Adsorption of organic acids on magnetite nanoparticles, pH-dependent colloidal stability and salt tolerance

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ColloidsandSurfacesA:Physicochem.Eng.Aspectsxxx (2013) xxx–xxx

ContentslistsavailableatSciVerseScienceDirect

Colloids and Surfaces A: Physicochemical and Engineering Aspects

j ou rn a l h o m e pag e :w w w . e l s e v i e r . c o m / l o c a t e / c o l s u r f a

Adsorption of organic acids on magnetite nanoparticles, pH-dependent colloidal stability and salt tolerance

E. Tombácz

^a,∗

, I.Y. Tóth

^a

, D. Nesztor

^a

, E. Illés

^a

, A. Hajdú

^a

, M. Szekeres

^a,∗

, L.Vékás

^b

aDepartmentofPhysicalChemistryandMaterialsScience,UniversityofSzeged,Hungary

bCenterofFundamentalandAdvancedTechnicalResearch,RA-TD,Timisoara,Romania

h i g h l i g h t s

Organic acids either stabilize or destabilize oxide nanoparticles in naturalwaters.

The stabilizing/destabilizing effect dependsonpH,salinityandorganic concentration.

Speciﬁcconﬁgurationof carboxylic groupsisnecessarytosurfacecom- plexation.

Surfacecomplexationleadstohigh- afﬁnityadsorptionisotherms.

Higher molecular weight organic acids provide better stability than smallerones.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Articlehistory:

Received1October2012

Receivedinrevisedform15January2013 Accepted18January2013

Available online xxx

Keywords:

Carboxylatedmagnetitenanoparticles Smallandmacromolecularorganic polyacids

Adsorption

Nanoparticlestabilization Overcharging

a b s t r a c t

TheadsorptionofdifferentorganicacidsandtheirinfluenceonthepH-dependentcharging,salttol- eranceandsothecolloidalstabilityofmagnetitenanoparticlesarecompared.Adsorptionisothermsof citricacid–CA,gallicacid–GA,poly(acrylicacid)–PAA,poly(acrylic-co-maleicacid)–PAMandhumic acid–HAweremeasured.ThepH-dependentchargestateofMNPswascharacterizedbyelectrophoretic mobilityandtheiraggregationbydynamiclightscattering.Thesalttolerancewastestedincoagulation kineticexperiments.Althoughtheadsorptioncapacities,thetypeofbonding(eitherH-bondsormetal ion-carboxylatecomplexes)andsothebondstrengthsaresignificantlydifferent,thefollowinggeneral trendshavebeenfound.SmallamountoforganicacidsatpH<∼8(thepHofPZCofmagnetite)–relevant conditioninnaturalwaters–onlyneutralizesthepositivecharges,andsopromotestheaggregationand sedimentationofnanoparticles.Greateramountsoforganicacid,abovethechargeneutralization,cause thesignreversalofparticlecharge,andathighoverchargingpromotestabilizationanddispersing.The thickerlayerofPAA,PAMandHAprovidesbetterelectrostericstabilitythanCAandGA.GAundergoes surfacepolymerization,therebyimprovingstabilization.Theorganicacidsstudiedhereeliminatecom- pletelythepHsensitivityofamphotericmagnetite,butonlythepolyanioniccoverageprovidessignificant increaseinresistanceagainstcoagulatingeffectsofsaltsatneutralpHcommonlyprevailinginnatural waters.

∗Correspondingauthorsat:DepartmentofPhysicalChemistryandMaterialsSci- ence,UniversityofSzeged,Aradivt.1,H-6720Szeged,Hungary.Tel.:+3662544212;

fax:+3662544042.

E-mailaddresses:tombacz@chem.u-szeged.hu(E.Tombácz), szekeres@chem.u-szeged.hu(M.Szekeres).

