MicrostructuralChangesinSDSMicellesInducedbyHydrotropicSaltP.A.Hassan, - PDF document

MicrostructuralChangesinSDSMicellesInducedbyHydrotropicSaltP.A.Hassan,SrinivasaR.Raghavan,andEricW.Kaler*CenterforMolecularandEngineeringThermodynamics,DepartmentofChemicalEngineering,UniversityofDelaware,Newark,Delaware19716ReceivedSeptember13,2001.InFinalForm:January4,2002Theadditionoflowconcentrationsofthehydrotropicsalt-toluidinehydrochloride(PTHC)tosolutionsoftheanionicsurfactantsodiumdodecylsulfate(SDS)promotesthetransitionfromsphericaltorodlikemicelles.NMRmeasurementsconfirmthatthehydrotropeadsorbsatthemicellewaterinterface,therebyscreeningelectrostaticrepulsionsbetweenthesurfactantheadgroups.Thesphere-to-rodtransitionindilutesolutionsisfollowedusingquasielasticlightscattering,andinthesemidiluteconcentrationrangedynamicrheologyisusedtoprobetheviscoelasticnatureofthesolutions.Thescalingofthezero-shearviscosityandtheplateaumoduluswithsurfactantconcentrationindicatesthepresenceofelectrostaticinteractionsbetweenthemicelles.Theadditionofinorganicororganicsaltstoanionicsurfactantsolutionfacilitatesthetransitionfromsphericaltorodlikemicellesbyscreeningtherepulsionsbetweenthechargedheadgroups.Inorganiccounterions(e.g.,,Br)bindmoderatelytocationicmicellesandthusleadtogradualmicellargrowth,withthemolarratioofsalttosurfactanttypicallyabove1.OrganiccounterionsthatbindstronglytothemicellarsurfacearehighlyefficientinpromotingmicellargrowthandinducewormlikemicellesatsignificantlylowerratiosofsalttoTypicalªbindingºcounterionsforcationicmicellesincludesalicylate,-toluenesulfonate,hydroxynaphthalenecarboxylate,andalkylMostoftheseadditivesbelongtothefamilyofi.e.,weaklysurface-activecompoundsthatcanincreasethesolubilityoforganicsubstancesinaqueoussolution.Thesecounterionsinteractwiththemicelle-formingsurfactantelectrostaticallyaswellashydropho-bically,andtheorientationofthehydrophobeatthemicellarsurfaceisimportant.Forexample,NMRmea-surementsshowthatthesalicylateionisstronglyadsorbedtocetyltrimethylammoniumbromidemicellesinacon-figurationthatallowsthecarboxylicandhydroxylgroupstoprotrudefromthemicelle.Theorthoisomer,salicylate,ismoreeffectivethanotherisomersofhydroxybenzoatesindrivingmicellargrowth.Wormlikemicellesformedbycationicsurfactantsuponadditionofhydrotropicsaltssuchassodiumsalicylatehavebeenstudiedextensively.Thesemicellescangrowaslongasseveralmicrons,withradiioftypically2Thepresenceofsuchsupramolecularstructuresimpartsstrongviscoelasticitytothesolution,andtherheologyofthesefluidsissimilartothatofsolutionsofflexiblepolymers.However,becausethemicellesareindynamicequilibriumwiththeirmonomers,theycanbreakandrecombinerapidlyandthusarecalledªlivingpoly-Catesandco-workershavedescribedthelinearandnonlinearviscoelasticpropertiesofsuchwormlikemicellarsolutionsonthebasisofamodifiedreptationmodelincorporatingtheeffectofreversiblescissionWhereashydrotropeeffectsforcationicsurfactantshavebeenstudiedextensively,therearefewreportsofanalo-gouseffectsforanionicsurfactants.Anionicwormlikemicelleshavebeenreportedmainlyinthepresenceofhighconcentrationsofhydratedsalts(e.g.,0.6MNaCl)orupontheadditionofacationicsurfactant.Webelieve *Towhomcorrespondenceshouldbeaddressed.E-mail:kaler@che.udel.edu.Tel.:(302)831-3553.Fax:(302)831-6751.Permanentaddress:NovelMaterials&StructuralChemistryDivision,BhabhaAtomicResearchCentre,Mumbai400085,India.Permanentaddress:DepartmentofChemicalEngineering,UniversityofMaryland,CollegePark,MD20742.