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JournalofMembraneScience226(2003)227–236Determiningtheζ-potentialofplanemembranesfromtangentialstreamingpotentialmeasurements:effectofthemembranebodyconductanceP.Fievet∗,M.Sba¨ı,A.Szymczyk,A.VidonneLaboratoiredeChimiedesMatériauxetInterfaces,16RoutedeGray,25030BesançonCedex,FranceReceived28April2003;receivedinrevisedform5September2003;accepted11September2003AbstractTheζ-potentialisanimportantandreliableindicatorofthesurfacechargeofmembranesanditsknowledgeisessentialforthedesignandoperationofmembraneprocesses.Thestreamingpotentialremainsthemostwidelyusedtechniquefordeterminingtheζ-potentialofmembranes.Inthecaseofasymmetricorcompositemembranes,theζ-potentialisnowfrequentlydeterminedfromtangentialflowstreamingpotentialmeasurements.However,theinterpretationofdataintermsofζ-potentialisusuallycarriedoutbydoingtheimplicitassumptionofnon-conductingsubstrates.Inthispaper,weinvestigatetheelectrokineticpropertiesofaceramicmembraneclosetotheNFrange,theporousstructureofwhichaffectsthestreamingpotentialbecausestreamingandconductioncurrentsinvolvedinthestreamingpotentialprocessdonotflowthroughidenticalpaths(thestreamingcurrentflowsonlythroughtheslitchannelformedbythetwomembranesamplesfacingeachotherwhereasanon-negligiblepartoftheconductioncurrentislikelytoflowthroughthemembraneporesfilledwithelectrolytesolution).Inthiscase,theso-calledFairbrotherandMastin(F–M)procedureaccountingforthesurfaceconductancetocorrecttheHelmholtz–Smoluchowski(H–S)relationisnotsuitableandleadstoanerroneousζ-potential.Thecorrectζ-potentialvalueisdeterminedfromanextrapolationmethodforwhichasetofmeasurementswithvariouschannelheightsisrequired.Averygoodagreementisobtainedwithζ-potentialvaluesdeducedfromconsecutivestreamingpotentialandtotalconductancemeasurements.Thecontributionofthemembranebodytothetotalconductivityofthemembrane/channel/membranesystemappearstobeindependentofthepHofsolution,probablyduetothefactthattheelectricconductiontakesplacethroughthemembraneporousbodyinsideathicklayerdominatedbythesupportlayerwhichthenenforcesitsconductivity.©2003ElsevierB.V.Allrightsreserved.Keywords:Zetapotential;Tangentialstreamingpotential;Electricconductivity;Membranebodyconductance1.IntroductionMostmembranesacquireanelectricsurfacechargewhenbroughtintocontactwithanaqueoussolution.Itisnowwellrecognisedthatchargeorelectricalpoten-tialpropertiesofmembraneshaveaverysubstantial∗Correspondingauthor.Fax:+33-3-81-66-20-33.E-mailaddress:patrick.fievet@univ-fcomte.fr(P.Fievet).influenceontheirfiltrationperformances.Thatisthereasonforwhichthereismuchinterestincharacter-isingtheseproperties.Theelectricpotentialatthesolidsurfacecannotbemeasureddirectly;however,theelectricalpotentialatthehydrodynamicplaneofshear,calledthezeta(ζ)potential,canbedeterminedfromstreamingpotentialmeasurement.Streamingpotentialoccurswhenthereisarelativemotionbetweenafluidcontainingcharged0376-7388/$–seefrontmatter©2003ElsevierB.V.Allrightsreserved.doi:10.1016/j.memsci.2003.09.007228P.Fievetetal./JournalofMembraneScience226(2003)227–236speciesandachargedsurfacecausedbyanhydrostaticpressuregradient.Althoughtheζ-potentialissome-whatdifferentfromthesurfacepotential,itisanim-portantandreliableindicatorofthemembranesurfacechargethatinteractswithitssurroundings.Streamingpotentialmeasurementscanbeper-formedintwodifferentways:byflowthroughthemembranepores(transmembranestreamingpotential)[1–10]orbyflowacrossthetopsurfaceofthemem-brane(tangentialstreamingpotential,TSP)[10–16].Thefirstmethodhastheadvantageofexperimentalsimplicitybutinterpretationofexperimentaldatamaybedifficultwhenmeasurementsareperformedwithmultilayermembranes(supportlayer(s)+skinlayer)[17–19]and/ormembraneshavingselectivelayers[20–22].Insuchcases,TSPmeasurements(alongthesurfacemembrane,ontheactivelayerside)appearasanalternativemethodprovidingdirectinformationaboutthemembraneskinlayer(i.e.theactivelayer).TheTSPmethodconsistsofapplyingapressuredif-ferenceacrossasmallchannelformedbyclampingtogethertwoidenticalflatmembranesseparatedbyaspacer.Severalmade-labclampingcellsarepresentedintheliterature.Amongtheseones,theadvancedflatplatestreamingdevicedevelopedbyWerneretal.[23]hastheadvantagetoallowadjustmentofthechannelheightbymeansofapiezo-drivenpositioningunitwithoutde-wettingofthesamples.Recently,anovelasymmetricclampingcellwaspresentedbyWalkeretal.[24].ThedesignofthiscellallowsmeasurementofTSPofanyflatsurfaces,eithersolidsorflexiblesheetswithouthavingtocutorshapethetestsurfacestofitthemeasuringcelldimensions.Also,commer-cialelectrokineticanalysersareroutinelycapableofprovidingthestreamingpotentialofflatsheets(asFig.1.Rectangularslitchannelfortangentialstreamingpotentialmeasurements.wellasgranularorfibrousmaterials).Inadditiontotheζ-potential,someofthesedevices(commercialornot)allowtodeterminethecellconductance.Currently,mostofusersofelectrokineticanalysersuseeitherthewell-knownHelmholtz–Smoluchowski(H–S)formulaoraversionofthisformulawhichin-cludesthesurfaceconductance[13,14,25].Veryrecently,YaroshchukandRibitsch[26]havepointedoutthatforcertainconductingmaterials