Modulating Interfacial Energy Dissipation via Potential-Controlled Ion Trapping - Tivony, Zhang, Klein - 2021 - Unknown

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ThisisanopenaccessarticlepublishedunderaCreativeCommonsAttribution(CC-BY)License,whichpermitsunrestricteduse,distributionandreproductioninanymedium,providedtheauthorandsourcearecited.pubs.acs.org/JPCCArticleModulatingInterfacialEnergyDissipationviaPotential-ControlledIonTrapping‡‡RanTivony,YuZhang,andJacobKlein*CiteThis:J.Phys.Chem.C2021,125,3616−3622ReadOnlineACCESSMetrics&MoreArticleRecommendations*sıSupportingInformationABSTRACT:Asametal(gold)surfaceatagiven,butvariablepotentialslidespastadielectric(mica)surfaceatafixedcharge,acrossaqueoussaltsolutions,twodistinctdissipationregimesmaybeidentified.InregimeI,whenthegoldpotentialissuchthatcounterionsareexpelledfrombetweenthesurfaces,whichthencometoadhesivecontact,thefrictionaldissipationishigh,withcoefficientoffrictionμ≈0.8−0.9.InregimeII,whenhydratedcounterionsaretrappedbetweenthecompressedsurfaces,hydrationlubricationisactiveandfrictionismuchlower,μ=0.05±0.03.Moreover,thedissipationregimeasthesurfacescontactislargelyretainedevenwhenthemetalpotentialchangestotheotherregime,attributedtotheslowkineticsofcounterionexpulsionfromorpenetrationintothesubnanometerintersurfacegap.Ourresultsindicatehowfrictionaldissipationbetweensuchaconducting/nonconductingcouplemaybemodulatedbythepotentialappliedtothemetal.21,231.INTRODUCTIONhydrationlubricationmechanism.ThisremarkableabilityWhenevertwocontactingsurfacesareinrelativemotion,ofhydratedionstoactasalubricatingelementoffersadirectenergywillbedissipatedattheirinterfaceandmanifestedasaandreadilyattainedapproachforcontrollingandmanipulatingslidingfrictionforce.Suchfriction,whiledesirableinsomefrictionaldissipationbetweenbaresurfaceswithouttheneedcasessuchasforimprovingtractionorcouplingbetweentheforapriorchemicalmodificationorcoating,whichisoftensurfaces,mayalsoleadtolargeenergylossandwear,1,2whichpronetowearandtear.Here,weexploitcontrolledchangesinisgenerallyundesirable,sothattheabilitytocontrolfrictioninsurfacepotentialtoeitherexpelortraphydratedionsbetweenDownloadedvia102.252.65.34onMay14,2021at10:00:00(UTC).situisclearlyatapremium.Indeed,achievingfacileexternalcompressedsurfacesandtherebystronglymodulatefrictional3dissipationbetweenthemastheyslidepasteachother.controlofthefrictioncoefficientbetweenslidingsurfacesisoneoftheholygrailsinthefieldoffrictionandlubrication.Inourproof-of-conceptmodelsystem,asmooth(rmsTheshearforcesbetweentwocontactingsurfacesarelargelyroughness=0.3nm)metalsurface(gold)atavariable,dictatedbysurfaceinteractionssuchaselectrostatic,vanderexternallycontrolledpotentialinteractswithanatomicallySeehttps://pubs.acs.org/sharingguidelinesforoptionsonhowtolegitimatelysharepublishedarticles.4Waals,andhydrophobicforces.Thissetsapreferableroutesmoothsingle-crystaldielectric,amicasurfaceataconstantforcontrollingfrictionforcesthroughmanipulatingsurface(negative)surfacecharge.Bytuningthesurfacepotentialofinteractions.Sofar,alterationofsurfaceinteractionshasmainlythegold,itsinteractionacrossaqueouselectrolytesolutionbeenattainedusing“smartsurfaces”thatrespondtovariouswiththenegativelychargedmicaismanipulatedbetweentwoexternalstimulisuchaspH,solvent,temperature,electricregimes:24,25Anelectrostaticattraction(regimeI),wherethe3,5potential,andlight.approachingsurfacescomeintostrongadhesivecontactandModulationoftheinterfacialfrictionviacontroloftheexhibithighfrictionaldissipationonsliding;andahydrationpotentialofconductingsurfacesisanespeciallyattractiverepulsionregime(II),wherehydratedcounterionsaretrappedmethod,asitmayprovideaneasilyapplied,rapid,andbetweenthesurfaces,preventingtheiradhesion.Inthislatter6−18reversibleswitchingoftheirphysicochemicalproperties.Inaqueoussystems,thepotentialofinteractingsurfacescontrolstheinterfacialdistributionofions,andthroughthatthenatureReceived:December18,2020oftheinterfacialforcesanddissipation.19Inparticular,theRevised:January18,2021trappingofhydratedcounterionsbetweenlike-potentialPublished:February3,2021surfacescanreadilyovercomethevdWattractionsbetween20−22themandcanmassivelyreducetheinterfacialenergydissipationasthesurfacesslidepasteachother,viathe©2021TheAuthors.PublishedbyAmericanChemicalSocietyhttps://dx.doi.org/10.1021/acs.jpcc.0c112643616J.Phys.Chem.C2021,125,3616−3622

1TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleFigure1.(A)InteractionprofilesFn(D)/Rbetweencurvedgoldandmicasurfaces,meanradiusofcurvatureR,underdifferentappliedpotentialsΨapp(color-codedtomatchcurves)andacross2mMLiClO4.Emptygraysymbolsrepresentanout(retraction)profiletakenimmediatelyafterapproach(filledgraysymbols)whilemaintaininganappliedpotentialof−0.3V.GoldsurfacepotentialsΨgoldwereextractedfromfits(blackcurves)tonumericalsolutionsofthenonlinearPoisson−Boltzmann(PB)equationd2Ψ/dx2=(8πen/εε)sinh(eΨ/kT)withconstantpotentialo0B(gold,Ψ)andconstantcharge(mica,σ=5.23mC/m2forallfittingcurves)boundaryconditions,augmentedbyvdWattractionF(D)/R=goldmican−A/6D2,whereA=9×10−20Jisthegold−water−micaHamakerconstantestimatedfromfits,nisthebulkionconcentration(numberofionsHH0perunitvolume),ε0isthepermittivityoffreespace,εisthedielectricconstantofthesolvent,kBistheBoltzmannconstant,Tistheabsolutetemperature,andeistheelectroniccharge.Lowerandupperinsetsareschematicrepresentationsofthetwodifferentinteractionregimesasdescribedinthetext.(B)Adhesionenergybetweengoldandmicaacross1mM(diamonds)and2mM(circles)LiClO4atdifferentgoldsurfacepotentialsΨgold.Eachsymbol(solid)colorrepresentsadifferentexperimentwithanewsetofsurfaces,whileemptysymbolsrepresentdifferentcontactpositionsinthesameexperiment.Foreachexperiment,surfacepotentialvalueswereobtainedthroughfitsofthegold−micainteractioncurvestothenonlinearPBequation,assimilarlydepictedin(A).regime(II),hydrationlubricationisactiveandtheinterfacialabsoluteseparationDbetweenthesurfacesisaccuratelydissipationonsliding,monitoredviathefrictioncoefficientμdeterminedwithaca.3Åresolutionthroughanalysisofthe(=[forcetoslide]/[loadcompressingthesurfaces]),is“fringesofequalchromaticorder”(FECO;seeinsetinFiguremassivelyreducedcomparedwithregimeI.S1A),formedbetweenthetworeflectivesurfaces,usingthe27“multilayermatrixmethod”.Externalpotentialsareapplied2.METHODStothegoldsurfaceacrossthethree-electrodeconfiguration2.1.Materials.PurewaterwithatotalorganiccontentofshowninFigureS1Bwiththegoldsurfaceastheworkinglessthan1ppb(TOC<1ppb),aresistivityof18.2MΩcmelectrode(W)andtwoplatinumwires(Kurt,JLesker,(so-calledconductivitywater),andpH5.8waspreparedby99.95%)ascounterand(quasi)-referenceelectrodes.Toavoidpassingtapwatertwicethroughareverseosmosissystem,thenanyelectricleakage,quartzboatandTeflonlowerlensholderspassingthroughanion-exchangecolumn,andthenthroughareusedinallmeasurements.Beforeeachmeasurement,mechanicalfiltersofmeshsize5and2μmbeforeprocessinginplatinumwiresarewashedinhotnitricacid(30%,∼80°C)foraBarnsteadNanopureDiamondUV/UFsystem.Goldpellets,atleast30minandthenpassedthroughaflamethreetimesto99.999%pure,werepurchasedfromKurtJ.LeskerCo.andoxidizeandremovetheadsorbedorganicresidues.Theevaporatedfromagraphitecrucible.Lithiumperchlorate,electrodesareconnectedtoapotentiostat(CHI600C,CHLiClO4,99.99%pure,waspurchasedfromMerckMilliporeInstruments,Inc.),whichservesasacontrolunit.andusedasreceived.Nitricacid65%(HNO3,pKa=−1.4)was2.4.NormalForceMeasurements.Intheexperiments,purchasedfromMerckMilliporeandusedasreceived.quasi-staticanddynamicforcemeasurementsareusedto2.2.AtomicForceMicroscopy(AFM).AFM(MFP-3D,determinethenormalizednormalforceFn/R(RistheradiusofAsylumResearch)isusedforthemorphologicalanalysisof24,25curvature