Accepted Manuscript Title: Surfactant-aided electrospraying of low molecular weight carbohydrate polymers from aqueous solutions Author: Roc´ıo P´erez-Masi´a Jose M. Lagaron Amparo L´opez-Rubio PII: DOI: Reference:
S0144-8617(13)00924-7 http://dx.doi.org/doi:10.1016/j.carbpol.2013.09.032 CARP 8121
To appear in: Received date: Revised date: Accepted date:
26-7-2013 11-9-2013 13-9-2013
Please cite this article as: P´erez-Masi´a, R., Lagaron, J. M., & L´opez-Rubio, A., SURFACTANT-AIDED ELECTROSPRAYING OF LOW MOLECULAR WEIGHT CARBOHYDRATE POLYMERS FROM AQUEOUS SOLUTIONS, Carbohydrate Polymers (2013), http://dx.doi.org/10.1016/j.carbpol.2013.09.032 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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SURFACTANT‐AIDEDELECTROSPRAYINGOFLOWMOLECULARWEIGHT
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CARBOHYDRATEPOLYMERSFROMAQUEOUSSOLUTIONS
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RocíoPérez‐Masiá,JoseM.Lagaron,AmparoLópez‐Rubio*
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NovelMaterialsandNanotechnologyGroup,Instituteofa*grochemistryandFood
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Technology(IATA‐CSIC),Avda.AgustinEscardino7,46980Paterna(Valencia),Spain
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*Correspondingauthor:Tel.:+34963900022;fax:+34963636301.
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E‐mailaddress:[emailprotected](A.Lopez‐Rubio)
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Abstract
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Inthisworkitisdemonstrated,forthefirsttime,thatitisfeasibletodevelop,usingthe
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electrosprayingtechnique,lowmolecularweightcarbohydrate‐basedcapsulemorphologies
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fromaqueoussolutionsthroughtherationaluseofsurfactants.Twodifferentlowmolecular
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weightcarbohydratepolymerswereused,amaltodextrinandacommercialresistantstarch.
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Thesolutionpropertiesandsubsequenthighvoltagesprayabilitywasevaluateduponaddition
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ofnon‐ionic(Tween20,andSpan20)andzwitterionic(lecithin)surfactants.Themorphology
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andmolecularorganizationofthestructuresobtainedwascharacterizedandrelatedtothe
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solutionproperties.Resultsshowedthat,whileunstablejettinganddroppingoccurredfrom
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thepurecarbohydratesolutionswithoutsurfactant,theadditionofsomesurfaceactive
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moleculesabovetheircriticalmicelleconcentrationfacilitatedcapsuleformation.Higher
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surfactantconcentrationsledtosmallerandmorehom*ogeneouscapsulemorphologies,
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relatedtolowersurfacetensionandhigherconductivityofthesolutions.
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Keywords:Electrospraying,electrospinning,encapsulation,surfactant,aqueoussolution,
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carbohydrate
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1.Introduction
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Thedevelopmentofmicro‐,submicro‐andnanostructuresfrombiopolymersforfunctional
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foodapplicationsisanemergingareaofinterest.Apartfromtheconventional
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microencapsulationtechniques,suchasspraydryingorcoarcervation,electrospinninghas
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beenrecentlysuggestedtobeasimpleandstraightforwardmethodtogeneratesubmicron
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encapsulationstructuresforavarietyofbioactivemolecules(Xie,Li&Xia,2008;Lopez‐Rubio
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&Lagaron,2012;Bock,Dargaville,&Woodruff,2012).Electrospinningisaprocessthat
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producescontinuouspolymerfiberswithdiametersinthesubmicrometerrangethroughthe
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actionofanexternalelectricfieldimposedonapolymersolutionormelt.Theelectrospun
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nanostructuresmorphologyisaffectedbythesolutionproperties(mainlybytheviscosity,
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surfacetensionandconductivityofthepolymersolution)andbytheprocessparameters
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(voltage,flowrateofthesolution,tip‐to‐collectordistance).Forcertainmaterials,size‐reduced
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capsulescanbeobtainedwhenloweringthepolymerconcentrationand/orincreasingthetip‐
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to‐collectordistance.Inthiscase,theelectrospinningprocessisnormallyreferredtoas
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“electrospraying”duetothenon‐continuousnatureofthestructuresobtained.Todate,a
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widevarietyofpolymersandpolymerblendshavebeenelectrospun,withsyntheticpolymers
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yieldingthebestresultsintermsofphysicalpropertiesanduniformity.Ontheotherhand,
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electrospinningofbiopolymersolutionshasbeenproventobedifficultduetoseveralfactors
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suchasthepolycationicnatureofmanybiopolymers,thelowchainflexibilitywhich
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complicateschainentanglements(essentialforfiberformation)andtheirgenerallypoor
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solubilityinorganicsolvents(Kriegel,Kit,McClements,&Weiss,2009).Moreover,unlike
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syntheticpolymers,anaturalpolymerderivedfromdifferentsourcespresentswidelyvarying
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propertiesandithasbeenobservedthattheviscosityofthesolutionsmayvarywithtimedue
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to,forinstance,aqueoushydrolysisofthebiopolymer(Bhattarai&Zhang,2007).