(1)Mazer,N.A.;Benedek,G.B.;Carey,M.CJ.Phys.Chem,1075.(2)Hoffmann,H.;Rehage,H.;Platz,G.;Schorr,W.;Thurn,H.;Ulbricht,W.ColloidPolym.Sci.,1042.(3)Porte,G.;Appell,J.J.Phys.Chem.,2511.(4)Kern,F.;Lemarechal,P.;Candau,S.J.;Cates,M.E.,437.(5)Khatory,A.;Lequeux,F.;Kern,F.;Candau,S.J.,1456.(6)Aswal,V.K.;Goyal,P.S.;Thiyagarajan,P.J.Phys.Chem.B,2469.(7)Aswal,V.K.;Goyal,P.S.Phys.Rev.E,2947.(8)Rehage,H.;Hoffmann,H.Mol.Phys,933.(9)Buwalda,R.T.;Stuart,M.C.A.;EngbertsJ.B.F.N.,6780.(10)AitAli,A.;Makhloufi,R.ColloidPolym.Sci,270.(11)Rehage,H.;Hoffmann,H.J.Phys.Chem.,4712.(12)AitAli,A.;Makhloufi,R.Phys.Rev.E,4474.(13)Soltero,J.F.A.;Puig,J.E.,2654.(14)Carver,M.;Smith,T.L.;Gee,J.C.;Delichere,A.;Caponetti,E.;Magid,L.J.,691.(15)Hassan,P.A.;Candau,S.J.;Kern,F.;Manohar,C.,6025.(16)Mishra,B.K.;Samant,S.D.;Pradhan,P.;Mishra,S.B.;Manohar,C.,894.(17)Oda,R.;Narayanan,J.;Hassan,P.A.;Manohar,C.;Salkar,R.A.;Kern,F.;Candau,S.J.,4364. (18)Balasubramanian,D.;Srinivas,V.;Gaikar,V.G.;SharmaM.J.Phys.Chem.,3865.(19)Manohar,C.;Rao,U.R.K.;Valaulikar,B.S.;IyerR.M.Chem.Soc.,Chem.Commun.,379.(20)Valaulikar,B.S.;Mishra,B.K.;Bhagwat,S.S.;Manohar,C.J.Colloid.InterfaceSci.,304.(21)Cates,M.E.;Candau,S.J.J.Phys.Condens.Matter(22)Clausen,T.M.;Vinson,P.K.;Minter,J.R.;Davis,H.T.;Talmon,Y.;Miller,W.G.J.Phys.Chem,474.(23)Cates,M.E.,2289.(24)Magid,L.J.;Li,Z.;Butler,P.D.,10028andreferencestherein.(25)Barker,C.A.;Saul,D.;Tiddy,G.J.T.;Wheeler,B.A.;Willis,J.Chem.SocFaradayTrans,154.10.1021/la011435iCCC:$22.002002AmericanChemicalSocietyPublishedonWeb02/23/2002 thatthepresentstudyisthefirstexampleofhydrotrope-inducedmicellargrowthinanionicsurfactantsolutions.Thehydrotropeconsideredhereisanaromaticsalt,-toluidinehydrochloride(PTHC),whichfeaturesahydrophobiccationthatcanbindstronglytoanionicmicelles(Scheme1).Wefocusonthegrowthofsodiumdodecylsulfate(SDS)micellesinwaterasafunctionofaddedPTHCconcentration.Theorientationofthehy-drotropiccounterionsonSDSmicellesismonitoredbyNMRspectroscopy.Avarietyoftechniquesincludingquasielasticlightscattering(QLS),capillaryviscometry,andrheologyisusedtocharacterizethemicelles.ExperimentalSectionMaterialsandMethods.Sodiumdodecylsulfate(electro-phoresisgrade)waspurchasedfromFisherScientific,and-toluidinehydrochloridewasobtainedfromAldrich.Allchemi-calswereusedasreceived.StocksolutionsofSDSandPTHCwerepreparedindeionizedwaterandmixedinvaryingratios.Forlightscatteringmeasurements,thesolutionswerefilteredthrough0.2-mnitrocellulosefilters(Acrodisc)andflamesealedin2-mLampules.Thesealedampuleswerekeptatroomtemperaturefor1dayforequilibration.QuasielasticLightScattering(QLS).QLSmeasurementsweremadeusingaBrookhaveninstrumentwithaBI200SMgoniometerandaBI9000ATdigitalcorrelator.ThelightsourcewasaLexelargonionlaseroperatedatanoutputpowerof100mWat488nm.Thescatteringampuleswereimmersedinanindex-matchingfluidthermostatedat25ÉC.Theaveragedecayratewasobtainedfromthemeasuredautocorrelationfunctionusingthemethodofcumulantsemployingaquadraticfit.Themagnitudeofthescatteringvectorisgivenby)sin-/2),whereistherefractiveindexofthesolvent,isthewavelengthoflight,andisthescatteringangle.Measurementsweremadeatfivedifferentanglesrangingfrom50Éto130É.DiluteSolutionViscometry.TherelativeviscositiesofdilutemicellarsolutionsweremeasuredusingaCannon-Ubbelohdecapillaryviscometerimmersedinaconstant-temperaturewaterbathat25ÉC.Thesampleintheviscometerwasequilibratedfor15minpriortomeasurement.Capillariesofvariousdiameterswereusedsothattheflowtimesweresufficientlyhightoavoidtheneedforkineticenergycorrections.Rheologicalmeasurementswereconductedat25ÉCusingaBohlinCS-10stress-controlledrheometerinacouettegeometry(cupofdiameter27.5mmandbobofdiameter25mmandheight37.5mm).Frequency-sweepexperimentswereperformedataconstantstress(choseninthelinearviscoelasticrange)overafrequencyrangeof0.01to100rad/s.NMRSpectroscopy.NMRmeasurementswereperformedat25ÉConaBrukerAC250NMRspectrometer(resonancefrequencyof250MHzforH)operatingintheFouriertransformmode.ForNMRmeasurements,sampleswerepreparedinD(CambridgeIsotopes).ResultsandDiscussion1.PhaseBehaviorStudies.Theelectrostaticinterac-tionsbetweenSDSandPTHCleadtocomplexphasebehaviorinaqueoussolutions.Intheabsenceofacompletediagram,apseudo-ternaryphasemapoftheSDSwatersystemcanbeuseful.Figure1showssuchaphasemapinthewater-richcornerat25ÉC.Alongthebinarywateraxis,micellaraggregatesareformedabovethecriticalmicelleconcentration(cmc),andthesolutionisisotropicandoflowviscosity.ForlowSDSconcentra-tions,thesolutionremainsclearandisotropicatallmolarratiosofPTHC.AthigherSDSconcentrations,addingPTHCmakesthesolutionsviscousand,insomecases,viscoelastic(trappedairbubblesrecoil).Asthemolarratio[PTHC]/[SDS]approaches1,aturbidprecipitateisformed.Atwo-phaseregion(shaded)persistsathigherPTHCcontent.Withinthetwo-phaseregion,themor-phologyvaries:acrystallineprecipitatemightcoexistwithanaqueousphase,orthesamplemightformaturbidAlongthebinaryPTHCwateraxis,PTHCissolubleinwaterovertheconcentrationrangestudied,andthisleavesanarrowone-phaseregioninthePTHC-richside.AddingsmallamountsofSDStoPTHCgivesatranslucentliquidthateventuallytransformsintoatwo-phasemixtureofsmallneedlelikecrystalsandalow-viscosityaqueousphase.Thecrystalsapparentlycorrespondtotheequimo-larsaltofSDSandPTHC.Interestingphasebehavioroccursathighersurfactantandsaltconcentrations,althoughthedetailswerenotexplored.At150mMSDS,theadditionofPTHCfirstleadstoaviscoelasticphase,thentoabiphasiccoacervate(twoliquidphasesinequilibrium),thentoastronglybirefringentandturbidphase(possiblyalamellardisper-sion),andfinallytoaprecipitateneartheequimolarratio.ThisrichphasebehaviorisaresultofstronginteractionsbetweentheSDSheadgroupandtheoppositelychargedandhydrophobic-toluidiniumcounterioninPTHC.Thecounteriontendstoadsorbatthemicellewaterinterfacewithitsaromaticringintercalatedinthehydrophobicinteriorofthemicelle(aswillbeshownlaterbyNMRspectroscopy).Thus,thecounterionformsa1:1complexwiththesurfactantandsoreducestheeffectivechargedensityatthemicellesurface.Inturn,thereisashifttomicrostructuresoflowercurvature.Notealsothesimi-laritiesbetweenthephasediagramshownhereforahydrotropemixtureandthosereportedformixturesofcationicandanionicsurfactants,e.g.,octyltrimethylammoniumbromideandSDS.Thecommonfeatureisstrongassociationbetweentheoppositelychargedmoieties.2.MicellarGrowthintheDiluteRegime:QLSandQLSwasusedtoprobethevariationinmicellarsizeandshapeuponadditionofPTHCto50mMSDSsolutions.Theaveragedecayratefromthesecond-ordercumulantexpansionoftheelectricfieldcorrelation)varieslinearlywith(Figure2).TheslopeScheme1.Structureof-ToluidineHydrochloride