)asafunctionofseparationD.Inthequasi-staticsurfaces.Scansareperformedimmediatelyafterexposureofapproach,theupperlensismovedtowardthelowerlensinathesurfacetowaterorsalinetoavoidcontaminationandtimestepwisemannerandatafixedstepdistance(ΔDPZT)usingaeffect.Allscansareconductedintappingmodeusingsiliconpiezotube(PZT).Duringtheapproach,theactualchangeinnitridetips(AsylumResearch)withaspringconstantof∼80distance(ΔD),measuredusingFECOfringes,isdeterminedN/m.andthenormalforcebetweenthesurfacecouldbethen2.3.SurfaceForceBalance(SFB).NormalandshearforcemeasurementswereperformedusingasurfaceforcecalculatedbyΔFn=Kn(ΔDPZT−ΔD).Inthedynamicapproach,thelowerlensismovedcontinuouslytowardsthebalance(SFB)modifiedwithacustom-designedthree-electrodeelectrochemicalcell,asschematicallyshowninupperlensataconstantvelocityusingastepmotorwhileFigureS1andaspreviouslyelaboratedelsewhere.25,26Briefly,recordingtheFECOfringesataframerateofeither30or60molecularlysmoothgold(upperlens)andback-silveredmicaframes/s(XR60camera,Sony).Throughvideoanalysisofthe(lowerlens)surfacesaremountedonfusedsilicalensesinainterferencefringesmotion,thesurfaceseparationD(t)canbecross-cylinderconfiguration,equivalenttothegeometryofdeterminedatanytimeandthenormalforcecanbethensphereoveraflatsurface,withtheupperlensmountedonacalculatedbysolvingtheequationofmotion(includingsectoredpiezotube(PZT)andlowerlensonaleafspringofhydrodynamicforces)relevantforourconfigurational28,29springconstantKn=81.5±2.7N/m(FigureS1A).Thesetup.3617https://dx.doi.org/10.1021/acs.jpcc.0c11264J.Phys.Chem.C2021,125,3616−3622

2TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleFigure2.Measurementoffrictionalforcesbetweengoldandmicasurfacesacross1and2mMLiClO4solutions,atdifferentloadsFninregimeIandregimeII.(A)TypicalshearforceFsvstimetraces,takendirectlyfromtheSFBinregimeII,atdifferentloads(correspondingmeancontactpressuresshown)in2mMLiClO4.ForregimeI,themuchhigherfrictionwasmeasureddifferently,asdetailedintheSupportingInformation(SI).(B)Summaryoffrictionvsloadinthetworegimes(regimeI:1mMLiClO4(stars);regimeII:1mMLiClO4(squares)and2mMLiClO4(trianglesandcircles)).Eachsymbolrepresentsadifferentexperimentoradifferentcontactposition(differentcolors).TheshadedareaillustratesthefrictioncoefficientrangemeasuredinregimeII,anditsaverageisindicatedbythereddashedline.Theblackandredsolidlinescorrespondtofrictioncoefficientsin1mMLiClO4andinpurewaterunderregimeI.Alsoshown(diamonds)isthefriction−loadvariationinwaterwithnoaddedsalt,wherethesurfacesalwayscometoadhesivecontact.TheinsetshowsthelowloadregimeIIdataonanexpandedscale.2.5.ShearForceMeasurements.Shearforcemeasure-values.AtpositiveΨgoldvalues(redandgreendatapointsinmentsareperformedbylaterallymovingtheupperlens(gold)Figure1),theinteractionispurelyattractiveandthesurfacesusingeitheraPZToradifferentialmicrometer(FigureS4),jumpintoadhesivecontact,withawell-definedadhesiondependingontherequiredmagnitudeofappliedshearforce.energyWA,expellingallelectricdoublelayer(EDL)counter-Theupperlens,mountedonthepiezotube(lateralshearionstothebulk.WhenΨgoldisslightlymorenegativethanthemotionrange,ca.1.5μm),isconnectedtoverticalleafspringssurfacepotentialofmicaΨmica,theinteractiononsetsasan(springconstantKs=400N/m),anditslateralmotioniselectrostaticrepulsion(duetocounterionosmoticpressure),monitoredinrealtimeusinganairgapcapacitanceprobe.butgraduallyturnstoelectrostaticattractionduetosurfaceShearforcesFsbetweenthesurfacescanthenbemeasuredchargeinversionatthegold,whichisthenfollowedbyajump-25throughmonitoringofthebendingoftheverticalspringsΔdintoadhesivecontact(bluecurve),asdiscussedindetail(FigureS4).Incaseswherealargeappliedshearmotion(>1.524earlier.However,whenΨgoldismuchmorenegativethanμm)isrequired(e.g.,diamondandstardatapointsinFigureΨmica(graycurve),theelectrostaticinteractionispurely2B),theupperlensmountisgentlymovedusingadifferentialrepulsiveatallmeasuredcompressions.Inthiscase,themicrometer.Inthisway,alateralforcecanbeappliedtotheinteractionenergycurveFn(D)/Risalsofullyreversibleonupperlensbybendingtheverticalleafspringsasschematicallyseparatingthesurfaces(emptygraycircles).SimilarresultsshownanddescribedindetailinFigureS4.Alloftheglasswareiscleanedwithpiranhasolution(30:70werealsoobtainedin1mMLiClO4(FigureS2).Such20,21reversibilityisaclearsignatureofhydrationrepulsion,H2O2/H2SO4),thenrinsedwithwaterandethanol.whereshort-rangehydrationforcesbetweenconfinedhydratedionsovercomethevdWattractionatallseparations,hindering3.RESULTSANDDISCUSSIONthesurfacesfromreachingaprimaryminimumintheFn(D)3.1.NormalandAdhesionForces.Interactionsbetweencurve(i.e.,anadhesivecontact).Overall,thisindicatestwothecurvedgoldandmicasurfaces,atclosestseparationDapartinteractionregimesatwhicheitheradhesion(regimeI)oracrossaqueoussaltsolution,underexternallysetgoldhydrationrepulsion(regimeII)occursasthesurfacescomepotentials,weremeasuredusingasurfaceforcebalanceintocontact.TheinsetsinFigure1Aschematicallyillustrate(SFB)custom-designedasathree-electrodeelectrochemical25theinterfacialconditionfollowingapproachtocontactinthecell(FigureS1),asdescribedindetailelsewhereandaretworegimes.showninFigure1.Figure1AshowstypicalnormalizednormalWithinregimeI,theadhesion,asquantifiedbyanadhesionforceFn(D)/Rprofilesbetweengoldandmicain2mMenergyperunitareaWA,mayalsobecontrolledbyvaryingtheLiClO4,wheredifferentpotentialsΨappareappliedtothegold30,31relativetoaplatinumquasi-referenceelectrode.Theactualgoldsurfacepotential.WAwasevaluatedbymeasuringthegoldsurfacepotentialΨgoldisnotidenticaltoΨapp,butisforceFpulloffrequiredtoseparatethetwocurvedsurfaces(micaextracted,asdescribedearlier,24byfittingtheF(D)/Rcurvesandgold)fromadhesivecontactatdifferentvaluesofΨgold,n32tothesolutionofthenonlinearPoisson−Boltzmann(PB)accordingtotheJohnson−Kendall−RobertsexpressionWAequationwithconstantpotential(gold)andconstantcharge=−2Fpulloff/(3πR),andisshowninFigure1Bfortwodifferent(mica)boundaryconditions,augmentedbyvdWattraction.24saltconcentrations.Whilethereissomescatter,possiblyThevaluesofΨgoldcorrespondingtodifferentΨapparegiveninarisingfromthedifferentextentsofgoldroughnessatdifferentFigure1A.contactpoints(SI),atbothsaltconcentrations,WAdecreasesThemeasuredforceprofilesshowagradualchangeoftheasΨgoldbecomesmorenegativeuptoavaluewherenointeractionfromattractiontorepulsionfollowingachangeofadhesionismeasuredduetohydrationrepulsion,i.e.,oncethegoldsurfacepotentialΨgoldfrompositivetonegativeregimeII(|Ψgold|≫|Ψmica|)isreached.3618https://dx.doi.org/10.1021/acs.jpcc.0c11264J.Phys.Chem.C2021,125,3616−3622

3TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleFigure3.VariationofinsitufrictionalforceFsbetweenmicaandgoldsurfacesacross2mMLiClO4(traces(ii)and(iii)in(A))and1mMLiClO4(traces(i)and(ii)in(B))astheupper(gold)surfaceismovedbackandforthlaterally,andtheappliedpotentialistoggledperiodically(asmarkedbyarrows).Shearforcevaluesforeachtraceareindicated.(A)(i):appliedlateralmotionΔX0tothegoldsurface;(ii)and(iii):surfacescompressedbyFninregimeII(Ψapp=−0.3V).Trace(ii):Fn=314μN,Ψapptoggledfrom−0.3to−0.2V(correspondingΨgold=−0.18to−0.058V).Trace(iii):Fn=255μN,Ψapptogglesfrom−0.3to−0.15V(correspondingΨgold=−0.18to+0.004V).