表面活性剂胶束形状随浓度转变的核磁共振研究_英文_

1、物理化学学报(WuliHuaxueXuebao)JulyArticleActaPhys.-Chim.Sin.,2009,25(7)：1357-表面活性剂胶束形状随浓度转变的核磁共振研究卢星宇1,2蒋艳1,2崔晓红1,2毛诗珍1,*2刘买利1,*杜有如1(1中国科学院武汉物理与数学研究所,波谱与原子分子国家重点实验室,武汉430071;中国科学院研究生院,北京100049)摘要：运用核磁共振一维氢谱和自扩散实验方法研究了聚乙烯乙二醇异辛酚醚(TX-100)、十二烷基苯磺酸钠(SDBS)和十四烷基三甲基溴化铵(TTAB)三种不同类型的表面活性剂在重水溶液中的胶束形状转变,发现它们在临界胶束浓度以上

2、的各自相应浓度都有胶束形状的变化(由球状转变为椭球状或棒状).在常温常压和没有其他添加剂的情况下,表面活性剂溶液浓度高于其临界胶束浓度时,球状胶束开始形成.核磁共振一维氢谱和自扩散实验的结果显示,当溶液浓度继续增加到一定程度时,溶液中表面活性剂分子的化学位移和自扩散系数的变化速率都有明显的转折,这说明溶液中球状胶束开始发生转变.进一步通过仔细分析对比核磁共振一维氢谱中各基团谱峰,发现表面活性剂胶束亲水表面上的质子的化学位移变化速率要远高于其疏水内核中的质子,据此推测胶束形状很可能由球状转变为椭球状或棒状.关键词：表面活性剂;中图分类号：O647核磁共振;胶束形状转变;第二临界胶束浓度NMRSt

3、udyofSurfactantMicelleShapeTransformationwithConcentrationLUXing-Yu1,2JIANGYan1,2CUIXiao-Hong1,2MAOShi-Zhen1,*LIUMai-Li1,*DUYou-Ru1(1StateKeyLaboratoryofMagneticResonanceandAtomicandMolecularPhysics,WuhanInstituteofPhysicsandMathematics,2ChineseAcademyofSciences,Wuhan430071,P.R.China;GraduateUnivers

4、ityoftheChineseAcademyofSciences,Beijing100049,P.R.China)Abstract：Themicelleshapetransformationoftert-octylphenyphenolpolyethyleneglycolether(TX-100),sodiumdodecylbenzenesulfonate(SDBS)andtetradecyltrimethylammoniumbromide(TTAB)inheavywatersolutionswasstudiedby1Hnuclearmagneticresonance(NMR)spectros

5、copy,includinganNMRself-diffusionexperiment.Theseexperimentsshowedthatthesurfactantsformedseveralshapesofmicelles(spherical,ellipsoidal,androdlike)attherespectiveconcentrationswhichwerefarabovetheircriticalmicelleconcentration(cmc).Atambienttemperature,normalpressureandwhentheconcentrationofthesurfa

6、ctantwasabovethecmcwithoutotheradditives,sphericalmicelleswereformed.Athighersurfactantconcentrations,1HNMRspectraandself-diffusionexperimentsshowedthatabruptchangesoccurredintheirchemicalshifts()andintheirself-diffusioncoefficients(D).Thisindicatedthatsphericalmicellestransformedintolargermicelles.

7、Bycarefulexaminationofeveryprotonresonancepeakinthe1HNMRspectra,wefoundthattheprotonsneartheheadgroupexperiencedlargerchemicalshiftchangesthanprotonsnearthehydrophobicgroup.Thissuggeststhattheshapeofsurfactantmicellesmostlikelytransformsfromsphericaltoellipsoidalorrodlike.KeyWords：Surfactant;NMR;Mic

8、elleshapetransformation;criticalmicelleconcentrationIIAsthemostbasalandordinaryformoforganizedsurfactant*molecularassemblies,micellesshowcomparativelysteadyReceived:January16,2009;Revised:March20,2009;PublishedonWeb:April24,2009.Correspondingauthors.Email:maosz,ml.liu;Tel:+86-27-87197126;Fax:+86-27-

