Higher Salt Hydrophobicity Lengthens Ionic Wormlike Micelles and Stabilizes Them upon Heating - Isabettini et al. - 2021 - Unknown

《Higher Salt Hydrophobicity Lengthens Ionic Wormlike Micelles and Stabilizes Them upon Heating - Isabettini et al. - 2021 - Unknown》由会员上传分享，免费在线阅读，更多相关内容在学术论文-天天文库。

pubs.acs.org/LangmuirArticleHigherSaltHydrophobicityLengthensIonicWormlikeMicellesandStabilizesThemuponHeatingStephaneIsabettini,LukasJ.Böni,MirjamBaumgartner,KeitaSaito,SimonKuster,PeterFischer,*andVivianeLutz-BuenoCiteThis:Langmuir2021,37,132−138ReadOnlineACCESSMetrics&MoreArticleRecommendationsABSTRACT:Tuningtherheologicalpropertiesofsurfactantsolutionsbychargescreeningisaconvenientformulationtoolincosmetic,household,oilrecovery,drag-reduction,andthickeningapplications.Surfactantsself-assembleinwater,anduponchargescreeningandcoreshielding,theygrowintolongwormlikemicelles(WLMs).Thesearevaluablemodelsystemsforsoftmatterphysics,andthemostexploredformulationishexadecyl-trimethylammoniumbromide(CTAB)andsodiumsalicylate(NaSal).ReplacingNaSalwitharomaticsaltsofalteredhydrophobicityresultsindifferentpenetrationoftheadditiveintheCTABmicellarcore.Thisalteredpenetrationdepthwilldeterminetheanisotropicmicellargrowththattailorstheviscoelasticresponse.Sodium4-methylsalicylate(mNaSal)isahigherhydrophobicityalternativetoNaSal,requiringlessadditivetoinducestrongchangesintheviscoelasticproperties.Herein,weprovideacomparativestudyofthemNaSal/CTABsystemwiththereferenceNaSal/CTABoverarangeoftemperaturesandsaltconcentrations.Thefindingsfromthewell-knownNaSal/CTABpairaretransferredtothemNaSal/CTABsystem,revealingtheoriginsoftheWLMsolution’sviscoelasticpropertiesbydiscerningcontributionsfromchargescreeningandmicellarcoreshieldinguponsmalldifferencesinhydrophobicity.■INTRODUCTIONconcentrationinsolution,alsothestericalandsurfacechargepropertiesofthesalt,i.e.,itsabilitytopenetrateintotheSurfactantsolutionsareemployedinhouseholdandcosmeticmicellarcoredeterminestheviscoelasticityandthemicellarproducts,detergents,pharmaceuticals,andemulsifiers.A1,3,4,10−13core−shellarchitecture.ThedepthofthecoreproblemoftenfoundisthattheviscosityisnotmaintainedpenetrationofthedifferentsaltsintothemicellarcoreathighertemperaturesandthattheviscositymightchangedeterminesthestructureandstabilityofWLMsandinfluencesDownloadedviaUNIVOFNEWMEXICOonMay16,2021at06:52:35(UTC).whenothercomponents,suchasfragrances,areaddedtothethesalt-curve.Thesurfactant-additivepairdefinesthepackingformulation.Atypicalformulationtoolistheadditionofsalttoparameter,anditwillgovernthetransitionfromglobulartoenhancetheviscosityofthesolution,whichcausesanisotropic14Seehttps://pubs.acs.org/sharingguidelinesforoptionsonhowtolegitimatelysharepublishedarticles.micellargrowthtowormlikemicelles(WLMs)andincreaseslongwormlikemicelles.1,2TheimpactoftemperatureonthefeaturesofWLMthepolydispersity.Aconvenientwaytodisplaytheimpactof6,13,15−19solutionswasstudiedbyseveralauthors.OneexampleadditivesontherheologicalpropertiesofsurfactantsolutionisisaddingmethylsalicylicacidtoCTABsolutionsattheso-calledsalt-curve.Inthesalt-curve,theviscosity,ormoreconcentrationsslightlylargerthantheequimolarone.Thispreciselythezero-shearviscosity,isplottedasafunctiononthesystemformsunilamellarvesicles,whichtransformintoWLMsmolarratioR=Csalt/Csurf,whereCsaltisthesaltconcentrationaboveacriticaltemperature,switchingfromaNewtonianfluidandCsurfisthesurfactantconcentration.Dependingonthe15toaviscoelasticshear-thinningbehavior.Fischerandinteractionsbetweenasaltandasurfactantmolecule,thesalt-16Rehagestudiedtime−temperatureaswelltime−concen-curvefeatureseitheroneortwoviscositymaxima,i.e.,a1,3,4trationsuperpositionofflow-curvestoelucidatetherheologicaldromedaryorcamelshape.Similarcurvesareformedbytheadditionofcosurfactantsorfragrancestosurfactantsolutions.5−8TheincreaseinviscosityiscommonlylinkedtoReceived:September2,2020theincreaseofmicellarlengthbeforethefirstmaximum,Revised:December11,2020causedbyneutralizationandcoreshieldingwitharomaticions.Published:December28,2020Thedecreaseinviscosityathighersaltconcentrationisassociatedwiththeenhancedscreeningandpossiblecharge9inversionattheisoelectricpointofWLMs.Besideits©2020AmericanChemicalSocietyhttps://dx.doi.org/10.1021/acs.langmuir.0c02608132Langmuir2021,37,132−138

1Langmuirpubs.acs.org/LangmuirArticle18referencesystem.Sodium4-methylsalicylate(mNaSal)waspreparedandstructuralparametersofWLMs,whileTungetal.focusedonthetemperatureeffectonthepackingparameterbytitratingtherespectiveacidformswithanequimolaramountofandthusthemicellarlength.HartmannandCressely17sodiumhydroxide.Thesaltswererecoveredbydryingundervacuumforaminimumof48h.Asolutionof100mMofhexadecyldemonstratedthattheviscosityofsemidiluteNaSal/CTAB6trimethylammoniumbromide(CTAB)inmilli-Qwater(Milliporesolutionsdecreaseswithincreasingtemperatures.Yixiuetal.SynergyUV,MilliporeAG,Switzerland)wasprepared.Theadditivesinvestigatedtheinfluenceofsaltandcosurfactantontheweremixedtothisstocksolutionwithmolarratios(R)between0.3anionicsurfactantsodiumerucateatelevatedtemperature.and0.7.AllsolutionswerevigorouslystirredandlefttoequilibrateatHowever,studiesthatincludetherheologicalresponseof30°Cforatleast1weekbeforemeasurementstoremoveentrappedWLMsolutionsuponheatingremainscarcefordifferentsaltairbubblesfromhighviscositysolutions.13Rheology.Steadyshearandsmallamplitudeoscillatorymeasure-structures.Weaddresstheseshortcomingswitharheologicalcharacter-mentswereperformedwitharheometer(MCR501andMCR502,izationoftheMaxwellianbehaviorofthemNaSal/CTABAntonPaar,Austria)equippedwithaconeplategeometry(CP50-1,radius=25mm,coneangle=1°).Thetemperaturewascontrolledsysteminthesemidiluteregime.Thisisundertakenoverafrom25to45°byaPeltierelement(PTD150,AntonPaar,Austria)rangeoftemperaturesandconcentrationswithfrequency-andawaterbath(Minichiller,Huber,Germany).Aplasticsolventsweepexperiments.Ourfindingsarecomparedtothewell-trapavoidedevaporation.Thelinearviscoelasticregion(LVE)wasstudiedNaSal/CTABsystemtohypothesizeontheimpactofevaluatedthroughanamplitudesweepatafixedangularfrequencyofsalthydrophobicityontherheologicalpropertiesuponheating.10rads−1.ThefrequencysweepwasmeasuredinalogarithmicThechargedfunctionalgroupandvalencyoftheNaSalandfrequencyrampintherangefrom10−2to102rads−1applyingamNaSaladditivesarethesame(Figure1a),thuswecansolelydeformationwithintheLVEregime.