(B)Trace(i):surfacescompressedbyFn=250μNinregimeII(Ψapp=−0.3V)thentoggledfromΨapp=−0.3to+0.2V(correspondingΨgold=−0.17to+0.098V).Trace(ii):surfacescompressedbyFn=462μNinregimeI(Ψapp=+0.2V)thentoggledfromΨapp=+0.2to−0.3V(correspondingΨgold=0.092to−0.165V).3.2.Potential-RegulatedFrictionalDissipation.Inter-formationmaybeasourceofthescatterobservedinourFsvsfacialenergydissipationbetweenthecontactingmicaandgoldFndata(insettoFigure2B).Thisisbecauseonlyasperitiessurfacesismanifestedbythefrictionforcebetweenthemaswhoseheightiscomparabletoorhigherthanthediameter(ca.36+theyslidepasteachother,andmaybemodulatedbychanging0.72nm)ofthetrappedhydratedLiionswouldbeexpectedthegoldpotential,asshowninFigure2.ThefrictiontomakevdWcontactwiththeopposingmicasurface,whilethecoefficientμ=μ(I)≈0.8whenthesurfacesapproachto“valleys”betweensuchasperitiesaccountingformostofthevdWcontactinadhesiveregimeI.WhentheyarecompressednominalcontactregiontrapthehydratedLi+ions(inregimeinregimeII,however,inwhichhydratedcounterionsareII).Thearealnumberdensityofsuchhighasperities,asseenintrappedbetweenthesurfaces,thefrictionaldissipationthegoldheightprofiles(FigureS3),isrelativelysmallsothatdecreasesdramatically,yieldingμ=μ(II)≈0.05±0.03.theirvarianceacrossthedifferentsmallcontactregions(withFigure2Ashowstypicalfrictiontracesin2mMLiClO4diametersoforder10μm)islikelytobecorrespondinglylarge.solution,recordeddirectlyfromtheSFBduringslidingofAlsoshownarethefrictionvsloaddatabetweenslidingmicathetop(gold)surfacepastthelower(mica)one,fromwhichandgoldsurfacesinadhesivecontact,whenimmersedinwaterthefrictionforcesFsareextracted.Figure2Bsummarizeswithnoaddedsalt,revealingalargefrictioncoefficientμ≈0.9,frictionasafunctionoftheloadbetweenthesurfaces.ThesimilartoregimeIinthe1mMLiClO4saltsolution(seemuchlowerfrictionaldissipationonslidinginregimeIIisMethodsandFigureS4foradetaileddescriptionofFsattributedtohydrationlubricationbythehydratedLi+measurement).Insuchalargelysalt-freemedium,therearecounterionstrappedbetweenthecontactingsurfaces,andasnotrappedhydratedionsbetweenthecontactingsurfacesevenseeninFigure2B,itisthereforethesameforboth1and2mMat|Ψgold|≫|Ψmica|,whichinasaltsolutionwouldcorrespondto25saltconcentrations(unliketheadhesionenergyinregimeI;seetrappedcounterionsinregimeII.ThisisbecausethealsoSI).Wenotehoweverthatthevalueofμ(II)(betweencounterionsarepredominantlyhydratedprotons,whichreadilygoldandmica),whilelow,isnonethelessaboutanorderofcondenseintoandneutralizethemicasurface,leadingtomagnitudehigherthanthatbetweentwoatomicallysmoothadhesivevdW(aswellaselectrostatic)interactionbetweenthe30micasurfacesacrosstrapped,hydratedLi+ionsundersimilarsurfaces.Thesimilarityofthefrictionaldissipationinthetwoloads,forwhichμ≈(4±2)×10−3,asreportedinref33.Thiscases(regimeIinLiClOsolution,andwaterwithnoadded4suggeststhat,despitetheverylowRMSroughness(∼0.3nm)salt)isattributedinbothtostrongadhesion,wheretrapped26ofthetemplate-strippedgold(FigureS3),asperitycontactscounterionsareabsent(orlargelyso),andtheassociatedformbetweenthegoldandmica(suchcontactsareabsentdissipationonsliding(FigureS4).frommica−micacontact,wherebothsurfacesareatomically3.3.ShearForceTuning.Furtherinsightintothenaturesmoothsingle-crystalcontact).Suchcontacts,acrosswhichoftheinterfacialdissipationisprovidedbyexaminingthehydratedionsmaynotbetrapped,wouldleadtoavailabilityofchangeinfrictioninsitu,i.e.,duringslidingofthecompressed4,34additionaldissipativepathwaysonsliding,inparticularsurfacespasteachother,asafunctionoftheappliedpotentialhystereticbreakingandreformingofvdWbonds,andalsoonthemetalsurface,asshowninFigure3.Figure3Ashowspossiblyplasticdeformationofthemetalasperitiesandsometheeffectonthefrictionofmildlychangingthepotential35wear.appliedtothegoldsurface.Initially,thesurfacesapproachtoNotably,sinceeachgold−micacontactregionhasslightlycontactwhenthepotentialappliedtothegoldisΨapp=−0.3differentgoldroughness,thevarianceintheasperitycontactVwellinregimeIIwherehydrationrepulsionandhydration3619https://dx.doi.org/10.1021/acs.jpcc.0c11264J.Phys.Chem.C2021,125,3616−3622

4TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticlelubricationapply,seeFigure1AandarecompressedbyahydrationlubricationbytrappedhydratedLi+ionsoccurssoforceFext.ThefrictionforceFsrequiredtoslidetheupperrapidly(<1s),showsunambiguouslythatthehydratedsurface(gold)againstthelowerone(mica)yieldsaneffectivecounterionsarelargelytrappedbetweenthecompressedfrictioncoefficient(Fs/Fext)≈0.05,consistentwithμ(II)surfacesthroughout.(Figure2B),whereFnisFext.ThepotentialΨappappliedtotheThisscenarioiscorroborated,asshowninFigure3B(ii),goldisthenchangedtoeither−0.2V(Figure3A(ii))or−0.15whenthesurfacesarebroughttoinitialcontactwhileinregimeV(Figure3A(iii)).AsseeninthecaptionofFigure1A,thisI(Ψapp=0.2V),compressedbyFext,andthepotentialtoggledresultsingoldsurfacepotentialΨgold=−0.058and+0.004V,repeatedlybetweenregimeIandregimeIIwhilesliding.Onrespectively,wherethesurfacesareinregimeI.UnderthetogglingtoregimeII(Ψapp=−0.3V),thefrictionforceshouldbereduced,ifhydratedLi+ionsweretoentertheintersurfaceappliedloadFext,inequilibrium,counterionsshouldbeexpelledfrombetweenthesurfacesandthefrictioncoefficientgap(whichistheequilibriumstateinregimeII),toavalueshouldbehigh.However,despitetheincreaseinfrictionasthecorrespondingtoμ=μ(II).Weobservefromthetraceingoldpotentialchanges(Figure3A(ii)and(iii)),thefrictionalFigure3B(ii),however,thatnoslidingoccurs.ThisimpliesadissipationremainscharacteristicofregimeII.frictionforcelargerthanthemaximalappliedshearforceFs,maxToseethis,weestimatetheadditionalforcebetweenthe(=228μN)andthusafrictioncoefficientμ>(Fs,max/Fn)>surfacesarisingfromthechangeingoldpotential.Thismaybe0.25,consistentwithμ(I)butinanycasefarlargerthanμ(II)approximatedasthepull-offforceFpulloffbetweenthesurfaces≈0.05±0.03(withFnestimatedasabove).Thisstronglyatthecorrespondingpotentials,availablefromtheWAvsΨgoldindicatesthatfewifanyhydratedcounterionsenteredthegapplotofFigure1B.ThetotalcompressionofthesurfacesmayafterbeingexpelledduringtheinitialapproachinregimeI.thenbeestimatedasFn≈Fext+FpulloffandtheeffectivefrictionThisisattributedasfortheoppositecase(Figure3B(i))coefficientasμ=(Fs/Fn).ThisyieldsfortracesFigure3A(ii)wherehydratedcounterionsweretrappedinthegaptotheand(iii)μ≈0.09and0.1respectively(SI).ThesevaluesoftheslowkineticsofhydratedionsenteringorleavingthenarrowfrictioncoefficientareslightlyhigherthantheregimeIIvalues,subnanometergapbetweenthegoldandmicasurfaces.Figure2B,forwhichμ(II)=0.05±0.03,butverymuchlowerthantheregimeIvalueμ(I)=0.8.Theimplicationofthis,to4.CONCLUSIONSbefurtherconsideredbelow,isthathydratedLi+ionsareToconclude,wehaveshownthatfrictionaldissipationacrosstrappedbetweenthegoldandmicasurfacesevenatgoldaqueoussaltsolutionbetweenasmoothmetalsurface(gold)atpotentialscorrespondingtoregimeI,therebygreatlyreducingacontrolledpotential,slidingpastadielectric(mica)atafixedthefrictionaldissipation,throughthehydrationlubricationcharge,dependsstronglyonthehydratedcounterionstrappedmechanism.betweenthesurfaces,whosepresenceinturndependscruciallyInFigure3B,weexaminetheeffectonthefrictionofonthegoldpotentialwhenthesurfacesapproachtocontact.Inapproachingthesurfacestocontactwiththegoldpotentialparticular,hydratedionsintheinterfacialgapcanstronglycorrespondingtoeitherregimeIIorregimeIandthentogglingreducetheslidingfrictionthroughthehydrationlubricationthegoldpotentialdeeplyintotheotherregime.Figure3B(i)mechanism,tofrictioncoefficientsaslowasμ=0.05±0.03,showsgoldvsmicainitiallycompressedbyFextatΨapp=−0.3comparedwiththehighfrictionobserved(μ≈0.8−0.9)whenVwellinregimeIIwherehydrationlubricationappliesasionsareexcludedfromtheinterfacialgap.Inthisscenario,thesurfacesslidepasteachother,yieldingμ=(Fs/Fext)≈0.04,mostofthedissipationisattributedtoslidingofasperityinlinewithearliervalues(Figure2).Thepotentialisthencontactsbetweenthegoldandmicasurfaces,wherehydratedtoggledtoΨapp=0.2V,stronglyinregimeI(Figure1A),andionspreventvanderWaalsadhesioninthevalleysseparatingbackagain,repeatedly.Thelargerfrictionforceinthiscasetheasperities.Importantly,withintheparametersofour(Ψapp=0.2V)exceedsthemaximalappliedshearforceFs,maxexperiments,theinitialequilibriumpresenceorabsenceofsothatthesurfacesdonotslide,andonecanonlysurmisethatinterfacialionsoncompressingthesurfacesatagivenpotentialμ>(Fs,max/Fn),whereFn≈Fext+Fpulloff(whereFpulloffisislargelymaintainedduringslidingalsowhenthepotentialofevaluatedfromWA)asabove,yieldingμ>0.095.Thisvalue,themetalischangedtofavoradifferentequilibriumstate.ThiswhileconsistentwiththevaluesseeninFigure3A(ii)and(iii),isattributedtotheslowkineticsofhydratedionsleavingorforwhichμ≈0.09−0.1andwhichindicatethepresenceofenteringthegap,and,forthecasewherehydratedionsaretrappedcounterions,cannotinitselfconfirmsuchapresence.trappedbetweenthesurfacesoninitialcontact,enablesfacile,Thisisbecauseitisalowerlimit,anditmightalsobemuchreversible,insitucontrolofthefrictionthroughchangesintheclosertoμ(I)≈0.8,wherenocounterionsaretrapped.metalpotential.Importantly,however,wenoteinFigure3B(i)thenear-instantaneousreversionofthefrictioncoefficienttoμ=(Fs/■ASSOCIATEDCONTENTFext)≈0.04ontogglingthepotentialbacktoΨapp=−0.3V.*sıSupportingInformationThisimmediate(<1s)reversiontothelowerfrictiondoesTheSupportingInformationisavailablefreeofchargeatstronglypointtothepresenceoftrappedcounterionsevenathttps://pubs.acs.org/doi/10.1021/acs.jpcc.0c11264.Ψapp=0.2V,deepintheregimewhere,inequilibrium,suchionsshouldbeexcluded.ThisisbecauseitisknownbothSchematicofthesurfaceforcebalanceinthethree-theoretically37,38andfromdirectexperiments29thatpene-electrodeconfiguration(FigureS1);gold−micainter-trationofionsintosuchanarrowslit(<1nm)betweentwoactionprofilesatdifferentappliedgoldpotentialssurfaces,evenwhenfavoredbytheirpotentials,iskineticallyindicatinghowthegoldsurfacepotentialisextractedlimited,andwouldbeexpectedtorequireoftheorderof102s(FigureS2);AFMmicrographsshowinggoldsurfaceorlonger(seealsoFigureS5).Thefactthatontogglingfromroughness(FigureS3);measurementoffrictionindeepinregimeI(Ψapp=0.2V)todeepinregimeII(Ψapp=adhesiveregimeIaswellasanestimationoftotalload−0.3V),thetransitiontothelowerfrictionwhichimplicatesandfrictioncoefficientontogglingpotentials(Figure3620https://dx.doi.org/10.1021/acs.jpcc.0c11264J.Phys.Chem.C2021,125,3616−3622

5TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleS4);andvariationofinsitufrictionalforceFsbetween(11)Labuda,A.;Hausen,F.;Gosvami,N.N.;Grütter,P.H.;micaandgoldacross2mMLiClOfollowingastepLennox,R.B.;Bennewitz,R.Switchingatomicfrictionbyelectro-4potentialappliedtothegoldover130s(FigureS5)chemicaloxidation.Langmuir2011,27,2561−2566.(12)Park,J.Y.;Ogletree,D.;Thiel,P.;Salmeron,M.Electronic(PDF)controloffrictioninsiliconpnjunctions.Science2006,313,186.(13)Li,H.;Wood,R.J.;Rutland,M.W.;Atkin,R.Anionicliquid■lubricantenablessuperlubricitytobe“switchedon”insituusinganAUTHORINFORMATIONelectricalpotential.Chem.Commun.2014,50,4368−4370.CorrespondingAuthor(14)Brandon,N.;Bonanos,N.;Fogarty,P.;Mahmood,M.TheJacobKlein−DepartmentofMaterialsandInterfaces,effectofinterfacialpotentialonfrictioninamodelaqueouslubricant.WeizmannInstituteofScience,Rehovot76100,Israel;J.Electrochem.Soc.1992,139,3489.orcid.org/0000-0001-6602-0694;Email:Jacob.klein@(15)Pashazanusi,L.;Oguntoye,M.;Oak,S.;Albert,J.N.;Pratt,L.weizmann.ac.ilR.;Pesika,N.S.Anomalouspotential-dependentfrictiononAu(111)measuredbyAFM.Langmuir2018,34,801−806.Authors(16)Li,H.;Rutland,M.W.;Atkin,R.Ionicliquidlubrication:RanTivony−DepartmentofMaterialsandInterfaces,influenceofionstructure,surfacepotentialandslidingvelocity.Phys.WeizmannInstituteofScience,Rehovot76100,IsraelChem.Chem.Phys.2013,15,14616−14623.YuZhang−DepartmentofMaterialsandInterfaces,(17)Li,S.;Bai,P.;Li,Y.;Chen,C.;Meng,Y.;Tian,Y.ElectricWeizmannInstituteofScience,Rehovot76100,Israelpotential-controlledinterfacialinteractionbetweengoldandhydro-philic/hydrophobicsurfacesinaqueoussolutions.J.Phys.Chem.CCompletecontactinformationisavailableat:2018,122,22549−22555.https://pubs.acs.org/10.1021/acs.jpcc.0c11264(18)Perez-Martinez,C.S.;Perkin,S.Surfaceforcesgeneratedbytheactionofelectricfieldsacrossliquidfilms.SoftMatter2019,15,AuthorContributions4255−4265.