9、87199291.TheprojectwassupportedbytheNationalNaturalScienceFoundationofChina(20610104,20635040).国家自然科学基金(20610104,20635040)资助项目鬁EditorialofficeofActaPhysico-ChimicaSinica1358ActaPhys.-Chim.Sin.,2009Vol.25shapesinthecomplexsolution,whichexhibitsvariousapplica-tionsinsimulationofbiomembrane,preparation

10、ofnanoparti-cles,andotherindustrialandtechnologicalprocesses1-4.Thema-jorfactorsthataffecttheshapeofmicellesarethenatureofthepolargroup,thelengthandthestructureofthehydrophobicchain,theadditivesuchaselectrolyteorpolarorganiccom-pound,thetemperature,andtheconcentrationofsurfactantinaqueoussolution5,6

11、.Althoughnumerousstudiesofmicellizationhaveappearedintheliteratureandthegrosscharacteristicsofmicellarcatalysisofvariouschemicalprocesseshavebeende-scribed6,relativelyfewstudieshavebeenconductedinwhichthemajoremphasiscenteredonthesizeandshapeofmicelles.Typically,ionicsurfactantsformsphericalmicelles

12、inaqueoussolution,inwhichthehydrophobicinteractionofthehydrocar-bontailwouldbalancetheelectrostaticrepulsionsbetweenthechargedheadsatthemicellesurface.Ithasbeenshown7,8thattheadditionofsaltscreenstheseelectrostaticrepulsions,allowingthechargedsphericalmicellestogrowintoelongated,rodlikestructures.Th

13、is“sphere-to-rod”transformationhasbeenwelldoc-umentedinthecaseofsodiumdodecylsulfatemicelles7-9.Micellesizeandshapemayalsoshowabruptchangeswhensurfactantconcentrationsincreasetovalueshigherthanthecmc,whichissometimescalledthecmcII,evenintheabsenceofotheraddi-tives10-13.Theincreaseofsurfactantmolecul

14、esinunitvolumemakesitpossibleforthemtoapproacheachothermorecloselyandformlargeraggregates,whichrequiresmuchmorespaceforthehydrophobicchains.Becauseasphericalmicellehasasmallervolume,itpreferschangingtolarger,mostlikelyellip-soidalorrodlikemicelletoincreasethevolume/surfaceratio.Theexistenceofellipso

15、idalorrodlikemicelleswasinferredfromexperimentsoflightscattering14-16andconfirmedbydirectobservationundertheCryo-TEM17-19forsomesystems.Howev-er,evidencefortheexistenceofmanyothersurfactantellip-soidalorrodlikemicellesisnotverystrong.Inthepresentwork,westudythreetypesofconventionalsur-factants:TX-10

16、0(nonionicsurfactant),sodiumdodecylbenzenesulfonate(SDBS,anionicsurfactant)andtetradecyltrimethylam-moniumbromide(TTAB,cationicsurfactant)atambienttem-peratureandnormalpressureinaqueoussolutionswithoutaddi-tive.Theshapetransformationsofthesethreesurfactantmicelles,whichhavedifferentpolargroupsandhyd

17、rophobicgroups,wereinvestigatedby1HNMRandNMRself-diffusionexperiments.Asapowerfulmethodinstudyingsurfactantaggregation,NMRspectroscopyisanoldtechniqueforthestudyofassociationofcolloids,butitsuseintheshapetransformationofmicellesisrare.Changesin1HNMRchemicalshiftsduetoalterationinthechemicalenvironme

18、ntsofsurfactantmoleculesprovidedirectandstrongevidenceofmicelleshapetransformations.Self-diffusioncoefficientsquantifythesizeofsurfactantmoleculesinaqueoussolutions,whichreflectsstateofthemolecules.NacalaiTesque.SDBS(MW=348.5)waspurchasedfromSinoph-armChemicalReagentwithapurityof98%.TTAB(MW=336.4)an