Figure1.Water-richcorneroftheSDSwaterphasemapat25ÉC.Solutionsintheone-phaseregionareunshaded.ThesolidlineindicatesthepathalongwhichmicellargrowthismonitoredatafixedSDSconcentration.ThedashedlineindicatesthepathatafixedmolarratioofPTHCtoSDS(Langmuir,Vol.18,No.7,2002Hassanetal. istheapparentdiffusioncoefficientofthemicelles,decreaseswithincreasing(Figure3).Forisintherangeexpectedforsmallglobularmicelles.UponfurtheradditionofPTHC,dropssteeply,reflectingasharpincreaseinthesizeofthemicelles.Theobserveddecreaseinsuggestsatransitionfromsphericalmicellestorodsofincreasinglength.Wewillpresentlyattempttorelatetotheaveragelengththerods.Notethat,inthecaseofrodlikescatterers,theintensitycorrelationfunctioncanbemodifiedbythecouplingbetweentranslationalandrotationaldiffusion.Contributionsfromtherotationaldiffusionoftherodsareexpectedatlargevaluesof,typically3,andthiswouldintroduceintoacomponentthatisinde-pendentof.Becausetheplotsofarelinearandpassthroughtheorigin(Figure2),dependssolelyonthetranslationalmotionofthemicelles.Toextractinformationaboutmicellesizefrom,theeffectofinteractionsbetweenneighboringmicellesmustbetakenintoaccount.Theapparentdiffusioncoefficientcanberelatedtotheinfinite-dilutiondiffusioncoef-isthediffusionvirialcoefficientandisthesurfactantconcentration.canbeexpressedasacompetitionbetweenanosmoticpressuretermthatenhancesthediffusionandaretardinghydrodynamicfrictionalterm.isthemicellemolecularweight,istheosmoticsecondvirialcoefficient,isthehydrodynamicfrictionvirialcoefficient,andisthespecificvolumeofthemicelle.ForarigidrodisAvogadro'snumber,isthediameter,therodlength,andThehydrodynamicfrictionvirialcoefficientforsuspen-sionsofunchargedrodsisFinally,thediffusioncoefficientcanberelatedtothelengthoftherodanditsaxialratioBroersma'srelationshipistheBoltzmannconstant,istheabsoluteisthesolventviscosity,andtheshapefactorisafunctionoftheaxialratio.TheexpressionsofTiradoetal.,whicharevalidforaxialratiosintherangeof230,relateTheaboveexpressionsallowforthecalculationofthediffusioncoefficient,andhencethemicellarlengthviaaniterativeprocedure,keepingthemicellediameterat3.3nm,whichistwicethelengthofthehydrocarbonchainofSDS.Thedata(Figure4)showthattheadditionofPTHCcausessignificantmicellargrowth.Amonotonic (26)Phalakornkul,J.K.;Gast,A.P.;Pecora,R.,3122.(27)Russo,P.S.;Karasz,F.E.;Langley,K.H.J.Chem.Phys.,5312. (28)Isihara,A.J.Chem.Phys,1446.(29)Peterson,J.M.J.Chem.Phys.,2680.(30)Lehner,D.;Lindner,H.;Glatter,O.,1689.(31)Tirado,M.M.;Martinez,C.L.;deLaTorre,J.G.J.Chem.Phys.,2047. Figure2.Averagedecayratesoftheintensitycorrelationfunctionasafunctionoffor50mMSDSwithvaryingmolarratioofPTHC,.Thecorrespondingvaluesofshownintheinset. Figure3.Apparentdiffusioncoefficient,,for50mMSDSasafunctionofthemolarratioofPTHC,PTHC,+kD(c-cmc)](1) Figure4.Averagelengthoftherodlikemicelle()andaverageequivalentspherehydrodynamicdiameter()asfunctionsofthemolarratioofPTHC,)(2) M2)f(3)f)1 4[1+L d(1+d 2L)(1+ðd 2L)](4)kf)(RT 3è)(L2 D0M)(3d 8L)2/3(5)D0)kBT (lnp)(6)MicrostructuralChangesinSDSMicellesLangmuir,Vol.18,No.7,2002 