1. Introduction

Inaqueousmedium,thecolloidalstabilityofdispersed mag- netitenanoparticlesasan exampleof environmentallyrelevant metaloxidesdependssensitivelyonnotonlythepH,butalsothe amountoforganicacidsoccurringmainlyinsurfacewaters.These 0927-7757/$–seefrontmatter© 2013 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.colsurfa.2013.01.023

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2 E.Tombáczetal./ColloidsandSurfacesA:Physicochem.Eng.Aspectsxxx (2013) xxx–xxx

organiccompoundscanmodifythesurfacechargepropertiesof magnetiteentirelyorpartiallydependingontheirchemistryand amountsadsorbed.

Themacromolecularhumicacids(HA)areanimportantfraction ofthenaturalorganicmatter(NOM)[1].Thesyntheticpolyacrylic acid(PAA)isoftenstudiedasHAanaloguemacromolecularcom- pound.Theycontainmainlycarboxylicacidicgroups,similarlyto thecitricacid(CA).Thenaturalhumicmacromoleculeshowever, have aromatic ringsbesides aliphaticparts of carbon skeleton, andsophenolicgroupshavetobealsoconsideredamongacidic functions[2].Gallicacid(GA)isoneofthecommonaromaticcom- poundshavingboth carboxylicandphenolic groups.It isfound frequentlyintheesterbondsoccurringinseveralnaturalantioxi- dantssuchasflavonoidsandgreentea,orasafreeacidreleased intotheenvironment. GA isa labile compound, it polyconden- satesspontaneouslyunderneutralpHconditions,especiallyinthe presence ofmineral particles [3].HA is ableto formappropri- atestabilizinglayeronmetaloxideparticles due toitsspecific affinitytometalions andpolyionic character[4,5].Besidesour severalyears’experiencewithaqueoussolutionsofhumicacids, theirinteractionwithmagnetite(Fe3O4,magneticironoxide)has beenalsostudied[6,7].TheCA,PAAandPAMinteractionswith magnetitenanoparticleshavebeenrecentlyinvestigated[8,9,16].

Besidestheirenvironmentalrelevance,theseareimportantfrom biomedicalapplicationpointofview.Althoughmagnetitenanopar- ticles can be easily prepared by co-precipitation of Fe(II) and Fe(III) salts in an alkaline solution, different coating layers on thesurfaceofparticleshavetobedevelopedtopreventparticle aggregationandtoimprovetheircolloidalandchemicalstability [10].Surfactantsareoftenusedtodispersenanoparticlesentirely inan appropriatemedium. Coatingof single-domainmagnetite nanoparticles(typical sizeof about10nm) withadouble layer ofsurfactants in anaqueousmedium resultsin stablecolloidal dispersions[11].Thecoverageofparticleswithadsorptionlayer providesenhancedresistanceagainsttheparticleaggregation.In aqueousmedium,electrostatic,stericorcombined(i.e.,electros- teric)stabilizationlayerscandevelop[5,12,13].Thethickercoating providesbetterstability[14].Magnetitenanoparticleswerestabi- lizedwithCAasawell-knowncomplexantof Fe OHsurfacesites [8,15],naturalpolycarboxylicacidHA [6,7,8]andartiﬁcialpoly- merspolyacrylicacid(PAA)andpoly(acrylic-co-maleic)acid(PAM) [9,16].

InthisworkweshowhowthepHsensitivityofamphotericmag- netitecanbeeliminatedandasignificantincreaseintheresistance againstsalt canbereachedbycoating ironoxidenanoparticles withtheabove polyanionicacids. Thebinding of thepolyacids tomagnetitesurfacewasstudiedinadsorptionexperimentsand thechargingandaggregationoftheparticlesinelectrokineticand dynamiclightscatteringmeasurements.Wecomparedtheeffectof theadsorptionofthedifferentorganicacidsonthepH-dependent charging,salttoleranceandsothecolloidalstabilityofmagnetite nanoparticles.Someoftheresultsusedherecanbefoundinourear- lierpublications[6–9,16],togetherwiththedetaileddiscussionson themechanismsofadsorption.Therefore,wedonotintendtogo intothosedetails,butweuseourearlierconclusionstosupport thefindingsofthepresentcomparativestudyonthestabilizing efficienciesofsmallandlargemolecularorganicacids.