‡R.T.andY.Z.contributedequallytothisstudy.(19)Israelachvili,J.N.IntermolecularandSurfaceForces:RevisedNotesThirdEdition;AcademicPress,2011.Theauthorsdeclarenocompetingfinancialinterest.(20)Pashley,R.DLVOandhydrationforcesbetweenmicasurfacesinLi+,Na+,K+,andCs+electrolytesolutions:Acorrelationof■double-layerandhydrationforceswithsurfacecationexchangeACKNOWLEDGMENTSproperties.J.ColloidInterfaceSci.1981,83,531−546.TheauthorsthanktheMcCutchenFoundation,theRothschild(21)Raviv,U.;Klein,J.Fluidityofboundhydrationlayers.ScienceCaesareaFoundation,andtheIsraelScienceFoundation−2002,297,1540−1543.NationalScienceFoundationofChinajointprogram(grant(22)Pashley,R.HydrationforcesbetweenmicasurfacesinaqueousISF-NSFC2577/17)fortheirsupportofthiswork.Thiselectrolytesolutions.J.ColloidInterfaceSci.1981,80,153−162.projecthasreceivedfundingfromtheEuropeanResearch(23)Klein,J.Hydrationlubrication.Friction2013,1,1−23.Council(ERC)undertheEuropeanUnion’sHorizon2020(24)Tivony,R.;Dan,B.Y.;Silbert,G.;Klein,J.DirectObservationresearchandinnovationprogramme(grantagreementno.ofConfinement-InducedChargeInversionataMetalSurface.743016).ThisworkwasmadepossibleinpartbythehistoricLangmuir2015,31,12845−12849.(25)Tivony,R.;Klein,J.ProbingtheSurfacePropertiesofGoldatgenerosityoftheHaroldPerlmanfamily.LowElectrolyteConcentration.Langmuir2016,32,7346−7355.(26)Chai,L.;Klein,J.Largearea,molecularlysmooth(0.2nmrms)■REFERENCESgoldfilmsforsurfaceforcesandotherstudies.Langmuir2007,23,(1)Hutchings,I.;Shipway,P.Tribology:FrictionandWearof7777−7783.EngineeringMaterials;Butterworth-Heinemann,2017.(27)Clarkson,M.T.Multiple-beaminterferometrywiththinmetal(2)Kim,S.H.;Asay,D.B.;Dugger,M.T.Nanotribologyandfilmsandunsymmetricalsystems.J.Phys.D:Appl.Phys.1989,22,MEMS.NanoToday2007,2,22−29.475−482.(3)Wu,Y.;Wei,Q.;Cai,M.;Zhou,F.Interfacialfrictioncontrol.(28)Chan,D.Y.;Horn,R.ThedrainageofthinliquidfilmsbetweenAdv.Mater.Interfaces2015,2,No.1400392.solidsurfaces.J.Chem.Phys.1985,83,5311−5324.(4)Singer,I.Frictionandenergydissipationattheatomicscale:A(29)Tivony,R.;Safran,S.;Pincus,P.;Silbert,G.;Klein,J.Chargingreview.J.Vac.Sci.Technol.,A1994,12,2605−2616.dynamicsofanindividualnanopore.Nat.Commun.2018,9,(5)Mendes,P.M.Stimuli-responsivesurfacesforbio-applications.No.4203.Chem.Soc.Rev.2008,37,2512−2529.(30)Tivony,R.;Klein,J.Modifyingsurfaceforcesthroughcontrol(6)Sweeney,J.;Hausen,F.;Hayes,R.;Webber,G.B.;Endres,F.;ofsurfacepotentials.FaradayDiscuss.2017,199,261−277.Rutland,M.W.;Bennewitz,R.;Atkin,R.Controlofnanoscalefriction(31)Fréchette,J.;Vanderlick,T.K.Electrocapillaryatcontact:ongoldinanionicliquidbyapotential-dependentioniclubricantPotential-dependentadhesionbetweenagoldelectrodeandamicalayer.Phys.Rev.Lett.2012,109,No.155502.surface.Langmuir2005,21,985−991.(7)Drummond,C.Electric-field-inducedfrictionreductionand(32)Johnson,K.;Kendall,K.;Roberts,A.Surfaceenergyandthecontrol.Phys.Rev.Lett.2012,109,No.154302.contactofelasticsolids.Proc.R.Soc.London,Ser.B1971,324,301−(8)Pranzetti,A.;Mieszkin,S.;Iqbal,P.;Rawson,F.J.;Callow,M.E.;Callow,J.A.;Koelsch,P.;Preece,J.A.;Mendes,P.M.An313.electricallyreversibleswitchablesurfacetocontrolandstudye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6TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticle(36)Nightingale,E.,JrPhenomenologicaltheoryofionsolvation.Effectiveradiiofhydratedions.J.Phys.Chem.B1959,63,1381−1387.(37)DeLevie,R.Onporouselectrodesinelectrolytesolutions:I.Capacitanceeffects.Electrochim.Acta1963,8,751−780.(38)Biesheuvel,P.;Bazant,M.Nonlineardynamicsofcapacitivecharginganddesalinationbyporouselectrodes.Phys.Rev.E2010,81,No.031502.3622https://dx.doi.org/10.1021/acs.jpcc.0c11264J.Phys.Chem.C2021,125,3616−3622