19、dD2OwerepurchasedfromAcroswithapurityof99%andadeuterationof99.8%.Theabove-mentionedreagentswereusedasreceived,withoutanyfurtherpurification.D2Owasusedassolventinsteadofwaterinordertoweakenthewatersignal.ABrukerAVANCEspectrometerwithaprotonfrequencyof500.13MHzwasusedintheNMRstudy.Me3SiCD2CD2CO2Na(TSP

20、)wasusedastheexternalreference.Forassuringcompleterecoveryofmagnetizationvector,asmallpulseflipangle30°wasusedratherthan90°intheconventionalsinglepulsese-quence.Thebipolargradientlongitudinaleddy-currentdelay(LED-BPP)sequence20wasusedtomeasuretheselfdiffusionco-efficientD.AllNMRexperiments

21、wereperformedat25andnormalpressure.2ResultsanddiscussionChemicalstructuresandprotonnumberingofthethreesur-factantsstudiedareshowninScheme1.Fromthe1HNMRspec-tra,themajorinformationweobtainedisthechemicalshift,whichissensitivetotheelectronicenvironment.Sinceneigh-boringmoleculescontributetotheelectron

22、icstructure,NMRissensitivetothesolventenvironmentofamolecule.This“sol-venteffect”providesmolecularinformation.Inaqueoussolu-tions,themotionalbehaviorofthesurfactantmoleculesdependsonnotonlywhethertheyexistasmonomersormicelles,butalsothesizeofthemicelles.Meanwhile,asimilarargumentcanbemadeforself-dif

23、fusioncoefficientsofthesurfactantmolecules.Becausethetransla-tionalmobilitiesaregreatlyaffectedbyassociationphenomena,theobservedself-diffusioncoefficientsofthesurfactantmolec-ulescanbeofconsiderablehelpinthedeterminationofsolutionstructure.Insurfactant-watersystems,thereisalwaysafinitemonomerconcen

24、trationandthesemonomerscancontributesig-nificantlytotheobservedaveragediffusion.Sincetheexchangeoftheindividualsurfactantmoleculesbetweenmonomersandmicellesisusuallyfast(theinverseofthedifferenceinthecorrespondingresonancefrequenciesistypical-lyintheorderoforbelow10-3s),theNMRchemicalshifts(orself-d

25、iffusioncoefficients)foramicellarsystemcanbewrittenasapopulation-weightedaverage21,22,Aobs=AmicPmic+Amon(1-Pmic)(1)inwhich,AobsistheNMRobservedvalueofchemicalshifts(or1ExperimentalTX-100(molecularweight(MW)=646.9)waspurchasedfromScheme1Chemicalstructuresandprotonnumberingofthethreestudiedsurfactants

26、No.7LUXing-Yuetal.:NMRStudyofSurfactantMicelleShapeTransformationwithConcentration1359self-diffusioncoefficients);AmonandAmicarethemonomericandmicellizedchemicalshifts(orself-diffusioncoefficients),respec-tively;Pmicisthefractionalpopulationofthemicellizedsurfac-tants.Ifitisassumedthattheconcentrati

27、onofmonomersisaconstantandequaltothecmc,Eq.(1)canbeexpressedasAobs=AmicPmic+Amon(1-Pmic)=(Amic-Amon)Pmic+Amon=(Amic-Amon)(c-cmc)/c+Amon=(Amon-Amic)cmc/c+Amic(2)inwhichcisthetotalconcentrationofthesurfactant.Therefore,whenthechemicalshiftortheself-diffusioncoefficientisplottedagainstthereciprocalconc

28、entration,astraightlinewouldbeobtainedforthemicellarsolution.Theslopeis(Amon-Amic)cmc,andtheinterceptisAmic.Forthechemicalshiftortheself-diffusioncoefficientofmonomers,Amoncanbeseemedasaconstant,boththeslopeandtheinterceptwoulddependonAmic.WhenNMRisusedtostudysolutionsofsurfactants,thecmccanbereadil