increaseinmicellarlengthoccursupto70nm(anaxialratioof20)atthehighestsaltcontentinvestigated.Thus,theQLSdatasuggestatransitionfromsphericalmicellestorigidrodlikemicelles.TheflexibilityofthemicelleshasbeenneglectedinthisanalysisbecausetheirlengthisexpectedtobeonlyafewtimesthepersistenceTheconsistencyofthemicellarlengthsdeterminedusingQLScanbecheckedbydilutesolutionviscometry.Foragivenvolumefractionofsuspendedrods,therelativeviscosityofadilutecolloidalsuspensionisgivenbyistheviscosityofthesuspension,istheviscosityofthesolvent,isashapefactor,andaccountsforpossiblehydrodynamicinteractions.Theshapefactorand,hence,arefunctionsoftheaxialratio.Inthelimitoflowisgivenbyby(-1/p)]0.5andk1)0.75forprolateForlongerrodsinsemidilutesolution,theeffectofmultiparticleinteractionsiscapturedbytheDoiEdwardsmodel,whichgivesthefollowingexpressionfor(inthelimit,beingthenumberdensityoftherods)Thenumberdensityofthemicellescanbecalculated),whereisthevolumeofthesurfactantmonomer.Theseequationsallowforthepredictionofforasolutionofmicellesofagivenlength.Forlengthswascalculatedusingeq10,andforwascomputedusingeqs8and9.ThevaluesofcalculatedusingthelengthsmeasuredbyQLSwerethencomparedwithexperimentalmeasurementsofafunctionof(Figure5).ThereisgoodagreementbetweenthemeasuredandpredictedvaluesexceptathighmolarratiosofPTHC.ThisvalidatestheQLSanalysisbasedontherigidrodmodel.Notethatthesphere-to-rodtransitioninducedbytheadditionofPTHCresultsina10-foldincreasein.Thus,datafortwoindependentlymeasuredquantities,i.e.,viscosityanddiffusioncoef-ficient,togetherrevealthegrowthofmicellesupontheadditionofPTHCtoSDS.Thissimpleanalysistakesnoaccountofthelengthpolydispersityofrodlikemicelles.3.WormlikeMicellesintheSemidiluteRegime:RheologicalStudies.ConsidertherheologyofSDS/PTHCmixturesatafixedmolarratioofsalttosurfactant,0.6.Figure6showsthezero-shearviscosityafunctionofsurfactantvolumefractionand25ÉC.Thereisarapidincreaseinbyafewordersofmagnitude,followedbyamaximumin0.014(i.e.,75mM).Theinitialrapidincreaseinviscosityreflectstheexponentialgrowthofthecylindricalmicellesatlowconcentrations.Themicelleseventuallydevelopintolong,flexibleentitiesandformatransiententanglednetwork,owingtowhichthesystembecomesviscoelastic.Dynamicrheologicalstudieswereperformedontheviscoelasticmicellarsamples.Therelaxationtimeofthesystemcanbeestimatedfromthedynamicfrequencyspectraas1/,whereisthefrequencyatwhichthedynamicmoduli,,crossover.Forconcentratedsamples(0.015),therheologyconformstotheMaxwellmodel.Accordingly,thestoragemodulusandlossaregivenbyistheplateaumodulus.AnalternateestimateoftherelaxationtimeforMaxwellfluidsisgivenbytheratioofthezero-shearviscositytotheplateaumodulus,thatis,Figure7representstheevolutionofthedynamicrelaxationtime(),aswellastheratioof),asafunctionofthesurfactantvolumefraction.Bothdescribeamaximumasafunctionof.ThetwoaredifferentatlowbutbecomenearlyequalatthehighervalueswherethesampleisaMaxwellfluid.Notethatthestressrelaxationinanentangledmicellarmeshisgovernedbyacompetitionbetweentwoeffects.Thefirstisthereptationofthemicellarchainswiththecharac-teristictime,andthesecondisthebreakingandrecombinationofthemicelleswiththecharacteristictime.Thesamplesatvolumefractionsbelowthemaximum (32)Magid,L.J.J.Phys.Chem,4064.(33)Nagarajan,R.JColloidInterfaceSci,477.(34)Matheson,R.R.,643.(35)Doi,M,;Edwards,S.F.J.Chem.Soc.,FaradayTrans.2,918. 