

band.31ThedeconvolutionprocessinGaussianbandsappliedtotheOHbandofwatermoleculesoftheseperturbedaggregatesgivesresultssimilartothoseintheabsenceofpolymer,Figure2.TheaforementionedfourGaussianfunctionsfittheOHspectralcontour.Thepositionofthesebandsisthesameasinnonperturbedaggregatesandtheyhavealsobeenascribedtothehydrogen-bondedpolymericchain,thedimerandthemonomerwatermoleculesattheinterface,andthefreewatermolecules,respectively.

Inthefirstplacetheeffectoftheadditionofwatertomicroemulsionscontainingconstantpolymercontenthasbeenanalyzed.ThefractionsofeachkindofmoleculeatdifferentwaterandpolymercontentshavealsobeendeterminedasthequotientbetweentheareaofeachGaussianwithrespecttothetotalpeakarea.Thesevaluesarerepresentedversusw0,ateachpolymercontent.Figure6showssomeoftheseresults.

Somesignificantdifferencesasregardsresultsobtainedinw/omicroemulsionswithoutpolymershavebeendetected.Theadditionofpolymersdecreasesthefractionofhydrogen-bondedpolymericchainwatermolecules,Figure6a.Thisfactcanbeunderstoodifitconsidersthatpolymersaresolubilizedinthewaterpoolregionwherethesekindsofwatermoleculesarelocalized,thelayerbetweentheinterfaceandtheinnercore.Thus,thepresenceofpolymersinthisregionpreventstheforma-tionofthewaterpoolstructuredlayerdecreasingthenumberofhydrogen-bondedpolymericchainwatermol-ecules.Asinw/omicroemulsionswithoutpolymers,thefractionofthiskindofwatermoleculeincreaseswithw0andreachesaconstantvalueathighwatercontents,intherangeofw0)5-10.

(31)Togetadeeperinsightoftheseresultsitisimportanttoknowtheratiooffilledmicellestoemptymicellesatdifferentw0values.ThisratiocanbecalculatedbyusingthePoissondistribution,butitisnecessarytoknowtheaggregationnumberofaggregateswithdifferentwaterconcentrations.Aroughlyestimationofthisratiocouldbecalculatedassumingthat,ateachw0value,theaggregationnumberoffilledmicellesisthesametothatcorrespondingtoemptyreversemicelles.Takingintoaccountthisassumption,valuescalculatedshowthatPVPK15filledmicellesareonlypredominantathighvaluesofwaterandpolymercontents.However,whenPSSisaddedtoreversemicelles,emptymicellesarealwayspredominant,i.e.,77%ofemptymicellesinmicroemulsionswith20%PSSandw0)30.Thesefactsarenotconsistentwithresultsobtainedinthiswork.ElectricalconductivityvaluesandIRspectraofaggregatescontaining2%ofPSSor2%ofPVPK15showsignificantchangesasregardstotheemptymicroemulsions.Therefore,filledreversemicellesmustbepredominantevenatthelowestpolymercontentusedinthiswork.Thisisonlypossibleiftheaggregationnumberoffilledaggregatesdecreasesascomparedtothatoftheemptymicelles.

Table1.VariationoftheFractionofEachKindofWaterMoleculewiththePolymerContentataFixedw0Value

of20hydrogen-bondedpolymericchain0.170.070.060.060.070.090.110.100.080.09

dimersattheinterface0.520.560.550.560.560.550.560.540.550.56

monomersattheinterface0.230.280.290.290.270.290.270.310.300.30

polymercontent0,002%PVPK153%PVPK155%PVPK1520%PVPK152%PSS5%PSS12%PSS14%PSS20%PSS

freemonomers0.080.090.100.090.100.070.060.050.070.05

Fractionsofdimerwatermoleculesincreaseascom-paredtothosecorrespondingtononperturbedmicro-emulsions,Figure6b.Aweaklyincreaseofwatermonomermoleculesattheinterfacewasalsoobserved.Theseresultsareconsistentwithaforementionedchangesonhydrogen-bondedpolymericchainwatermolecules.Iftheadditionofpolymersgivesasignificantreductionofthesemolecules,watermoleculesmustbedisplacedfromthestructuredwaterpoollayertootherwaterpoolregions,inthiscasetothemicellarinterface.

Ontheotherhand,fractionsofbothdimerandmon-omerwatermoleculesattheinterfaceremainconstantwiththewatercontent,whereasinnonperturbedmicro-emulsionsthesefractionsdecreasewithwaterconcentra-tionuntilreachingaw0valueof7,wheretheyremainconstant.Atthepresenttimeonlyaspeculativeexplana-tionofthiseffectcanbegiven.Inordertoobtainthermodynamicallystableaggregates,polymermoleculesmightforceanorganizationontheinterfacewhichisnotalteredbytheadditionofwater.