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Electrospinningfromaqueoussolutionsisbeneficialfromanenvironmentalpointofview.
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Furthermore,theuseofwaterdoesnotgeneratetoxicityproblems.Onthecontrary,organic
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solventsmaybeevenprohibitedforcertainapplications,suchasinthecaseoffoodproducts
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(Kriegel,Kit,McClements,&Weiss,2010).Thatissuefurthercomplicatestheelectrospinning
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processduetotheionizationofwatermoleculesathighvoltagesinanairenvironment,which
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maycausecoronadischarge.Besides,aqueoussolutionspresenthighsurfacetensionvalues
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whichhindertheformationofstablejetsduringtheelectrospinning.Moreover,polymersthat
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havelowaqueoussolubility,lowMwpolymersandpolymerswithrigidorglobularstructures
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thatdonotgeneratesufficientviscosityarenoteasilyelectrospunwhentheyareinan
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aqueoussolution(Nagarajan,Drew,&Mello,2007;Stijnman,Bodnar&Tromp,2011).
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Differentsurfactantshavebeenaddedtotheelectrospinningsolutionsforvariouspurposes,
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likeenzymestabilization(Herricksetal.,2005),creationofmesoporousstructures(Hong,Fan,
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&Zhang,2009;Houetal.,2009),ortomakecompatiblehydrophilicfillerswithhydrophobic
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matrices(Kim,Lee,&Knowles,2006).However,moreimportantly,surfactantshavebeenseen
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toimprovethespinnabilityofpolymersolutions,whichisnormallyaconsequenceofthe
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reductionintheirsurfacetension(Boninoetal.,2011).Tothebestofourknowledge,allthe
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studiescarriedouttodateinthisarea,relatetofiberformationandithasbeendemonstrated
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thatadditionofsurfactantsreducefiberdefects,butdonotpromotefiberformationfor
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solutionswhicharenotreadilyspinnable(Aceituno‐Medina,Lopez‐Rubio,Mendoza,&
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Lagaron,2013).However,theeffectofsurfactantadditiononthesprayabilityorcapsule
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formationfrombiopolymersolutionsisunknown.
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Inthisstudy,wehypothesizethatadditionofsurfactantstoaqueousbiopolymersolutionsmay
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provetobeaconvenientmethodtoproduceencapsulationstructuresbymodulatingthe
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electrosprayingconditions.Totestthishypothesis,varioussurfactants(azwitterionicandtwo
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nonionicsurfactants)wereaddedtotwodifferentlowmolecularweightcarbohydratepolymer
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solutions.Solutionsweresubjectedtoelectrosprayingandtheinfluenceofsurfactanttypeand
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chargeonsolutionpropertiesandonthemorphologyofthesubmicronstructuresgenerated
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wereevaluated.
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2.MaterialsandMethods
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2.1Materials
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AmaltodextrinwithaDEvalueof16.5‐19.5waspurchasedfromSigmaAldrich.Acommercial
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resistantstarch(derivedfromcornstarch)withtradenameFibersol®(www.fibersol.com)
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manufacturedbyADM/Matsutani(Iowa,USA)wasused.Thenon‐ionicsurfactants,
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polyoxyethylenesorbitanmonolaureate(Tween20)andsorbitanmonolaureate(Span20),and
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thezwitterionicsurfactant,L‐α‐phosphatidylcholine(lecithin)weresuppliedbySigma‐Aldrich.