15(9) 10ln d)(10) Figure5.Relativeviscosities()of50mMSDSsolutionsasafunctionofthemolarratioofPTHC,.Thecirclesrepresentthepredictionsbasedonthelightscatteringdataassumingthemicellesarerigidrods. Figure6.Variationofthezero-shearviscosity,,withsurfactantconcentrationatafixedmolarratioofPTHC, 1+ö2ô2andG¢¢(ö))G0öô Langmuir,Vol.18,No.7,2002Hassanetal. inFigure7showdynamicbehaviorthatisanalogoustothatofpolydispersepolymersolutions,withthestressrelaxationbeingdescribedbyastretchedexponential.Inthatcase,.Theconcentratedregime,ontheotherhand,isdescribedbytheMaxwellmodelandsocorre-spondstothefastbreakinglimit,i.e.,.Here,thestressrelaxationcanbedescribedbyasingleexponential,andthesampleisaMaxwellfluidwiththesinglerelaxationtime Figure8showsthevariationoftheplateaumoduluswithsurfactantvolumefractionatafixedmolarratioincreasesmonotonicallywithinapower-lawrelationship;however,theobservedscalingexponent(5.2)ismuchhigherthanthatexpectedforsemidilutemicellarsolutions(2.25).Suchalargeexponentmightreflectthechargednatureofthewormlikemicelles.Notethattheplateaumodulusisrelatedtothecorrelationforstrainfluctuations().Thepresenceofelectrostaticinteractionsincreasestherangeofthesecorrelations,whichmightsuppressthevalueofcomparedtothatforunchargedmicelles.Indeed,insomechargedwormlikemicellarsystems,thecorrelationlengthfromrheologyissignificantlyhigherthanthestaticcorrelationlengthobtainedfromsmall-angleneutronscattering(SANS).Inthosesamples,withtheadditionofsalt,thevaluefromrheologydecreasesandeventuallyapproachesthevalueobtainedfromSANS.Thus,theobservedincreaseinthescalingexponentofisprobablyduetothepresenceofelectrostaticinteractions.Thenotablefeaturesoftherheologyarethemaximaintherelaxationtimeandthezero-shearviscosity.FromtheMaxwellrelation,,itisclearthatthetwomaximatrackeachother(asincreasesmonotonically,Figure8).Apossibleexplanationforthemaximaisthatitreflectsatransitionfromlineartobranchedmicelles.Branchedmicellesareexpectedtoshowalowerviscositythanlinearmicellesbecausetheyhaveanadditionalavenueforstressrelaxation(involvingtheslidingofbranchpointsalongthemicelle).ThequestiontheniswhySDSmicellesinthepresenceofPTHCwoulddevelopbranches.ItistruethattheinsertionofthePTHChydrotropeintotheSDSmicellewilllikelyincreasethesurfactantpackingparameteranddecreasethesponta-neouscurvatureofthesurfactantmonolayer.Thus,therelativeenergyofformationofhighlycurvedendcapswillincrease,endcapswillbelesslikelytoform,andsmallmicelleswillgrowwithaddedPTHC.Atfixedincreasingsurfactantconcentration,itispossiblethattheincreasingdensityofmicellesegmentswillallowbranchformationasthecurvatureatthebranchpointswillbelowerthanthatatanendcap.4.CounterionAdsorptionandMicellarDynam-ics:NMRStudies.Themolecularoriginofrodlikemicellegrowthisbelievedtobetheadsorptionorinsertionofthehydrotropeintothemicelle.NMRspectroscopyisanidealprobeoftheaveragepositionandorientationofcounterionsonthemicellarsurface.PastNMRstudiesofaromaticcounterionsoncationicmicellesshowedanupfieldshiftinprotonresonancesofthearomaticmoiety,fromwhichitwasinferredthatthearomaticringsinsertedintotheHere,protonNMRspectraofPTHCweremeasuredinthepresenceandabsenceofSDS(Figure9).TheNMRspectraof20mMPTHCinDOshowsthetwoorthoprotonsasadownfielddoublet(7.32and7.35ppm)becauseofcouplingwiththemetaprotons.Thetwometaprotonsappearslightlyupfield(7.23and7.25 (36)Schmitt,V.;Lequeux,F.J.Phys.II,193.