Thefreemonomermoleculesfractionispracticallyunaffectedbytheadditionofpolymers,remaininigconstanttoavaluearound0.08.

Tostudytheeffectofthepolymerconcentrationonreversemicelles,thevariationoffractionscorrespondingtodifferentkindsofwatermoleculeswiththepolymerconcentrationatafixedw0valuehasbeenanalyzed.Table1presentssomeoftheseresults.Inordertocompareresultscorrespondingtoswollenmicelleswithoutattractiveinteractions,thew0valueselectedwas20,justtoavoideffectsofveryhighorveryloww0values.

ResultsofTable1confirmthebehaviordiscussedabove,i.e.,thedecreaseofhydrogen-bondedpolymeric

1944Langmuir,Vol.13,No.7,1997chainwatermoleculesandthesimultaneousincreaseof

dimerandmonomerwatermoleculesattheinterfaceinducedbytheadditionofpolymers.Thefreemonomerwatermoleculesdonotshowsignificantvariationswithrespecttotheemptymicelles.

Finally,itcanbenotedthatvaluesoffractionsforeachtypeofwatermoleculeremainconstantwiththeadditionofdifferentpolymerconcentrations.Thisfactsuggeststhatpolymermoleculescanbeincorporatedtoaggregatesandthattheincorporationtomicellesgivesawaterpoolstructureunaffectedbythepolymerconcentrationinthewholerangeofpolymercontentsusedinthiswork.TheCarbonylBands.Theadditionofthesewater-solublepolymerstotheAOT/toluene/watermicroemul-sionsdoesnotmodifythespectralcontourofthecarbonylbandofthesurfactantasregardsthecorrespondingonetononperturbedmicroemulsions;i.e.,theasymmetricalbandisdeconvolvedintwoGaussianfunctionscenteredaround1737and1720cm-1,respectively,Figure4.Thesebandswerealsoascribedtothegaucheandtransconformations,respectively.Thefractionofeachcon-formerhasthesamevalueasintheabsenceofpolymer,63%ofgaucheand27%oftransconformers,respectively.Thepositionandtheareaofeachbandarealsounaffectedbytheadditionofdifferentpolymerandwatercontents.Thisshowsthatpolymersarenotlocalizednearthecarbonylgroup,sothatnochangesonthestereospecificpositionaroundthecarbonylgroupwereexpected.

TheSulfonateBands.Theantisymmetricdoubletandthesymmetricstretchingsulfonatebandsofmicroemul-sionsofAOTperturbedbytheadditionofpolymerswereinvestigated.Figure5showsthepositionofthesymmetricstretchingbandateveryw0andataRvalueof2%forPVPK15andPSS.Twodifferentbehaviorsasafunctionofthekindofpolymerwereobserved.InthecaseofaggregatescontainingPSS,thevariationofpositionofthisbandwithw0showsatrendsimilartothatcorre-spondingtoaggregateswithoutpolymer.Themaximunofthebandisshiftedfrom1051to1048cm-1asw0increasesuntilavalueofaround4.Uptothisvaluethepositionofthebandremainsconstant.Asmalldiffer-enceinthebandpositionwasdetectedathighw0values.Thebandiscenteredaround1047cm-1inemptymicro-emulsionsandaround1048cm-1inthefilledones.Thisindicatesthattheseparationbetweenthesurfactantsulfonateandthesodiumionsissmallerinperturbedaggregatesthaninnonperturbedones.Thisbehaviorisobservedatallpolymerconcentrationsstudied.

WhenPVPK15isaddedthepositionofthebanddoesnotchangewiththeincreaseofw0,remainingconstantatavalueof1050cm-1.Thisvalueisintermediatebetweenthecorrespondingonestomicroemulsionsandlowaggregationnumberaggregateswithoutpolymer.Thiscorrelateswellwiththevalueofthesplittingoftheantisymmetricstretchingsulfonatebands.InthepresenceofPSS,thebehaviorissimilartothatcorrespondingtononperturbedaggregates.HoweverwhenPVPK15isadded,thesplittingofbandsremainsconstantatavalueof29cm-1.Thisvalueisalsointermediatebetweenthosecorrespondingtomicellesoflowaggregationnumberandnonperturbedmicroemulsions.Alltheseresultsmayindicatethat,inPVPK15filledmicroemulsions,theseparationbetweentheNa+counterionandthesulfonatesurfactantionisintermediatebetweenthecorrespondingonetolowsizemicellesandswollenreverseaggregateswithoutpolymersandisunaffectedbychangesinthewatercomposition.