29、yobtainedbyplottingthechemicalshiftortheself-diffusioncoefficientversusthereciprocalconcentration.There-sultsofthemethyl(H1)protonchemicalshiftmeasurementsforTX-100areplottedinFig.1.Whentheconcentrationisbelowthecmc(cmc=0.3mmol·L-123),normallyonlymonomersexistinthesurfactantsolution,andthusthec

30、hemicalshiftshouldnotchange.ButasshowninFig.1,thereisatinyvariationofchemi-calshiftbelowthecmc,whichispossiblybecauseoftheforma-tionofoligomers(premicelles)24,25.Toremovethisuncertainty,wewouldliketoanalyzesituationsabovethecmcinlatterdis-cussions.Thevariationsofself-diffusioncoefficientandmethylpro

31、ton(H1)chemicalshiftwiththereciprocalofconcentrationforTX-100areshowninFig.2.Asprovedabove,wheneitherthechemicalshiftortheself-diffusioncoefficientisplotteda-gainstthereciprocalconcentration,astraightlinewouldbeob-tainedforthemicellarsolution,andtheslopeandtheinterceptdependonAmic(micorDmic).InFig.2

32、,wefindthatbothlinesshowL-1abruptchangesatthesameconcentration,about2.85mmol·(9.5cmc),namelyitscmcII9.ThevaluesofmicandDmichavechanged,andtheslopesofthetwolineschangefrom0.017to0.032,and19to39,respectively,asshowninFig.2.TheseresultsFig.2Variationsoftheself-diffusioncoefficientandthemethylproto

33、n(H1)chemicalshiftversusreciprocalofconcentrationforTX-100Thedataonlinesrepresenttheslopesoffoursegments.indicatethatthereisanimportanttransformationatcmcIIforthesolventenvironment,thesize,sizedistribution,andshapeofmicelles.WhentheconcentrationishigherthancmcII,themethylproton(H1)chemicalshiftmoves

34、morequicklytoahigherfield.Thisdenotesthattheintermoleculararrangementbecomescloser.Synchronously,theself-diffusioncoefficientdiminishesmorerapidly,whichindicatesthatthesizeandshapeofmicellesincreasealot,andsphericalmicellestransformtolargerones.Here,therearetwokindsofmicellesintheaqueoussolution:sph

35、ericalandthelargerones,thentheobservedchemicalshiftorself-diffusioncoefficientwouldbeexpressedasAobs=Amic1Pmic1+Amic2Pmic2+Amon(1-Pmic1-Pmic2)(3)Herecomponentsofthesurfactantsolutionbegintogetcom-plexbecausemoleculesexchangeamongthesethreestates,andoneofthesethreestateswouldtakealeadingpositioninthi

36、sdynamicequilibrium.However,thisresultisalsoastageinmi-celletransformation.Whentherangeofsurfactantconcentrationreaches30mmol·L-1(100cmc),weobserveanewabruptpoint(cmcIII)around9mmol·L-1(30cmc)asshowninFig.3,whichindicatesthattherecouldbeafurthertransformationwithin-creasingtheconcentration

37、.Heretheobservedchemicalshiftorself-diffusioncoefficientwouldbeexpressedasFig.1Variationofthemethylproton(H1)chemicalshiftversusreciprocalofconcentrationforTX-100Fig.3Variationofthemethylproton(H1)chemicalshiftversusreciprocalofconcentrationforTX-100inalargerconcentrationrangefrom0.5to30mmol·L-

38、11360ActaPhys.-Chim.Sin.,2009Vol.25Fig.4Variationsoftheself-diffusioncoefficientandthemethylproton(H1)chemicalshiftversusreciprocalofconcentrationforSDBSThedataonlinesrepresenttheslopesoffoursegments.Aobs=Amic1Pmic1+AmicNPmicN+Amon(1-Pmic1-PmicN)(4)NMRexperimentsofSDBS(Fig.4)andTTABatvariouscon-cent

39、rationsinD2Oat25alsoshowsimilarphenomenatothoseofTX-100.WhentheconcentrationofSDBSsolutionis12mmol·L-1(cmc=1.2mmol·L-123)andthatofTTABis26mmol·L-1(cmc=2.2mmol·L-123),thecurvesofboththechemicalshiftandtheself-diffusioncoefficientplottedagainsttherecip-rocalconcentrationcometoaturn