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Allproductswereusedasreceivedwithoutfurtherpurification.
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2.2Determinationofthecriticalmicelleconcentrations(CMC)foreachsurfactantbyplate
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tensiometry
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TheCMCofsurfactantsintheabsenceandpresenceofthelowmolecularweightcarbohydrate
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polymerswasdeterminedbymeasuringthesurfacetensionasafunctionofsurfactant
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concentrationthroughadigitaltensiometer(modelEasyDyneK20,KrüssGmbH,Hamburg,
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Germany)usingtheWilhemyplatemethod.Anamountof30gofeachtestsolutionwas
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pouredintoan80mmdiameterglassbeaker.Theglasshadbeenpreviouslyrinsedwith
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absoluteethanolanddeionizedanddistilledwaterandthendriedat70ºCtoremoveany
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surface‐activematerial.Allmeasurementsweredoneintriplicateafterequilibratingthe
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solutionsat25ºC.
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2.3Preparationofcarbohydrate‐basedsolutions
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Carbohydrate‐basedsolutionswerepreparedbydissolvinga20wt.‐%ofthematerialsin
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distilledwaterthroughgentlestirringatroomtemperature.Differentconcentrationsofthe
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varioussurfactants(0,5,and30wt.‐%withrespecttothebiopolymerweight)wereaddedto
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thesolutions.
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2.4Characterizationofthecarbohydrate‐basedsolutions
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Theapparentviscosity(ηa)ofthepolymericsolutionsat100s‐1wasdeterminedusinga
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rotationalviscositymeterViscoBasicPlusLfromFungilabS.A.(SanFeliudeLlobregat,Spain)
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usingaLowViscosityAdapter(LCP).Thesurfacetensionofthebiopolymersolutionswas
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measuredusingtheWilhemyplatemethodinanEasyDyneK20tensiometer(KrüssGmbH,
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Hamburg,Germany).Bothtestswerecarriedoutintriplicate.Theconductivityofthesolutions
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wasmeasuredusingaconductivitymeterXSCon6(Labbox,Barcelona,Spain).All
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measurementsweremadeat25ºC.
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2.5Preparationofcarbohydrate‐basedcapsulesthroughelectrospraying
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Theelectrospinningapparatus,equippedwithavariablehigh‐voltage0‐30kVpowersupply,
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wasasingleneedleFluidnatek®basicsetupfromBioiniciaS.L.(Valencia,Spain).Thesyringe
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containingthecarbohydratesolutionswasplacedhorizontallytothecollector.Thedistance
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betweentheneedleandthecollectorwassetat10cm.Theexperimentalsetupwashousedin
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alaminarflowsafetycabinet.Theelectrosprayedcapsuleswereobtainedusingavoltageof9
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kVandaflowrateof0.15mL/h.
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2.6Infraredspectroscopy
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Attenuatedtotalreflectanceinfraredspectroscopy(ATR‐FTIR)experimentswereperformedin
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acontrolledchamberat21oCand40%RHcouplingtheATRaccessoryGoldenGateofSpecac
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Ltd.(Orpington,UK)toaBruker(Rheinstetten,Germany)FTIRTensor37equipment.Allthe
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spectrawerecollectedwithinthewavenumberrangeof4000–600cm‐1byaveraging15scans
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at4cm‐1resolution.AnalysisofthespectraldatawasperformedbyusingGrams/AI7.02
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(GalacticIndustries,Salem,NH,USA)software.
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2.7Scanningelectronmicroscopy(SEM)
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SEMwasconductedonaHitachimicroscope(HitachiS‐4100)atanacceleratingvoltageof10
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KVandaworkingdistanceof15mm.Theelectrosprayedcapsulesweresputteredwithagold‐
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palladiummixtureundervacuumbeforetheirmorphologywasexaminedusingSEM.Capsule
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diametersoftheelectrosprayedmaterialsweremeasuredbymeansoftheAdobePhotoshop
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CS3extendedsoftwarefromtheSEMmicrographsintheiroriginalmagnification.