2. Experimental

2.1. Materials

Syntheticmagnetite(Fe₃O₄)waspreparedbyalkalinehydroly- sisofiron(II)-andiron(III)-salts.Themethodwasusedtoprepare superparamagneticmagnetitewithparticlesizebelow10nm.The

detailsofpreparationandthecharacterizationofmagnetiteitself canbefoundinthepaperspublishedbefore[7,17,18,19].

Reagent grade citric acid(CA) and gallic acid(GA), and the polyelectrolytes polyacrylic acid (PAA, Mw=1800DA) and poly (acrylic-co-maleic)acid(PAM,Mw=3000DA,50wt.%inH2O)were purchasedfromSigma–Aldrich.

Humicacid(HA)wasobtainedfrombrowncoal (Tatabánya, Hungary)byatraditionalalkalineextractionprocedureusing0.1M NaOHsolution.TheashcontentofrawHAwasreducedbyHF/HCl treatmentbelow 1%.The dried, ground HA wasextracted with benzene/ethanol in a Soxhlet apparatusfor 72hto remove tar components.Na–humate solutionwaspreparedfrom thedried HAsampledissolvedinacalculatedamountofNaOHequivalent tothetotal acidityof HA measuredbypotentiometric titration [6].Theamountofhumicacidsinmolescannotbegiven,because themolecularweightofthesenaturalmaterialsisundeﬁneddue totheirpolydisperseand fractalnature[1].Because mainlythe acidicfunctionalgroups(carboxylandphenolicOH)takepartin thecomplexationreactionsandadsorptionprocesses,itisstraight- forwardtoexpresstheamountofHAinrelationtotheamount ofthesegroups.Thewholeamountoftheacidic groupsrelated totheunitmassofHA(i.e.,thetotalacidityofthesample)was 3.5mmol/g,whichwasusedtogivetheconcentrationofHAsolu- tionsinmmol/Lunit.Theamountoftheothertwopolyelectrolytes PAAand PAM(Mw3000Da forboth)wasrelated tothenum- berofcarboxylicgroupsinthemonomerunits: COOH/AA=1and COOH/AM=3.Themolarweightsofthemonomersare72(AA) and 188(AM)g/mol. We did notuse theamountof carboxylic groupstoexpressconcentrationinthecaseofsmallmoleculesCA andGA,becausetheirmolecularweightisexactanditallowstoget aclearmolecularpictureoftheinteractions.

NaCl,HCl and NaOH,usedtosetthe pHand ionicstrength, wereanalyticalgradeproductsofReanal(Hungary).Milli-Qwater wasused.Allexperimentswereperformedatroomtemperature (25±1^◦C).

2.2. Methods 2.2.1. Adsorption

TheadsorptionisothermsofthepolyacidsatpH6andcon- stantsaltconcentrationof0.01MNaClweredeterminedbybatch method.Themagnetitesuspensions(1–20g/L)wereequilibrated withtheseriesofpolyacidsolutionsupto10mmol/Lconcentra- tionin closed testtubes for 24hat roomtemperature.ThepH wasadjustedto6.5±0.1byaddingsmallportionsofeitherNaOH orHClsolutionsandcheckedafteradsorptiontimefor24h.The equilibrium concentrations were determined by measuringthe absorbanceofsupernatantsat260nm(GA),450nm(HA)orthe differentialabsorbanceat223and250nm(PAAandPAM)inan USB4000spectrometer(OceanOptics)afterperfectseparationof thesolidparticlesbycentrifugingat13000rpmfor1h.Athigher polyacidconcentrationstheseparationwasassistedbyaperma- nentmagnetandmembraneﬁltration(0.22␮mMILLEX-GP).The equilibrium concentration of CAwas determined bycerimetric titrationusingferroinindicator[15].

2.2.2. Electrophoreticmobilitymeasurement

Electrophoreticmobilitiesofthepure(naked)andthepolyacid coatedmagnetitesamplesweremeasuredat25±0.1^◦Cinadispos- ablezetacell(DTS1060)ofNanoZS(Malvern,UK)apparatus.The settingsoftheinstrumentwerecheckedbymeasuringastandard latexsamplewiththezetapotentialof55±5mV.Themeasure- mentswereperformedunderoptimalscatteringcondition(10⁵ countspersecond)applyingeither0.05or0.1g/Lmagnetitecon- tentdependingontheaggregationstateofthedispersions.The rangeofpHwasbetween3and10.Themeasurementswere

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E.Tombáczetal./ColloidsandSurfacesA:Physicochem.Eng.Aspectsxxx (2013) xxx–xxx 3

startedafteronehourequilibrationtime.Inoneseriesofexperi- mentstheeffectoftheaddedamountsofpolyacids(expressedas theamountsofacidicgroupsforHA,PAAandPAM)upto0.6mmol relatedto1gmagnetitewasmeasuredatpH6.Then, thepH- dependencewasinvestigatedinthepresenceofvariousselected amountsofpolyacidsrangingfrom0.05to1.8mmol/g.Theexper- imentswereperformedatconstantionicstrengths0.005M(CA), 0.002M(HA)and0.01M(GA,PAAandPAM)setbyNaCl.

2.2.3. Particlesizing–dynamiclightscattering(DLS)

Measurements were performed using a NanoZS apparatus (Malvern, UK) with a He-Ne laser (=633nm), operating in backscatteringmodeatangle173^◦.Thestocksolofmagnetitepar- ticles wasdilutedwithNaCl electrolyteto achieve 0.1g/L solid content.ThepHofthesystemswasadjustedintherangeof3–10, measureddirectlybeforeplacingthesampleinthemeasuringcell.

ThepH-dependentparticleaggregationwasmeasuredatconstant ionicstrength,0.005M(CA),0.002M(HA)and0.01M(GA,PAAand PAM),setbyNaCl.Thestabilizingeffectoftheadsorptionofpoly- acidswasinvestigatedatdifferentaddedamountsofthemsimilarly tothatintheelectrophoreticmobilitymeasurements.Allmeasure- mentswereperformedatagivenkineticstateachievedby10sof ultrasonicationfollowedby110sofrelaxation.Theaverageval- uesofthehydrodynamicdiameterwerecalculatedfrom3rdorder cumulantﬁtsofthecorrelationfunctions.

2.2.4. Coagulationkineticmeasurement

Thesalt tolerance ofstabilized magnetite nanoparticleswas tested in coagulation kinetic measurements by using Zetasizer 4 (Malvern, UK) apparatus. NaCl concentration was changed graduallyfrom0mMto1000mMatpH6.Themagnetitesolcon- centrationtoachieveoptimumscatteringanddiffusionconditions was0.0025g/L.TheDLSmethodwasusedtofollowthesizeevolu- tionofaggregatesintime.Thecoagulationratewascalculatedfrom theslopeofkineticcurvesasexplainedbefore[7,12].Thestability ratio(W)wascalculatedfromtheinitialslopesofkineticcurves belongingtotheslowandfastcoagulationassuggestedinlitera- ture[20,21].Toensuretheonsetoffastcoagulationregime,atleast threedifferent,instantlycoagulatingconcentrationsofNaClwere applied.Thecriticalcoagulationconcentration(CCC)wasdeter- minedfrom thelog₁₀W versuslog₁₀c_NaCl (NaCl concentration) function.Ina typicalexperiment,changes inthehydrodynamic diameter(Z-averagevalues,Z_ave)weremonitoredforanhourwith atimeresolutionof2min.

3. Resultsanddiscussion

3.1. Adsorptionofcarboxylicacidsonmagnetitenanoparticles Theadsorptionisothermsofthedifferentlowandhighmolecu- larweightcarboxylicacidsareseeninFig.1.Theadsorbedamounts representmmolesofCAorGApergofMNP,andmmolesof COOH groupspergofMNPforthepolyelectrolytes,andthusonlythefea- tureoftheisothermsisdirectlycomparable.Alltheisotherms,with theexceptionofPAA,areofH-type,meaningthattheadsorption isofhighafﬁnity.Thisisexplained[22]byeitherthecooperative interactionsoccurringbetweenthemanyfunctionalgroupsofthe macromoleculesandthesurfacesitesoftheMNPsortheintrin- sichighafﬁnityoftheindividualcarboxylgroupstothesurface sites.Wehaveproventheformationofdirectmetal–carboxylate surfacecomplexesinthecaseofCA,PAMandHA[6,8,15,16],and onlyH-bondinginthecase ofPAA [9].Theresultssuggestthat surfaceFe–carboxylatecomplexbondscanformwhen thegeo- metricarrangementoftheneighboringcarboxylgroupsmatches thedistancebetweensurface Fe–OHsites.Wehavefoundthat carboxylicgroupsbelongingtoneighboring carbonatomsinCA