40、.Thentherearesimilarinfer-encestothoseforTX-100:sphericalmicellesofSDBSandTTABcouldtransformtolargeronesaround12mmol·L-1and26mmol·L-1,respectively.Accordingtoknownstudiesonthesurfactantmicelleusingothermethods14-19,wecansupposethatellipsoidalorrodlikemicelleswouldbethemajorstateinsurfactan

41、tsolutionwhentheconcentrationismuchhigherthanthecmc.However,evidencefortheexistenceofthesethreesurfactantellipsoidalorrodlikemicellesisnotverystrong.Withfurtherexaminationofeveryprotonresonancein1HNMRspectraofthethreesurfactantsstudied,wecandiscussdynamicsituationsofthetransformationofsurfactantmice

42、lles.Inthe1HNMRspectraofTX-100,wefindthatnotonlythemethylproton(H1)chemicalshifthaveaturnatcmcIIasthecon-centrationisvariedasreferredtoabove,buttheprotonchemi-calshiftsofothergroupsoftheTX-100moleculesalsobehavessimilarly.However,eachgroupvariestoadifferentextent,whichindicatesthatthechemicalenviron

43、mentofeachpartofthemoleculeisdifferent.Variationsofprotonchemicalshiftsofhydrophilic(H6)andhydrophobic(H1)groupswiththerecipro-calconcentrationforTX-100areshowninFig.5.AlthoughtheslopesofboththeOCH2(H6)groupandthemethyl(H1)groupalteredatthesameconcentration,2.85mmol·L-1,thehyderopho-bicmethylpr

44、oton(H1)hasasmallervariationof0.017to0.032comparedwith0.17to0.23forthehydrophilicOCH2(H6)group.Thisindicatesthatthechemicalenvironmentofthehydrophilicheadgroupsarrangedatthemicellesurfacechangesmoresig-nificantly.Thereforewesurmisethatwhenthesurfactantcon-centrationisabovethecmcII,thesurfacehydrophi

45、licgroupsoftheTX-100micellebecomecloser,andgroupsinthehydropho-biccorechangelessalthoughtheyhavethesametrend.Thesedifferentchangesbetweenheadgroupsatthesurfaceandhy-drophobicchaininthecorewouldtransformthestructureoftheTX-100micelle,asassumedinScheme2(a).Intheprocessofthistransformation,themicellesb

46、ecomelargerandtheshapetransformsfromsphericaltomostlikelyanellipsoidalshapetoin-creasethevolume/surfaceratio.Thisinducesthehydrophilicheadgroupsatthemicellesurfacetobeclosewitheachotherandatthesametimetherearesmallvariationsofthedensityofhydrophobicchainsinthecore,asindicatedfromtheresultsofthe1HNMR

47、experimentsabove.ThisprovidesevidencefortheexistenceofTX-100ellipsoidalmicelles.1HNMRexperimentsofSDBSandTTABalsoshowsimilarphenomenawiththoseofTX-100.Variationsofprotonchemicalshiftsofthehydrophilic(H5)andhydrophobic(H1)groupswiththereciprocalconcentrationforTTABareshowninFig.6.Wecanseethatthemethy

48、lproton(H1)chemicalshiftisalmostconstant·whenthesurfactantconcentrationbecomeshigherthan26mmol-1L,whichindicatesthatthechemicalenvironmentaroundhy-drophobicchainsissteady.ThislikelyindicatesanotherkindoftransformationasshowninScheme2(b).Rodlikemicelleshaveasteadycore,thuswhenthesurfactantconcen

49、trationincreaseson-lythelengthofthemicelleincreases,whichhardlychangestheFig.5Variationsofprotonchemicalshiftsofhydrophilic(H6)andhydrophobic(H1)groupsversusreciprocalofconcentrationforTX-100Thedataonlinesrepresenttheslopesoffoursegments.Scheme2Micelletransformationfromsphericaltoellipsoidalorrodlik

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