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3.ResultsandDiscussion
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3.1Criticalmicelleconcentration(CMC)ofthedifferentsurfactants
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Surfactantsareamphiphilicmoleculesthatreadilyabsorbatsurfaces,therebyloweringsurface
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orinterfacialtensionofthemediuminwhichtheyaredissolved.Moreover,aboveacritical
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concentration,theso‐calledcriticalmicelleconcentration,surfactantsself‐assembletoforma
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varietyofcolloidalstructures,whichhavedifferentpropertiesfromthoseofthedissolved
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surfactantmonomers,e.g.,solubility,surfacehydrophilicity,chargedensity.Previousstudies
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havedemonstratedthatadditionofnon‐ionicandionicsurfactantsabovetheircriticalmicelle
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concentrationtopolymersolutions,significantlyimprovedtheelectrospinningprocess
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generatingdefect‐freefibers(Kriegeletal.,2009).Therefore,inthisstudy,thefirstintention
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wastoadddifferentsurfactantsabovetheirCMCtostudytheirinfluenceonthesprayabilityof
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lowMwcarbohydrates.TheCMCinformsabouttheconcentrationofsurfactantnecessaryto
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formamonolayerofmoleculesorientedattheair‐waterinterface(Lin,Wang,Wang&Wang,
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2004;Chou,Krishnamurthy,Randolph,Carpenter&Manning,2005).Ontheotherhand,the
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concentrationneededforthepolymer‐surfactantassociationisthecriticalaggregation
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concentration(CAC)anditisusuallylowerthantheCMCbyafactorbetween1and10.Both
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thesurfactantconcentrationandthepolymer‐surfactantinteractionsmayresultinchangesin
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therheologyandconductivityofthesolutions,factorswhichgreatlyaffectthe
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electrospinning/electrosprayingprocess(Linetal.2004).
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Initially,thesurfacetensionfordifferentsurfactantconcentrationsinaqueoussolutioninthe
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absenceandpresenceofthelowmolecularweightcarbohydrateswasmeasuredandCMC
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valuesweredeterminedwhentheplateauinsurfacetensionwasobtained.Table1showsthe
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CMCvaluesforthedifferentsurfactantsassayedandtheconcentrationaddedinthesolutions.
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Forallthesolutionstesteditwasobservedthatverylowconcentrationsofthesurfactants
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wereneededtoreachtheCMC,regardlessofwhetherthecarbohydrateswerepresent.Itwas
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alsoobservedthatCMCincreasedwiththeadditionofthebiopolymersprobablybecausethe
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surfactantswerealsointeractingwiththebiopolymersinsolution.Itispossiblethatinthe
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presenceofcarbohydrates,theconcentrationofthesurfactantsinthesurfacedecreased,as
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partofthesurfactantwasboundtothecarbohydrates.Asaresult,theamountofsurfactant
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neededtoreachtheCMCincreased(Chouetal.,2005).Knowingthisplateauvalue,two
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differentconcentrationsofeachsurfactant(5and30wt.%)wereaddedtothecarbohydrate
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solutions,whichcorrespondedto28.9mMofSpan20,8.2mMofTween20and13.2mMof
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lecithinwhen5%ofsurfactantwithrespecttothebiopolymerweightwasadded;and173.2
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mMofSpan20,49.0mMofTween20and79.0mMoflecithinwhen30%ofsurfactantwith
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respecttothebiopolymerweightwasincorporated.Pleasenotethatbothconcentrations
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werewellhigherthantheCMCofthesurfactants.
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INSERTTABLE1ABOUTHERE
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3.2Solutionproperties
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Thephysicalpropertiesofthecarbohydrate‐surfactantsolutionsarecriticalinthesuccessful
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preparationoftheelectrosprayedstructures.Therefore,theconductivity,viscosityandsurface
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tensionofthedifferentsolutionsweremeasuredandtheresultsaresummarizedinTable2.