0 0.2 0.4 0.6 0.8 1

0 1 2 3 4

Amount adsorbed, mmol/g

Equilibrium concentration, mmol/L pH ~ 6 -6.5

0.01 M NaCl

CA HA

PAA

GA PAM

0 0.4 0.8

0 0.05 0.1 0.15

HA

Fig.1.Adsorptionisothermsofcarboxylicacidsonmagnetitenanoparticlesmea- suredatpH6–6.5and0.01Mionicstrength.TheenlargementoftheHAisotherm isseenintheinset.(Theamountofthemacromolecularpolyacids(HA,PAAand PAM)wasrelatedtothemolesofacidicgroups.Thelinesaredrawntoguidethe eyes.)

andinthecarbonbackboneofPAMandHAcantakepartinsuch interaction.TheneighboringcarboxylatesinPAAbelongtoevery secondCatomofthebackboneofpolyacidchain,ageometrically unfavorableconditionforFe–carboxylateformation.

InthecaseofGA,thehighadsorptionafﬁnitycanresultfrom

␲–electroninteractionswiththepolarsurfaceoftheMNPs,aswell asfromcomplexbondformationat Fe–OHsiteswiththepartici- pationoftwoneighboringphenolicOHgroupsofGA[23].Itshould benotedthatalthoughtheshapeofthePAA,CAandGAisotherms maylooksomewhatsimilaratlowequilibriumconcentrations,the high-affinitypartisdefinitelyabsentfromthePAAisotherm.The latterisaclearindicationthatthemechanismofPAAadsorptionis differentfromthatofCAandGA.TheadsorptionofPAA,PAMand HAreachesdefiniteplateauregionattheadsorbedamountsof0.6 [9],0.9[16]and0.85mmol/g,respectively.Thefullisothermsof PAAandPAMadsorption(uptotheirequilibriumconcentrationof 8and7mmol/L,respectively)areseeninRefs.[9]and[16].On thecontrary,oncethehigh-affinityadsorptionlimit(0.1mmol/g) hasbeenexceeded,theadsorptionofbothCAandGAincreases linearlywithoutlevelingoffataplateauvalue.Thelinearincrease intheadsorbedamountsisprobablyconnectedwiththepolymer- izationofthemoleculesintheadsorptionlayer.Itiswellknown thatGApolymerizeseasilyinsolution[24].Afteritsadsorption,the polymerizationcontinueswithanevengreaterrateatthesurface aswell[3].RegardingthelinearpartoftheCAisotherm,ourpre- liminarystudiesclearlyindicatetheappearanceofC Ovibrations intheFTIRspectrabelongingtoestergroups;theresultsbeingpub- lishedinaforthcomingpaperontheadsorptionmechanismofCA andGA.

3.2. Theeffectofdifferentorganicacidsonparticlechargeof magnetite

The addition of carboxylic acids to the MNP dispersions at pH6.5andI=0.01Mhadapronouncedeffectontheelectrokinetic potentialoftheparticles,asseeninFig.2.Duringtheadsorption, thepolyacidsCA,HA,PAMandPAAtakenegativechargestothe surfaceinexcessofthatnecessarytoneutralizetheoriginalpos- itivechargesofthemagnetiteatthegivenpHandionicstrength.