■RESULTSGeneralSalt-CurveFeatures.Bothsystems,NaSal/CTABandmNaSal/CTAB,developsimilartrendsforthezero-shearviscosityηasafunctionofmolarratio(R)asshown1inFigure1bandalsoknownasthesalt-curve.Thesesaltshavethesamecharge,sothedifferencesinFigure1barecausedmainlybytheadditionalmolecularhydrophobicityofmNaSal.ThepartitioncoefficientPofNaSalandmNaSalis0.328and0.827,respectively.Thisvaluehighlightsthedifferenceinhydrophobicity,asmNaSalprefersthehydrophobicenviron-mentofferedbythemicellarcore.ThesaltcurvealsorevealsthatmNaSalisanalternativetoNaSal,becauselessadditiveisrequiredtoinducethestrongincreaseinviscoelasticity.Weassumethatthemolarratioatthemaximumviscosity(ηmax)ismainlyinfluencedbythechargescreening,whiletheviscosityη(R)dependsonmolecularhydrophobicityandcoreshielding(Figure1a).Toseparatetheseinfluences,weinvestigatefoursamplecompositionsindicatedinFigure1b:(a)TwosolutionswiththesamemolarratioR=0.4,butdifferentsaltadditives.Theyarelocatedbeforetheηmax,calledhereS1formNaSalandS2forNaSal.Consequently,thereareresidualheadgroupchargesonthemicellarsurface,duetopoorsaltchargescreening(Figure1a).(b)TwosolutionswithsimilarviscosityandFigure1.(a)Sketchofamicellarcross-section.Thecationiclocatedafterηmax,withR=0.55formNaSal(S3)andR=0.7headgroupisscreenedbythenegativechargeofthesalt,calledchargeforNaSal(S4).Here,thechargescreeningbyNaSalorscreening.ThehydrophobiccoreisshieldedfrominteractionswithmNaSal,anditsimpactontherheologicalpropertiesarewaterbytheorganicpartofthesaltmolecule,calledcoreshielding.comparedafterthemicellarchargeinversion.TheonsetshowsthemolecularstructuresofNaSalandmNaSal,andCTABmicellesneedlowerconcentrationsofmNaSal(R=theirpreferentialorientationwhenpenetratingthemicellarcore.NotetheadditionalmethylgroupinmNaSal.(b)Salt-curvefor100mM0.45)thanofNaSal(R=0.5)toreachηmax.AlthoughηmaxisCTABwitheitherNaSalormNaSalasaromaticsaltadditives.Thereachedatdifferentmolarratios,thedifferenceisnotselectedsamplesS1,S2,S3,andS4aredescribedinthetext.EverydataexcessivelylargeasmNaSalandNaSalhaveanequivalentpointwasmeasuredintriplicate,yieldinganaverageerrorof±5%.charge.Moreover,themaximumviscosityreachedwithmNaSal(2000Pas)ishigherthantheonewithNaSalisolatetheimpactofhydrophobicityontheviscosity.By(1650Pas).ItisimportanttoremarkthatthisincreaseisselectingNaSalandmNaSalasadditivesforCTAB,weprovecausedexclusivelybytheincreaseinmolecularhydrophobicitythatevenasmallincreaseinmolecularhydrophobicityofmNaSal.TheadditionalmethylgroupofmNaSalallowsforenhancestheanisotropicmicellargrowthandstabilityofdeeperpenetrationintothemicellarcorethanNaSal,shieldingWLMsuponheating.thecorefromwatermolecules.Thisshieldingenhancesthecriticalpackingparameter(CPP)ofthesaltadditive/CTAB11,14■EXPERIMENTALSECTIONpairasmeasuredbyHNMR.OurhypothesisisthattheSamplePreparation.AllproductswerepurchasedfromSigma-largerhydrophobicportionofmNaSalpenetratesmoreAldrich(Buchs,Switzerland).Sodiumsalicylate(NaSal)wasusedasaefficientlytheavailablehydrophobic”space”inthemicellar133https://dx.doi.org/10.1021/acs.langmuir.0c02608Langmuir2021,37,132−138