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Fromthesedataitisobservedthattheadditionofresistantstarchtowaterdidnot
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considerablyincreasetheconductivityofthesolventbecausethismaterialdidnotpresentany
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electricalcharge.Onthecontrary,themaltodextrin‐basedsolutionspresentedenhanced
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conductivityvalues.Thisfactcouldbeduetomaltodextrinformingchargedionswhen
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dissolvedinwater.FromTable2,itisalsoobservedthatadditionofnon‐ionicsurfactantsto
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theresistantstarchsolutionsproducedaslightincreaseintheconductivity,probablydueto
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theexistenceofpolargroupsinthismolecule(Linetal.2004).However,whenSpan20and
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Tween20wereincorporatedtothemaltodextrinsolutions,theydidnotaffecttheconductivity,
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showingthattheeffectofthesesurfactantsinthesolutionconductivityisverylimitedanditis
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onlyrelevantwhenthesolutionpresentsverylowconductivity.Incontrast,additionoflecithin
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ledtohigherconductivityinbothcarbohydratesolutions.Thisfactwasrelatedtothe
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zwitterionicnatureofthelecithinwhichpresentsasymmetricpositiveandnegativeelectric
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charges.Thesechargesweredissociatedinaqueoussolutionandthus,ledtoanincreaseof
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theelectricalconductivity(Hunley,England&Long,2010).Concerningtheviscosity,itwas
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seenthatverylowvalueswereobtainedregardlesstheabsenceorpresenceofthe
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surfactants.Theseresultswereexpected,sincethelowmolecularweightcarbohydratesused
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inthisstudywouldrequiregreaterconcentrationstoachievecomparablesolutionviscosities
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tohighmolecularweightpolymers.Inparticular,theadditionofSpan20andTween20hardly
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increasedtheviscosityvalues.However,additionoflecithinincreasedthesolutionsviscosity
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fromca.2to5cP, probablybecausetheinteractionsbetweenthecarbohydratesandtheionic
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surfactantwerestrongerthanthosewiththenon‐ionicsurfactants.Nevertheless,lowviscosity
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valuesareneededforelectrospraying,sincehigherviscosityfavorstheformationoffibers
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insteadofsphericalcapsules(beads)(Fong,Chun&Reneker,1999).Finally,Table2showsthe
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surfacetensionofthedifferentsolutionsassayed.Itwasobservedthatsurfacetensionvalues
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ofsurfactant‐freesolutionswereover50mN/m,duetothehighsurfacetensionofwater,
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whichwasthesolventusedinthesolutions.Additionofthedifferentsurfactantsledtoa
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decreaseinsurfacetension,reachingtheplateauvaluesobtainedfortheCMCofthedifferent
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surfactants.Ingeneral,itcanbestatedthatincreasingthesurfactantconcentrationledto
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greaterconductivityandviscosityvalues.
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INSERTTABLE2ABOUTHERE
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3.3Morphologyoftheelectrosprayedcarbohydrates
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Theelectrosprayingofallthesolutionswasperformedunderthesameprocessingconditions
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(cf.section2.5).Initially,thecarbohydrate‐aqueoussolutionswithoutsurfactantswere
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electrosprayedanditwasobservedthatalthoughthecommercialresistantstarchformed
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sphericalcapsuleswithsizesrangingfromafewnmto∼2µmwithanaveragesizeof0.6±0.3
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µm(imagenotshown),extensivedroppingoccurredduetounstableelectrospraying.Onthe
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otherhand,itwasnotpossibletoobtainanyelectrosprayedstructurefromthemaltodextrin
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aqueoussolution.Theseresultscanbeexplainedbythephysicalpropertiesofthesolutions.As
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itwascommentedbefore,bothcarbohydratesolutionspresentedhighsurfacetensionand
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lowviscosityvalues;however,resistantstarchdidnotgreatlyincreasetheconductivityofthe
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solution,whiletheadditionofmaltodextrinproducedveryhighconductivityvalues.Whenthe
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highvoltage(typicallyintherangeof0–30kV)isappliedtothespinneretfromwherethe
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solutionisejected,thesurfaceofthefluiddropletheldbyitsownsurfacetensiongets
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electrostaticallychargedatthespinnerettip.Stableelectrosprayingorelectrospinningis
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knowntobeattainedwhentheelectrostaticforcesinsidethedroplet(arisingfrommutual
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electrostaticrepulsionbetweenthesurfacechargesandtheCoulombforceappliedbythe
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externalelectricfield),arestrongenoughtoovercomethesurfacetensionofthepolymer
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solution,forcingtheejectionoftheliquidjet(Zhang&Kawakami,2010).Beforetheejectionof
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theliquidjet,andasaconsequenceofthementionedelectrostaticinteractions,theliquid
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dropelongatesintoaconicalobjectknownastheTaylorcone.Thus,inthecaseofthe
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resistantstarch,theelectricalconductivityofthissolutionwasinsufficient,atthevoltage
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applied,toovercomethehighsurfacetensionand,consequently,theTaylorconedidnotform
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anddroppingofthesolutionoccurred.Incontrast,whenthecoulombicrepulsionsaretoohigh
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andovercometheviscoelasticforces,lesschainentanglementstakeplaceduring
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electrosprayingand,thus,verysmallparticlesornon‐definedstructuresareobtained(Bocket
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al.,2012).Thisseemedtobethecaseforthemaltodextrinsolution,asitsveryhighelectrical
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conductivitycompletelyhinderedtheelectrosprayingprocess.