Theamountofcarboxylicacidsatthepointofchargeneutralization (thezerovalueoftheelectrokineticpotential,akindofisoelectric

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Fig.2. Effectoftheadditionofcarboxylicacidsontheelectrokineticpotentialofthe MNPs,measuredatpH6–6.5,andatI=0.01M.(Theamountofthemacromolecular polyacids(HA,PAAandPAM)wasrelatedtothemolesoftheiracidicgroups.The linesaredrawntoguidetheeyes.)

point– IEP)isnearlythesameforallthefourmacromolecularpoly- acids;thesmalldeviationsarewithintheinaccuracyrangeofthe measurements(±5mV,Section2.2.2).Accordingtoourconception, theseamountsaretheactualmolesof COO⁻groupslinkingthe polyacidstothesurfacesites.Furtheradsorptionofthepolyions inexcessofsurfacechargeneutralizationcauseschargereversalof particles.AdditionofGA,amonocarboxylicacid,shouldnotinduce chargereversal,ifthecarboxylicgroupbecomescoordinatedor electrostaticallyattachedtothesurface.Thechangesintheelec- trokineticpotentialofMNPswithincreasingGAadsorptionarevery similartothatofthepolyacids,meaningthatGAadsorptionneu- tralizesandoverchargestheMNPs.Thissinglefactshowsthatthe carboxylicgroupsofGAarenotinvolveddirectlyinGAbonding toMNPs,whichisinlinewiththeﬁndingsinRef.[23].Theelec- trokineticpotentialoftheGA-coatedMNPschangeswithtime,as itispresentedinFig.3.Thisuniquebehaviorindicatesthatsurface polymerizationofGAproceeds,leadingtotheincreasingthickness ofthecoatingshellandthedecreasingabsolutevaluesofelectroki- neticpotential.Wewillgiveadetailedanalysisoftheadsorption, surfacepolymerizationandstabilizingeffectofGAonMNPsina forthcomingpublication.

We have alsostudied thestability of thedispersions in the functionof the added amountof the carboxylic acids bymea- suring thesize of the primary particles and aggregates in DLS

Fig.3.Time-dependenceoftheelectrokineticpotentialofGA-coatedMNPs.(The linesaredrawntoguidetheeyes.)

experiments.The resultsshowthatMNPs aggregateat pH6.5 and0.01Mionicstrengthandthehydrodynamicdiameterofthe aggregatesis300nm.Thesizeoftheaggregatesincreaseswith additionofsmallamounts(0.1–0.2mmol/g)ofcarboxylicacidsup tod1000orevento2000nm.Largeramountsofeachpolyacid decreasethehydrodynamicdiameterdowntothesizeofthepri- maryMNPs(around100nm)measuredfortheuncoatedparticlesat pH<5,wellbelowtheIEPofnakedMNPs(pH8[8]).Thus,allcar- boxylatedcoatingscanstabilizetheindividualMNPsatthemost commonpHsgenerallyprevailinginenvironmentalwaters.

3.3. Theeffectofsmallandmacromolecularorganicacidcoating onthepH-dependentchargestateandaggregationofmagnetite

Wehaveexaminedthestabilizingeffectofthedifferentcar- boxylicacidsintermsofthebreadthofthepHrange,inwhichthe coatednanoparticlesaredispersedindividuallyinacolloidallysta- blestate.AsitisseenontheleftsideinFig.4,theIEPofMNPsshifts frompH8tolowerpHvaluesuponadditionof0.1mmol/gofall carboxylicacids.Attheirhigheraddedamounts(1.2–1.8mmol/g, rightsideinFig.4)theelectrokineticchargeofthecoatedparticles wasprincipallynegativeinthewholerangeofpHstudiedhere.In thepresenceofthesmallmoleculesCAandGA,theIEPshiftedto pH3,andsothepH-rangeoftheirstabilityisnarrowerthanthatof thepolyacids.Thesizeoftheparticles(individualandaggregated) wasmeasuredinDLSexperimentsinparallelwiththeelectroki- neticpotentialmeasurements,tosupportthataggregationoccurs neartheIEPs.

ThepH-rangesofaggregationareshowninTable1togetherwith theIEPvalues.TheresultsshowthatthepH-dependentstability shiftsinparallelwithIEPineachcase,thedifferencesareonlyin theamountsthatcancompletelymasktheoriginalamphotericfea- tureofmagnetite.Inaddition,smalldeviationhasbeenfoundin theaveragehydrodynamicsizesofparticlescoveredbydifferent organicacidsduetothedifferenceinthestructureandthicknessof theadsorbedlayers.Highmolecularweightpolyelectrolytesgen- erallyledtolargervaluesofhydrodynamicdiameterthansmaller molecules,forexample,150nmforPAAandPAM[16,19],ascom- paredto100nmforCAandGAstabilizedsystems,measuredin dynamiclightscatteringexperiments.