2Langmuirpubs.acs.org/LangmuirArticleFigure2.FrequencysweepsforWLMsolutionsatR=0.4,consistingof40/100mMof(a)S1mNaSal/CTABand(b)S2NaSal/CTAB.TheelasticmodulusG′isplottedwithfilledsymbolsandthelossmodulusG″withhollowsymbols.Cole−Coleplotofthedynamicmoduli,G′andG″normalizedbyG0,asafunctionoftemperaturefor(c)mNaSal/CTABand(d)NaSal/CTAB.Figure3.FrequencysweepsforWLMsolutionsconsistingof100mMCTABand(a)55mMmNaSalS3and(b)70mMNaSalS4.TheelasticmodulusG′isplottedwithfilledsymbolsandthelossmodulusG″withhollowsymbols.Cole−Coleplotofthedynamicmoduli,G′andG″normalizedbyG0,asafunctionoftemperaturefor(c)mNaSal/CTABand(d)NaSal/CTAB.core,whilescreeningitfromtheinteractionwithwater.Theplateaumodulus(G0)remainsnearlyconstantasafunctionof20consequentchangeinmicellarcurvaturewithmNasalincitestemperature,anditisslightlyhigherforthemNaSal/CTABanisotropicmicellargrowthatloweradditiveconcentrationssystem.Cole−Coleplotsthatrelatethedynamicmoduli,thanforNaSal,whichisseeninFigure1b.storagemodulus(G′)andlossmodulus(G″)normalizedbyMaxwellBehaviorandItsTemperatureDependence.G0,tothetemperatureareshowninFigure2c,d,includingtheBeforethemaximumviscosity(ηmax),ionicWLMshaveidealMaxwellbehaviorrepresentedbydashedsemicircles.Aremainingsurfacechargesandarenotfullyneutralizedbysalt.perfectsemicircleshapeindicatesthatthesolutioniswell-Wemeasuretherheologicalresponseofthesesolutionsintherepresentedbyasinglestressrelaxationtime,whileaflattenedtemperaturerangeof25−45°C,toestimatethestructuralsemicircleshapeindicatesbroaddistributionsofrelaxation21stabilityatunscreenedconditions,i.e.,beforeηmax.Bothtimes.ApartfromdeviationsforG′/G0>2,theS1andS2additivesatR=0.4resultinWLMstructureswithsimilarsamplesformsemicircles,andcanberepresentedbyasingle16Maxwellbehavior,whichshifttowardhigherangularrelaxationtime.Thesedeviationsderivefromsolventfrequencies(ω)uponheat(Figure2a,b).Themagnitudeofcontributioninhighangularfrequencyregimes,whichlead16thesefrequencyshiftsissimilarforbothsystems,howeverthetotheupturnofG″afterreachingG″min(Figure2a,b).responseofmNaSal/CTABisslightlyshiftedtolowerAfterthemaximumviscosity(ηmax),thesurfacechargesoffrequencies.Thisshiftindicatesthatbettercoreshielding,ionicWLMsareexpectedtobeneutralizedbythesaltadditive.causedbymNaSal,leadstohigherrelaxationtimes.TheFrequencysweepsforsamplesS3andS4exhibitsimilar134https://dx.doi.org/10.1021/acs.langmuir.0c02608Langmuir2021,37,132−138

3Langmuirpubs.acs.org/LangmuirArticleFigure4.(a)Variationofmeshsizeξandentanglementlengthlefortheinvestigatedsamples.(b)AveragemicellarlengthL̅asafunctionoftemperature.(c)Relaxationtimeλasafunctionoftemperature.(d)Arrheniusplotofrelaxationtimesλatdifferenttemperatures.LinearfitsextracttheenergyofrelaxationEathroughtheArrheniusequation.