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Theadditionofsurfactantstothecarbohydrateaqueoussolutionsproducedadecreasein
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surfacetensionwhichfavoredtheformationofelectrosprayedstructures.Figure1showsthe
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SEMimagesandcorrespondingsizedistributionofthematerialsobtainedfromthe
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electrosprayingofthedifferentresistantstarchsolutions.FromFigures1Aand1Bitwasseen
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that,regardlessofconcentration,whenSpan20wasaddedtotheresistantstarchsolution,
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threedifferentcapsulesizepopulationswerefound,althoughthestructuresweresmallerand
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morehom*ogeneousinsizewhen30%ofthesurfactantwasadded.Figures1Cand1Dshow
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thattheadditionof5%ofTween20alsogeneratedthreepopulationswithrespecttothe
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capsulesdiameter.However,whentheconcentrationwasincreasedto30%,onlytwodifferent
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sizedistributionsandsmallercapsuleswereattained.Ontheotherhand,whenlecithinwas
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includedinthesolutions,onlyonepopulationwithrespecttothecapsule’sdiameterswas
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seen(cf.Figures1Eand1F).Moreover,theparticlesizewasgreatlyreducedwhencompared
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tocapsulesobtainedfromthecarbohydratewithoutsurfactant.Thus,theaveragesizeinthis
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casewas0.3±0.1µmand0.2±0.1µmwhen5%and30%oflecithinwasaddedrespectively.
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Thevariationsobservedbetweenthedifferentstructurescanbemainlyattributedtothe
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electricalconductivityofthesolutions.Itisknownthathigherconductivityleadstoadecrease
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insizebecauseCoulombicrepulsionforcescompetewiththeviscoelasticforcesofthesolution
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anddisentanglemoreeasilythepolymernetworkformedduringelectrospraying.Inother
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words,increasingconductivitymakesiteasierforthesolutiontobebrokenupintosmaller
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droplets(Gañan‐Calvo,Davila&Barrero,1997;Bocketal.,2012).
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INSERTFIGURE1ABOUTHERE
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Regardingthemaltodextrinstructures,Figure2showstheSEMimagesandcorrespondingsize
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distributionofthematerialsobtained.Itisobservedthattheadditionofnon‐ionicsurfactants
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allowedtheformationofparticlesfromafewnmto500nm(cf.Figures2Ato2D).Therange
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ofsizedistributionwasconsiderablynarrowerthanfortheresistantstarchmaterialsand,in
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mostcases,morethan50%oftheparticleswerearound200nminsize.Thisfactwas
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explainedfromthesurfacetensiondecreaseproducedbythesurfactants.Viscoelasticand
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electricalforcesmustovercomethesurfacetensioneffectinordertoobtainadefined
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structure.Whensurfactantswerenotaddedtothemaltodextrinsolution,thedropletsformed
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ontheneedletipgrewuntilitsmasswaslargeenoughtoescapeandelectrosprayingcouldnot
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occur(Xu&Hanna,2006).However,theadditionofthenon‐ionicsurfactantsreducedthe
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surfacetensionand,thus,aconicalmeniscuswasformedontheneedletip.Themeniscus
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furtherdeformedandbrokeintodropletswithsmallparticlesizesandnarrowsizedistribution
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duetotheelectrostaticforceintroducedbythemaltodextrin.Nevertheless,when30%of
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Tween20wasaddedtothesolution,theelectricalconductivityincreasedanddifferentcapsule
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morphologieswereobtained,probablybecausethehighelectricalforcesfavoredweak
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entanglementsinthepolymer(Bocketal.,2012).Theadditionoflecithinproducedan
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excessiveincreaseintheconductivitywhichcompletelyhinderedcapsuleformation.