3.4. Salttoleranceofdifferentcarboxylatedmagnetite nanoparticles

SalttoleranceoftheMNPscoatedwithdifferentamountsof carboxylicacidswasmeasuredatpH6.5incoagulationkinetics experiments.We observedthat thecriticalcoagulation concentration(CCC)of thecoagulating NaClelectrolyteincreases with increasingamountofcarboxylicacid,iftheIEPoftheactualcar- boxylicacidcoatedMNPislowerthanpH6.5.Inthecasethat theaddedamountofcarboxylicacidsisinsufﬁcienttodecreaseIEP wellbelowpH6.5,theCCCdoesnotincreasecomparedtothatof thenakedMNPs.Atlowcoverage,thepartiallycovered(i.e.,deco- rated)particlescanaggregatebecauseoftheelectrostaticattraction betweentheoppositelychargeduncoatedandcoatedpatcheson theparticlesurfaces[7,25].ThehighestattainedvaluesofCCCand therespectiveamountsofaddedpolyacidsarecollectedinTable2.

ItisseenthatthesmallmoleculesCAandGAcannotstabilizethe MNPsatneutralpHtoresisthighersaltconcentrations,despitethe approximatelyidenticalvaluesofelectrokineticpotentials(within therangeof−35–−55mVatpH6.5,rightsideofFig.4).Onthe contrary,thethickercoatingshellspreparedwithmacromolecular polyelectrolytesPAA,PAM,HAandthesurfacepolymerizedPGA enhancethesaltresistanceequallyuptoCCC500mM.

It is worth mentioning that there are hardly any publica- tions giving CCC values for colloidalparticles, relevant for the

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Fig.4. ShiftinginthevaluesoftheelectrokineticpotentialoftheMNPsduetotheadditionofsmall(leftside)andlarge(rightside)amountsofcarboxylicacids,measured atdifferentpHsandatI=0.01M.(Thelinesaredrawntoguidetheeyes.)

Table1

EffectofthequalityandaddedamountsofcarboxylicacidsontheIEPandthepH-rangeofaggregationofMNPs.

Carboxylatedcoatingagents Addedamount^a(mmol/g) pHofIEP pH-rangeofaggregation

CA 0.13 6.0 4.5–9.5

0.65 3.0 <4.5

GA 0.02 6.8 <9

0.1 4.8 <7.5

0.6 4.2 <6

1.8 2.7 <5.2

PAA 0.1 6.5 4–7

0.48 3.8 3.5–5.5

1.15 3.0 <3.5

PAM 0.1 6.2 3.5–8.5

0.47 3.5 <5

1.3 2.5 <3.5

HA 0.08 4.3 3.5–8.5

1.54 3.2 <3

aTheaddedamountofHA,PAAandPAMwasrelatedtothemolesoftheiracidicgroups.

magnetite/polyelectrolytesystemsstudiedhere.InthepaperofHu etal.[26]theCCCvalueofmagnetitenanoparticlesisgiveninthe presenceof20mg/gofhumicacidatpH9.8as125.5mM(NaCl).

AtthishighpH,themagnetiteitselfhadconsiderableelectrostatic stability withCCC=23.8mM NaCl (for comparison, CCC=1mM NaClatpH6.5inourexperiments)andthechargeofbothMNPs andHAisnegative.Correspondingly,theadsorbedamountofthe humate mustbe low, i.e.,restricted tothespeciﬁc effects only (e.g.,Fe–carboxylatesurfacecomplexformation),whicharehardly affectedbypH.Ofcourse,theprobabilityofclosecontactofHA carboxylatesand Fe OHgroupsonMNPsurface,thusthechance ofcomplex formation,is reduced bytheincreased electrostatic repulsionat pH9.8.Unfortunately, theauthorsdidnotprovide adsorptiondata.Nevertheless,evenunderelectrostaticallyunfa- vorableconditions,thestabilizingeffectofHAonMNPshasbeen shown.