(e)ViscosityηandenergyofrelaxationEaasafunctionofmolarratioR.Maxwellianresponses,shiftingtohigherωuponheatingdiffer,thepropertiesextractedfromoscillatoryrheology(Figure3a,b).ThemaindifferencecomparedtoS1andS2remainsimilar,asaconsequenceofthesimilarmolecularsystemsisthemagnitudeofthefrequencyshiftwithchargeofNaSalandmNaSal,andthehigherhydrophobicityoftemperature.ThesemagnitudesaresmallerforthescreenedmNaSal.WeproposethatthehigherviscosityofmNaSal/systems.Theplateaumodulus(G0)remainsindependentofCTABisaresultofthehigherhydrophobicityofmNaSalandtemperature.ThestructureofS4(R=0.7)seemstobecoreshielding,whichleadstoanisotropicmicellargrowth.ForequivalenttotheoneofS3solution(R=0.55),eventhoughthatwemustthenestimatethemicellarlength.First,thethemagnitudeofchargescreeningdiffers.Theenhancedmicellarmeshsize(ξ)iscalculatedasafunctionoftheplateauchargescreeningofR4causesitshigheroverallelasticityandmodulus(G0,Figures2and3)andthethermalenergy(kBT),moduleG0.Cole−ColeplotsforS3andS4areshowninFigure22−24accordingtoeq1.Here,kBistheBoltzmannconstantand3c-d.S4followsanalmostperfectsemicircleshapethatTthetemperatureofthesolution.Bothadditive/CTABpairssuggestsMaxwellianstressrelaxationbehavior.Atthisleadtoaveragemeshsizesξof49.3±2.1nmwithoutconcentration,asinglerelaxationtimedescribestheWLMsintheentirefrequencyregime.S3exhibitsmoredeviationsatsignificantvariationswithtemperature(seeFigure4aformoreG′/G0>2(Figure3d);however,thesearelesssignificantthandetails).Theosmoticcompressibility,andconsequentlytheforS1andS2systems(Figure2c,d).meshsize,oftransientmicellarnetworksareknowntobe2DecreaseofMicellarLengthuponHeating.Eveninsensitivetotemperaturevariation.Consequently,weassumethoughthesalt-curvesofmNaSal/CTABandNaSal/CTABthattheentangledmicellarnetworkremainsthesameforboth135https://dx.doi.org/10.1021/acs.langmuir.0c02608Langmuir2021,37,132−138

4Langmuirpubs.acs.org/LangmuirArticleadditivesatalltemperatures,thusξshouldbemoredependentdynamicalpropertiesofWLMsareexpectedtohaveastrongeronthechargethanonthehydrophobicityoftheadditive.dependenceontemperatureatlowsaltconcentrations,when26thechargesarenotscreened.ThisdependenceisobservedkTBbythehigherslopesofλ(T)forsystemsS1andS2thantheξ=3G0(1)slopesforsystemsS3andS4(Figure4c).Therefore,thewormlikemicelleswillbreakandreformfasteratelevatedThepersistencelength(b)ofWLMs,i.e.,therigidity20,21temperatures,leadingtotheseshorterrelaxationtimes.modulusofacylinder-likestructure,isshowntodependTheshorterλuponheatingisthusdirectlyrelatedtothe23,25weaklyonsalttypeandconcentration.IfmeasuredbyshorteraveragemicellarlengthsL̅showninFigure4b.Inquasi-elasticlightscattering,bisabout15nmforNaSal/18addition,Tungetal.proposedthatduetofasterend-cap25CTAB.Weassumethatb≈15nmforNaSal/CTABandformation,shorterWLMsareformeduponheating,whichwillmNaSal/CTABsystems,tofacilitatetheirdirectcomparison.Itthenacceleratetherelaxationtime.shouldbeclearthatthisdescriptionisonlyvalidforflexibleToquantifythebehaviorofthesolutionsuponheating,we23WLMsthataremuchlongerthantheirb.bisusedtoplottherelaxationtimesλasafunctionoftemperature(1000/calculatetheentanglementlength(le)basedoneq2.TheT)intheArrheniusformshowninFigure4d.Therelaxationentanglementlengthisinaverage109.1±7.8nm(seeFiguretimehasamonoexponentialdecaywithtemperatureforboth4aformoredetails),beingabout7timeslargerthanthesaltadditives.Thisdecayistypicalofviscoelasticsolutionsthatconsideredb.Thisentanglementlengthlisusedtoestimate27efollowtheArrheniustheory.Fromeq4,weextracttheeffectsemiquantitativelythemicellarlength(L̅).oftemperatureTontherelaxationtimeλbycalculatingthe5/3relaxationactivationenergyEa.Here,Aisapre-exponentialξfactorandRisthegasconstant.le=b2/3(2)λ=AeERa/T(4)Theratiooftheminimumlossmodulusandtheplateaumodulus(Gmin″/G0,Figures2and3)isrelatedtotheThelinearfittingsinFigure4dcrossattemperaturesTc,dueentanglementlength(le)andtotheaveragemicellarlengthtotheirdifferingEa.Thiscrossovertemperatureisonly(L̅)ofWLMsinsolution,asshownineq3.22−25WhenG″isobservedforNaSal/CTABsystematTc=33°C,whileTcminoccursatT>45°CformNaSal/CTABsystems.Forobserved,thisratioincreaseswiththetemperatureforallstudiedsystemsandsupportsashorteningoftheaveragetemperatureslowerthanTc,thescreenedsolutions(S3andmicellarlengthL̅withthecalculatedentanglementlengthle,S4)relaxfaster,duetochargeneutralization.However,forwecanestimatetheaveragemicellarlengthL̅asafunctionoftemperatureshigherthanTc,themolecularandionicmotiontemperature,asshowninFigure4b.willincrease,andconsequentlytheS1andS2solutionsrelaxfaster.ThisinversionisbecauseofthehigheravailabilityofG′′minleionsathighersaltconcentration,whichenhancerelaxation≈G0L̅(3)processbymicellarscission.ThehigherhydrophobicityofmNaSalandcorepenetrationalsoimpactstherelaxationoftheTheslopesofthemicellarlengthL̅decayasafunctionofmicellarstructure.TherelaxationtimesofmNaSal/CTABtemperaturearesimilarforS1andS2samplesandforS3andS4.micellesofS1aremarkedlylongerthanthoseoftheNaSal/However,themicellarlengthofsampleswithhighersaltCTABsystemforthecorrespondingsampleS2.Thisdifferenceconcentration,andthusbetterchargescreening(S3andS4),isnotaspronouncedforbothsystemsS3andS4,respectively.variedlessuponheatingforbothadditives.ThesesimilaritiesConsequently,ahighercrossovertemperatureTcisobservedindicatethatmicellarshorteningasaresponsetoheatismostlyformNaSal/CTABversusNaSal/CTAB.governedbythechargescreeningratherthanbycoreshielding.TherelaxationactivationenergyEaisestimatedfromtheWestillseetheimpactofhighermNaSalhydrophobicityonslopesinFigure4d.TocomparethevariationofEatothesalt-themicellarlength,asitformslongermicellesthanNaSal/curvesshape(Figure1b),weexpandthemeasurementsofCTABindependentlyofR.ThebettermicellarcoreshieldingoscillatoryrheologyasafunctionoftemperaturetoarangeofbymNaSalresultsinlongerWLMs,whilechargeshieldingconcentrations.TheresultsareshowninFigure4e,wherethegovernsmainlythemicellarentanglementlengthlethatviscosityηandEaareplottedasafunctionofmolarratioR.28remainsnearlyconstantforoursamples.TheoftenreportedEarangeof70−300kJ/molfallsinaRelaxationMechanismsuponHeating.ThemicellarsimilarrangetothecalculatedEavaluesinFigure4e.TheEarelaxationtimedependsontemperature,formulation,concen-fallswithincreasingchargescreeningforbothsalt/CTABtrationandflowconditions.TheassumedMaxwellbehaviorissystemswhileRincreases.TheEaofthemNaSal/CTABasimpleandaccuratemodelforlowfrequencies,butdeviatessystemisalwayslowerthanthatofNaSal/CTABforsameathighfrequenciesduetoadditionalrelaxationmechanisms.molarratioR.Thisobservationsuggeststhat,duetoitsThecharacteristicrelaxationtime(λ)iscalculatedfromthedifferentpenetrationinthemicellarcore,mNaSalismoreinverseofthefrequencycrossover(ωc)betweenthestorageeffectiveatchargescreeningthanNaSal.Thisconclusiongoesmodulus(G′)andthelossmodulus(G″).Forallsolutions,inlinewiththeappearanceofaviscositypeakηmaxatlowerRbeforeandafterηmax,ωcshiftstowardhigherangularvaluesformNaSalversusNaSalinFigure1b.frequenciesuponheating,andtherelaxationtimebecomesWefurtherdividethedecayinηmaxasafunctionofRintwoshorter(Figures2and3).regions,limitedbythemaximumviscosityηmax(Figure4d).InFigure4ccomparestherelaxationtimeλasafunctionoftheregionbeforeηmax,Eaismoreaffectedbysaltaddition.Thistemperature.ItisclearthatthestructureofWLMsrelaxesimpactismorepronouncedforthemNaSal/CTABsystem.Infasterathighertemperatures,whichisexpectedfromthetheregionaftertheηmax,Eadecayssimilarlyforbothsalts.enhancedmolecularmotioncausedbyheat.Furthermore,theHere,themicellarsurfacechargesarefullyscreenedand136https://dx.doi.org/10.1021/acs.langmuir.0c02608Langmuir2021,37,132−138

5Langmuirpubs.acs.org/LangmuirArticlecharge-inversionplaysarole.BasedonEa(R),itbecomesclearhydrophobicityofaromaticsaltadditivesinionicwormlikethatthehydrophobicityofmNaSalbecomeslessimportanttomicellesoffersasimpleandpowerfulmeansoftailoringthestructuralstabilityathighersaltconcentrations.Athigherrheologicalresponseandresistancetoheat.saltconcentrations,thefallinEawithincreasingRbecomeslesspronounced.ForthesehighR,the”freespace”inthe■AUTHORINFORMATIONhydrophobicmicellarcoreisalreadyoccupiedbytheorganicCorrespondingAuthoradditive,thuscoreshieldingisnotsignificantlyincreasedwithPeterFischer−InstituteofFood,NutritionandHealth,ETHthepresenceofadditionalsalt.HereagaintheEaoftheZurich,8092Zurich,Switzerland;orcid.org/0000-0002-mNaSal/CTABsystemismarkedlysmallerthanthatofNaSal/2992-5037;Phone:+41446325349;CTABforasamemolarratioR.ThisgoesinlinewithEmail:peter.fischer@hest.ethz.ch;Fax:+41446321155evolutionofviscosityasafunctionofRinFigure1b.ThelowerEaobtainedformorescreenedWLMshighlightstheAuthorsshortenedrelaxationtimesathightemperatures.TheseWLMStephaneIsabettini−InstituteofFood,NutritionandHealth,systemsarelikelydominatedbyscission.Contrastingly,lessETHZurich,8092Zurich,Switzerland;orcid.org/0000-screenedWLMsexperiencelongerrelaxationtimesbyboth0003-3416-8103reptationandscission,whichremainsensitivetobothchargeLukasJ.Böni−InstituteofFood,NutritionandHealth,ETHandcorescreening.Zurich,8092Zurich,Switzerland;orcid.org/0000-0003-■4958-0037CONCLUSIONMirjamBaumgartner−InstituteofFood,NutritionandByselectingNaSalandmNaSalasadditivesforCTAB,weHealth,ETHZurich,8092Zurich,SwitzerlandprovethatevenasmallincreaseinmolecularhydrophobicityKeitaSaito−HeatandFluidEngineeringGroup,Departmentenhancestheanisotropicmicellargrowth.Thesurfactant-ofMechanicalEngineering,NagaokaUniversityofadditivepairdefinesthepackingparameterthatgovernstheTechnology,940-2188Nagaoka,Japanself-assemblingstructureandviscoelasticityofwormlikeSimonKuster−InstituteofFood,NutritionandHealth,ETHmicelles(WLMs).ThechargedfunctionalgroupandvalencyZurich,8092Zurich,SwitzerlandoftheNaSalandmNaSaladditivesarethesame,andthusweVivianeLutz-Bueno−InstituteofFood,NutritionandisolatetheimpactofhydrophobicityontherheologicalHealth,ETHZurich,8092Zurich,Switzerland;response.Thesalt-curvesofmNaSalandNasalindicatetheorcid.org/0000-0001-9735-5470twomainfactorscontributingtomicellargrowth:(a)TheCompletecontactinformationisavailableat:molarratioR,wherethemaximumviscosityηmaxisreached,ishttps://pubs.acs.org/10.1021/acs.langmuir.0c02608similarforbothsalts.ThissimilarityisaconsequenceoftheequivalentchargescreeningcausedbymNaSalandNaSal.(b)NotesTheηmaxishigherformNaSal,becauseofitshigherTheauthorsdeclarenocompetingfinancialinterest.hydrophobicity.Theshieldingofthemicellarcorefromwatermoleculesleadstoanisotropicmicellargrowth,thushigherviscosities.■ACKNOWLEDGMENTSWeselecttwosolutions,S1formNaSalandS2forNaSal,S.I.acknowledgesfinancialsupportfromSwissNationalbeforetheηmaxatR=0.4,andtwosolutions,S3formNaSalScienceFoundation(GrantNo.200021-150088).L.J.B.andS4forNaSal,afterηmax.AllsolutionsexhibitMaxwellacknowledgesfinancialsupportfromETHZurich(Grantbehaviorwithminordeviationsathighangularfrequencies.No.ETH-1914-1).K.S.acknowledgestheShort-TermOver-TheenhancedmicellarcoreshieldinginducedbymNaSalseasTrainingProgramforGlobalLeadership(NagaokaresultsinlongerWLMs,whilechargeshieldinggovernsmainlyUniversityofTechnology)supportinghisstayatETHZurich.themicellarentanglementlengthlethatremainsnearlyV.L.B.acknowledgesfinancialsupportfromETHZurichconstantforoursamples.Arrheniusplotsareconstructed(GrantNo.ETH-2212-2).fromtherelaxationtimesλoverthestudiedtemperaturerangeforagivensaltinCTAB.Thecorrespondingactivation■REFERENCESenergiesofrelaxationEaarecomputedandcomparedforboth(1)Lutz-Bueno,V.;Isabettini,S.;Walker,F.;Kuster,S.;Liebi,M.;salts,revealingtherelaxationmechanismslinkedtochargeFischer,P.IonicMicellesandAromaticAdditives:ACloserLookatscreeningandcoreshieldingofCTABmicellesbysalttheMolecularPackingParameter.Phys.Chem.Chem.Phys.2017,19,additives.Thus,Eafallswithincreasingchargescreening21869−21877.inducedbythepresenceofthearomaticsaltadditiveandis(2)Makhloufi,R.;Hirsch,E.;Candau,S.;Binana-Limbele,W.;Zana,particularlysensitivetotheadditive’shydrophobicitybeforeR.Fluorescencequenchingandelasticandquasi-elasticlightηmax.Athighersaltconcentrations,theeffectoftheadditive’sscatteringstudiesofelongatedmicellesinsolutionsofcetyltrimethy-hydrophobicityonthestructuralstabilityofWLMsislesslammoniumchlorideinthepresenceofsodiumsalicylate.J.Phys.pronouncedduetoadditionalrelaxationmechanismsthatexistChem.1989,93,8095−8101.athightemperatures.(3)Lutz-Bueno,V.;Liebi,M.;Kohlbrecher,J.;Fischer,P.Insummary,bycomparingNaSalandthemorehydrophobicIntermicellarInteractionsandtheViscoelasticityofSurfactantSolutions:ComplementaryUseofSANSandSAXS.LangmuirmNaSalasadditivestoCTABwecansolelyisolatetheimpact2017,33,2617−2627.ofhydrophobicityontheviscosity(ormoregenerally:onall(4)Gaudino,D.;Pasquino,R.;Stellbrink,J.;Szekely,N.;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