Streamingpotentialasatoolinthecharacterizationoful

ColloidsandSurfaces,36(1989)297-312297ElsevierSciencePublishersB.V.,Amsterdam--PrintedinTheNetherlandsStreamingPotentialasaToolintheCharacterizationofUltrafiltrationMembranesMARIANNENYSTR(3M1,MATTILINDSTR(~M1andEINARMATTHIASSON2~LaboratoryofPhysicalChemistry,DepartmentofChemicalTechnology,LappeenrantaUniversityofTechnology,P.O.Box20,SF-53851Lappeenranta(Finland)2DepartmentofFoodTechnology,TechnologicalInstituteofIceland,Keldnaholt,112Reykjavik(Iceland)(Received28July1988;accepted4January1989)ABSTRACTAnapparatusforthemeasurementofstreamingpotentialversuspressurerelationshipsofultra-filtrationmembranesispresented.ResultsforthreetypesofmembranesofvariousporesizesatdifferentpH,andionicstrengthsarereported.Thedatahavebeenusedtocalculateapparentvaluesforzetapotentialsofthemembranesandthepossibilitytocomparethesevaluestothetruezetapotentialsisdiscussed.Theinfluenceofmodification,thepresenceofsmallchargedionsandchargedpolyelectrolytesontheapparentzetapotentialsofthemembraneshasbeenmeasured.Finallytheeffectsofthesurfacechargesofthemembranes,characterizedbytheirapparentzetapotentials,areillustratedbyultrafiltrationexperimentsperformedwithchloroligninandwheyprotein.INTRODUCTIONInultrafiltration(UF)thepermeatefluxdeclinestronglydependsontheadsorptionofsolutecomponentsonthemembranesurface.Thisphenomenonoftencalledfoulingisusuallycausedbylargemoleculessuchasproteins[1,2]andpromotedbysmallonessuchasCa2+ions[3].Foulingismostoftenanundesirablephenomenonasitdecreasesthefluxofpermeatethroughthemembrane[4].However,sometimesfoulingmaynotbeinconvenient,iftheadsorbingcomponentonlybuildsupathinmodifyinglayeronthesurfaceofthemembrane.Thislayermaygivethemembranesuchfavorablepropertiesasbetterretention,raisedhydrophilicityoramoresuitablesurfacechargeden-sity[5].AnimportantfactorinUFisthestrengthanddirectionoftheinter-actionforcesbetweenthemacromoleculesinsolutionandthesurfacelayerofthemembrane.Theelectrostaticcomponentoftheseforcesstronglydependsonthechargedensityofthemembranesurfaceanditspores,whichisimpor-tanttoknowinordertopredicttheamountoffoulingcausedbyadsorption0166-6622/89/$03.50©1989ElsevierSciencePublishersB.V.298[6].Surfacechargedensityisrelatedtothezetapotentialofthemembrane,whichinprinciplecanbecalculatedfromstreamingpotentialdata.ThepurposeofthispaperistopresenttheconstructionofaninexpensiveapparatusforstreamingpotentialmeasurementsofUFmembranesequippedwithall-electronicsensorsanalog-to-digitalconversionofsignalsanddigitalsignalprocessingimplementedaroundanordinaryhomecomputer,andtogiveandinterpretthestreamingpotentialdataobtainedforanumberofcommer-cialUFmembranes.ThemeasurementswerecarriedoutbothwithvirginandfouledmembranesatdifferentpHandionicstrengths.THEORYWhenelectrolytesolutionisforcedtoflowthroughacapillaryorporousplugbymeansofhydraulicpressure,astreamingpotential,JE,acrosstheflowpathisgenerated.Therelationbetweenthismeasurablequantityandthezetapo-tential,5,whichismostoftenusedtocharacterizethechargedensityofthesteadyinterfaciallayers,isgivenforacapillarybythewell-knownHelmholtz-Smoluchowskiequation[7].ZIE~o~r~(1)AP-~/KInEqn(1)ZIPisthehydraulicpressureacrossthecapillary,eothepermit-tivityofvacuum,~rtherelativedielectricconstantofthesolvent,~]theviscos-ityandKtheconductivityofthesolution.Equation(1)canbeappliedonlyinanidealcase.Infact,agreatnumberofphenomenainterfereintheformationofthemeasuredstreamingpotential.TheinfluenceofsurfaceconductivityofthecapillarywasfirststudiedbyStock[8],whointroducedthecorrectiontermgiveninEqn(2)whereAsisthesur-faceconductivity,andRtheradiusofthecapillary.AE~o~r~(2)ziP-rl(K+22~/R)LaterRiceandWhitehead[9]suggested,thatatleastfornarrowcapillaries,acorrectioninEqn(1)shouldbemadewhichtakesintoaccountthedoublelayerthickness,rDH,ontheinterface.AsaresultofsolvingtheappropriatePoisson-BoltzmannequationtheyobtainedZIEeo~r~F(R/rDH)(3)AP-~]/(capInEqn(3)KcapistheconductivityoftheliquidinthecapillaryandthecorrectionfunctionFapproachesthevalueiatthelimitR/rDH~co.ThepropercorrectionofEqn(1)shouldalsodependonthefixedchargeonthecapillary299wall.However,RiceandWhiteheaddidnotconsiderthisfixedchargeorthesurfaceconductivity,intheirderivationofEqn(3).LaterOldhametal.[10]suggestedanumericalsolutionofthesameproblem,whichalsotakesintoaccountthesephenomena.TheproblemhassubsequentlybeenthoroughlystudiedbyLevineetal.[11]andAndersonandKoh[12].InadditionHildreth[13]haspointedouttheimportanceofthepresenceofadistributedconductanceacrossthecapillarychannelandthedifferenceintheionicmobilitiesoftheco-ionsandthecounter-ionsofthefixedcharge.Reportsonstreamingpotentialmeasurementsofmembranesarenotabun-dant.Someworkhasbeendonewithmicrofiltration(MF)membranes[14,15]andalsowithreverseosmosis(RO)membranes[16-18].InthepaperofBrozandEpstein[14]thezetapotentialswerealsocalculatedandcorrectedac-cordingtoRiceandWhitehead,buttheresultssufferfromdifficultiescausedbyretardationeffectsduetohighappliedpressures.TheauthorsoftheotheravailablereportsonMFandROdonottrytocalculatezetapotentialsorbasethecalculationsonthemostsimpleequation,Eq