Table2

CCCvaluesofuncoatedandcoatedMNPs,measuredatpH6.5.

Polyacids@MNP Addedamount^a(mmol/g) Approx.CCCNaCl,(mM)

NakedMNP 0 1

CA@MNP 0.3 70

GA@MNP 2 20^b

PAA@MNP 1.12 500

PAM@MNP 1.18 500

HA@MNP 1.5 500

PGA@MNP 2 500^c

aTheaddedamountofHA,PAAandPAMwasrelatedtothemolesoftheiracidic groups.

bMeasuredafter1hstanding.

c Measuredafter2weeksstanding(whileGAsurfacepolymerizationtookplace).

7. Conclusion

The colloidal stability of magnetite as an example among the environmentally relevant iron oxides dispersed in aqueous medium depends sensitively on not only the pH,but also the amountoforganicacidssuchashumicacidsoccurringmainlyin surfacewaters.Thesepolyanionicorganiccomplexantscanmodify thesurfacechargepropertiesofmagnetiteentirelyorinacertain degreedependingontheiramountadsorbed.

Theadsorptionofdifferentorganicacidsanditseffectsonthe pH-dependentcolloidalstabilityandsalt toleranceofmagnetite nanoparticleswerestudied.Theadsorbedamountsweregivenin themolaramountof acidicgroups perunit mass ofiron oxide formacromolecularacidsHA,PAA,PAMandPGA.Thisapproach madethequantitativecomparisonoftheamountsofacidicgroups of large organicacids (both thewell, and the undefined polyelectrolytes) in the adsorbedlayer withthe amountof surface chargeof magnetitepracticable.Thus, thecharge neutralization andchargereversalcouldbeinterpretedonchemicalbases.The specificchemicalfeatureoftheinteractingpartnershastobecon- sidered,becausechemicalreactionstakeplaceattheelectrified interface,i.e.,thefunctionalgroupsoforganicacidsinteractwith thecharged/unchargedsurfacesitesofmagnetite.Theexactfeature ofthespecificinteractionsdependsdefinitelyonthegeometryof complexinggroupsoforganicmolecules.

Traceamountsoftheorganicacidscandestabilizemagnetite dispersions,while theirhighloadingmaskstheoriginalsurface propertiesofmagnetiteandimprovescolloidalstabilityandsalt toleranceofdispersions.TraceamountsofCA,GA,PAA, PAMor HAonlyneutralizethepositivechargesofmagnetiteatpHlower

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thanitspHPZC8,andsopromoteaggregationbetweentheparti- cleshavingbothpositivesurfacesitesandnegativepatchescoated bytheorganicpolyanions.Theseconditions,i.e.,fineironoxide particlesdispersedinwaterwithneutralorslightlyacidicpHand onlytraceamountoforganicacidsdissolvedinit,arerelevantin naturalwaters.Inthepresenceofgreateramounts ofpolyacids (abovetheadsorptionsaturation)however,thesurfacecoverage ofmagnetitebecomescompletecausingasignreversalofparticle chargeandoverchargingofnanoparticles.Thethickerlayerofthe macromolecularcoatingshellprovidesbetterelectrostericstabil- itythanthatformedfromthesmallmoleculesofCAorGA.Ithas beenprovedthatthepHsensitivityofamphotericmagnetitecan becompletelyeliminatedbybothsmallandlargemolecularstabi- lizers,butonlythemacromolecularcoverageofparticlesincreases significantlyintheresistanceagainstsaltatneutralpHcommonly prevailinginnaturalwaters.Oneadditionalinterestingfindingis thatapparentlythereisnocorrelationbetweenthestabilizingeffi- ciencyofthecarboxylicacidsandtheconcentrationofthefully dissociatedcarboxylicgroups.

Acknowledgement

ThisworkwassupportedbyOTKA(NK84014)foundation.The ﬁnancialsupportbytheTÁMOP-4.2.2/B-10/1-2010-0012fundis gratefullyappreciated.

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