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INSERTFIGURE2ABOUTHERE
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Itisinterestingtonotethat,apartfromthecapsularmorphologygenerated,additionof
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surfactantsalsoledtoneedle‐likemorphologiesinbothcarbohydratematrices,thus
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confirmingthatadditionoftheseamphiphilicmolecules,whichdecreasedthesurfacetension
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oftheaqueoussolutions,considerablyenhancedchainentanglements.
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Ingeneral,fromthemorphologyofthestructuresobtained,itcanbestatedthatnon‐ionic
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surfactantsaremoresuitableforgeneratingencapsulationstructuresfromlowmolecular
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weightcarbohydratepolymers,andthatthesizeandsizedistributioncanbemodifiedbythe
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typeandamountofsurfactantadded.
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3.4Infraredspectraoftheencapsulates
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ATR‐FTIRanalysesweredoneinordertocharacterizethemolecularorganizationofthe
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structuresattained,aswellastoconfirmthepresenceofthesurfactantsinthecarbohydrate
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structures.Infirstplace,theregionfrom800to1200cm‐1wasanalyzedforallthematerials
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obtained.Thisareapresentsthecharacteristicvibrationalbandsofthecarbohydrates,
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correspondingtothestretchingvibrationsofC‐OandC‐Cgroups,andthebendingvibrationof
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C‐O‐H(Wolkers,Oliver,Tablin,&Crowe,2004;Kacurakova&Mathlouthi,1996).FromFigure3
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itwasobservedthatwhensurfactantswereaddedtotheresistantstarch,thesebandswere
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shiftedbyapproximately2‐6cm‐1suggestingthattherewasachemicalinteractionbetween
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thecarbohydrateandthesurfactants.Specifically,themostnotedshiftwasobservedforthe
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bandwhicharoseat1006cm‐1intheresistantstarch(cf.Figures3Ato3C).Thisbandwas
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shiftedtowardshigherwavenumbersinthesurfactant/polymercapsules,whichcouldmean
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strongerhydrogenbondingduetotheinteractionofthecarbohydratewiththesurfactants
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(Wolkersetal.,2004).Itisinterestingtonotethatgreaterbandshiftswererelatedtosmaller
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capsulemeandiameters,whichmaybeprobablyexplainedbythegreaterspecificsurface
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presentinthematerialcontainingsmallercapsules.Moreover,inthisspecificcarbohydrate
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polymer,i.e.theresistantstarch,aclearchangeinbandshapewasalsoobservedinthe
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spectralrange950‐1050cm‐1,whichalsoresultedinnarrowerbandsintheencapsulates
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containingthesurfactants,indicatingthatsurfactantadditionledtogreatermolecularorder.
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Onthecontrary,forthemaltodextrinstructures(Figures3Eto3F),thecharacteristic
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carbohydratebandshardlyshiftedwiththeincorporationofthesurfactants,indicatingthat
312
theirinteractionwiththepolymermaybelessintensethaninthepreviouscase.Nevertheless,
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itwasseenthatlecithinproducedthegreatestbanddisplacementsforbothpolymermatrices
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probablybecauseitisazwitterionicsurfactantwhichpresentedastrongerinteractionwiththe
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polymers(Linetal.2004).
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INSERTFIGURE3ABOUTHERE
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Furthermore,themostcharacteristicbandofthesurfactantswhichwasnotoverlappedwith
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thecarbohydratebandswasconsideredtodeterminetheeffectoftheconcentrationofthe
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surfactantsintheelectrosprayedmaterial.Figure4showsthecapsule’sspectrafrom1800to
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1600cm‐1wherethebandcorrespondingtothecarbonylgroup,ataround1740cm‐1,
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attributedtothesurfactantswaslocated.Fromthespectra,itwasobservedthatthe
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surfactantswereincorporatedinallthestructures,sincethispeakappearedinallthe
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materials.Itisworthnotingthatthelecithinbandshowedthegreatestshiftwhenitwas
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combinedwiththepolymers,thusconfirmingthestrongerinteractionbetweentheionic
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surfactantswiththepolymers.Moreover,thispeakcouldalsorevealtheamountofsurfactant
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includedintheinitialsolutions,sinceitwasmoreintensewiththeincreasingconcentrationof
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thesurfactant.