TheCH2WaggingBandsProgressionandtheCH2ScissoringVibrations.TheadditionofPVPK15andPSStomicroemulsionsmadewithAOTdissolvedintoluene

Gonza´lez-Blancoetal.

andwithdifferentwatercontentsdoesnotchangetheCH2scissoringvibrationsascomparedtocorrespondingonestononperturbedmicelles.Thisbehaviormayindicatenochangesonthepackingofthehydrocarbonchainwhenpolymersareadded.

Ontheotherhand,significantchangesontheCH2waggingbandswereobserved.Resultscanbesummarizedasfollows:(a)AtlowPVPK15concentration,R<5%,thewaggingspectrumiscomposedofthreebandscenteredaround1100,1338,and1361cm-1.Thisspectrumischaracteristicofahydrocarbonconformationwithsomeall-transsegments.Theconformationisthesametothatofaggregateswithoutpolymersandwithwatercontent,w0,intherangeof1-15.Theadditionofwaterdoesnotaffecttheconformation.(b)AthighconcentrationsofPVPK15,Rg5%,twokindsofconformationsappearasafunctionofthewatercontent.Atw0valuesbetween1and3,anewbandcenteredat1082cm-1appearsandasignificantincreaseonCH3symmetricdeformationband,1380cm-1,isobserved.Thebandat1082cm-1hasbeenobservedinsomephospholipidhydrocarbonchainsandisassignedtoshortall-transsegmentsconnectedbyasinglegauchebond.32SomeofthesegroupsofbondshaveaGTTconformation,responsiblefortheincreaseoftheintensityofthebandcenteredaround1380cm-1.Atw0>3,thebandcenteredaround1361cm-1isshiftedto1355cm-1,andsimultanouslytheintensityofbandscenteredat1338and1380cm-1decreases.ThisbehaviorischaracteristicofthepresenceofsomeGGconforma-tionsandshowsanincreaseoftheconformationaldis-orderatthesew0values.(c)ThepositionandtheintensityofwaggingbandsofthesurfactantcorrespondingtomicroemulsionswithlowPSScontents,Re12%,suggestthatthehydrocarbonconformationissimilartothatcorrespondingtoaggregateswithhighPVPK15contentandloww0values.Thecharacteristicofthishydrocarbonchainconformationhasbeendiscussedinsectionb.(d)InfraredspectraofaggregatescontainingPSSconcentra-tionsgreaterthan12%andw0>2havethefollowinggroupofbands:1100,1338,and1361cm-1.Thesebandsindicatethepresenceofall-transsegmentsinthesurf-actanthydrocarbonchain,inasimilarconfigurationtothatcorrespondingtononperturbedaggregateswithwatercontentinthew0rangeof1-15ortofilledmicelleswithlowPVPK15content.Finallyatw0<2,bandscenteredat1082and1368cm-1appearandsimultaneouslytheintensityofthebandat1380cm-1increases.Bandscenteredat1082and1380cm-1correspondtoGTTconfigurationsandwerediscussedinprevioussections,andthethirdband,centeredaround1368cm-1,corre-spondstothewaggingvibrationofaGTGconformation.27ThepresenceofthesebandsonthespectrummightindicatetheexistenceofbothGTGandGTTconforma-tionsinthesemicroemulsions.

Conclusions

AttractiveinteractionsbetweenreverseaggregatesofAerosolOTdissolvedintolueneathighwatercontents,w0differentg25,structureshavebeenindetected.nonperturbedFT-IRaggregates.spectradetectedThesestructureshaveastrongdependenceofthewatercontent.Noattractiveinteractionsbetweendropletswerede-tectedinpolymer-filledaggregatesatallthewaterandpolymercontentsstudiedinthiswork.ThisbehaviorshowsthattheadditionofPVPK15andPSSinreverseaggregatespreventstheinteractionbetweenthesereversemicelles.

(32)Gaber,B.P.;Yager,P.;Peticolas,W.L.Biophys.J.1978,21,161.

MicroemulsionsandPolymersLangmuir,Vol.13,No.7,19971945

FromFT-IRspectrathelocationofpolymersintheaqueouscorecanbemade.Polymersaresolubilizedinaregionlocalizedbetweentheinterfaceandthemostinternallayerofthewaterpool.ThesulfonatestretchingbandcorrespondingtothesurfactantmoleculeshowsthatPVPK15andPSSmoleculesforcethesodiumandsulfonateionstoagreaterclosenessthaninemptyaggregates.However,thiseffectismoreaccusedinPVPK15filledmicroemulsions.Finally,theadditionofthesepolymerstoaggregatesincreasesthedegreeofdisorder

intheconformationofhydrocarbonchainsofthesurfac-tant.Thekindofconformationhasagreatdependenceofthestructureofthepolymerandofthewaterandpolymercontents.

Acknowledgment.TheauthorsacknowledgetotheJuntadeCastillayLeo´n(ProjectSA13/93)thefinancialsupportofthisproject.

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