(37)Koehler,R.D.;Raghavan,S.R.;Kaler,E.W.J.Phys.Chem.B,11035. (38)Lequeux,F.Europhys.Lett,675.(39)Kreke,P.J.;Magid,L.J.;Gee,J.C.,699. Figure7.Twomeasuresoftherelaxationtime,,asfunctionsofsurfactantconcentrationatafixedmolarratioof Figure8.Evolutionoftheplateaumodulus,,asafunctionofsurfactantconcentrationatafixedmolarratioofPTHC, Figure9.HNMRspectraofthesaltPTHCandSDS/PTHCmixtures.TheSDSconcentrationinthemixtureiskeptconstantat50mM.MicrostructuralChangesinSDSMicellesLangmuir,Vol.18,No.7,2002 ppm),againasadoublet.Thethreemethylprotonsattheparapositionareobservedat2.33ppm.Inthecaseof10mMPTHCinthepresenceof50mMSDS,boththemetaprotonsandtheareshiftedslightlyupfieldfromtheiroriginalpositions,whereasthepositionsoftheorthoresonancesremainpracticallythesame.ThisclearlyindicatesthatthecationicadditivestronglybindstothesurfaceofthemicelleswithitsmetaandparapositionsinsertedintothemicellarinteriorandtheorthoprotonsandtheNHgroupprotrudingfromthemicellesurface.Anextrapeakalsoappearsinthespectra,atripletobservedaround3.91ppm,thatarisesfromtheoftheSDS(i.e.,theCHthatisdirectlybondedtotheOatomofthesulfateheadgroup).AllotherprotonresonancesofSDSappearbelow2ppmandshownosignificantchangesupontheadditionofPTHCexceptforlinebroadeningeffects.Inthecaseof20mMPTHCand50mMSDS,them-andp-protonresonancesareshiftedfurtherupfield,indicatinganincreasedpopulationofPTHCinthemicelles.Furthermore,theresonancelinesarebroadenedandthemultipletscannotberesolved,indicatingasignificantlylargermicellesize.Thelinebroadeningforprotonsismuchlessthanthatforthearomaticprotons.Thisindicatesthattherotationalmotionofthehydrotropeisanisotropic,thepreferentialaxisbeingperpendiculartothemicellarsurface.Similareffectshavebeenreportedforsalicylateanionsincationicmicelles.Also,theSDSprotonresonancesareshiftedupfieldbytheadditionofPTHC,furtherconfirmingtheinter-calationofaromaticringsamongtheSDSheadgroups.For30mMPTHCand50mMSDS,thespectraaresimilartotheabove,exceptthatthemandpprotonsandtheprotonsareshiftedslightlyupfield.Furtherad-ditionofPTHCcausesprecipitationoftheequimolarsalt.SDSmicellesgrowinthepresenceofahydrotropicsaltPTHC.Thephasebehaviorinthewater-richsideofthepseudo-ternaryphasediagramresemblesthatofatypicalanionicmixture,therebyindicatingthestrongassociationcharacteristicsofthesurfactantandsalt.NMRstudiesconfirmtheadsorptionofthecounterionfromPTHContothemicellarsurfaceandshowthatthearomaticringisintercalatedbetweenthehydrocarbonchainsintheinteriorofthemicelle.TheadditionofevenlowconcentrationsofPTHCtoSDSpromotesatransitionfromsphericaltorodlikemicelles.Consequently,thereisasharpdecreaseinthetranslationaldiffusioncoefficientmeasuredbyQLS,aswellasadramaticincreaseintheviscosityofthesolutions.AnalysisoftheQLSdatausingarigid-rodmodelyieldstherodlengthasafunctionofcomposition.ThedynamicrheologicalbehaviorispurelyMaxwellianathighsur-factantconcentrations.AtafixedmolarratioofPTHCtoSDS,therearemaximainthezero-shearviscosityandrelaxationtimeasafunctionofthesurfactantconcentra-tion.Themaximaarepossiblyduetoatransitionfromlineartobranchedmicelles.SupportofthisworkbytheNSF(CTS-9814399)isgratefullyacknowledged.P.A.HisthankfultoDr.C.Manohar,IIT,Mumbai,India,forfruitfuldiscussionsandtoDr.SteveBaioftheDepartmentofChemisty,UniversityofDelaware,forhelpwithNMR (40)Shikata,T.;Morishima,Y.,1931.Langmuir,Vol.18,No.7,2002Hassanetal.