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INSERTFIGURE4ABOUTHERE
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4.Conclusions
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Inthisworkitisdemonstratedthatadditionofsurfactantsconsiderablyimprovesthe
335
electrosprayingoflowMwcarbohydrateaqueouspolymersolutions.Specifically,ultrathin
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capsulesmadefromacommercialresistantstarchandamaltodextrinwithSpan20,Tween20
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orlecithinweredeveloped.Thiswasmainlyduetoareductioninthesurfacetensioncaused
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bysurfactantaddition,whichstabilizedtheelectrosprayingprocess.However,ithasalsobeen
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shownthatthetypeandamountofsurfactantgreatlyinfluencedthemorphologyandsize
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distributionoftheencapsulationstructuresgenerated.Ingeneral,itcanbestatedthatnon‐
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ionicsurfactantsweremoresuitablefortheelectrosprayingoflowMwcarbohydrate
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solutions,aselectricallychargedsurfactantsgaverisetofusedandtoosmallstructures.FTIR
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resultsshowedthatthesurfactantswereeffectivelyincorporatedinthecarbohydrate
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polymersandwhilegreatermolecularorderanddifferentcapsulesizeswereobtainedfrom
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resistantstarchsolutionsbychangingthetypeandconcentrationofsurfactant,onlyverysmall
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structureswereformedfrommaltodextrinsolutions,duetotheirhighelectricalconductivity.
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Theseresultsarehighlyinterestingforthedevelopmentofencapsulationstructuresforfood‐
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relatedapplicationswheretheuseofaqueoussolutionsismandatory.
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350
Acknowledgements
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A.Lopez‐RubioisrecipientofaRamonyCajalcontractfromtheSpanishMinistryofScience
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andInnovation.TheauthorsthanktheSpanishMICINNprojectsAGL2012‐30647,FUN‐C‐FOOD
353
(CSD2007‐00063),andtheEUprojectoftheFP7FRISBEEforfinancialsupport.Authorswould
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alsoliketoacknowledgetheCentralServicesforExperimentalInvestigationSupporting(SCSIE)
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oftheUniversityofValenciafortheelectronicmicroscopyservice.
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FIGURECAPTIONS
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Figure 1. Selected SEM images and their corresponding capsule size distribution of resistant
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starch‐based structures with the different surfactants: A) 5% Span20; B) 30% Span20; C) 5%
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Tween20;D)30%Tween20;E)5%lecithinandF)30%lecithin.
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Figure 2. Selected SEM images and their corresponding capsule size distribution of
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maltodextrin‐basedstructureswithdifferentsurfactants:A)5%Span20;B)30%Span20;C)5%
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Tween20;D)30%Tween20;E)5%lecithinandF)30%lecithin.
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Figure3.ATR‐FTIRspectrafrom1200to880cm‐1 forthepurecarbohydrate(dottedline),the
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surfactants(dashedline),thecarbohydratewith5%ofsurfactant(greyline)andwith30%of
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surfactant (black line) for: (A) resistant starch/Span20; (B) resistant starch/Tween20; (C)
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resistant starch/lecithin; (D) maltodextrin/Span20; (E) maltodextrin/Tween20; and (F)
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maltodextrin/lecithin.
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Figure 4. ATR‐FTIR spectra from 1600 to 1800 cm‐1 for the pure carbohydrate (dotted line),
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thesurfactants(dashedline),thecarbohydratewith5%ofsurfactant(greyline)andwith30%
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of surfactant (black line) for: (A) resistant starch/Span20; (B) resistant starch/Tween20; (C)
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resistant starch/lecithin; (D) maltodextrin/Span20; (E) maltodextrin/Tween20; and (F)
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maltodextrin/lecithin(F).
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Table1.Criticalmicelleconcentration(CMC)ofthedifferentsurfactantsinaqueoussolutionin
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absenceandpresenceofthecarbohydrates.
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CMCofsurfactant(mM) Span20 Tween20 Lecithin 0.04 0.01 0.12 0.1 0.03 0.16 0.1 0.05 0.16
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Carbohydrate(wt‐%) Aqueoussolution Resistantstarch20% Maltodextrin20%
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Table2.Conductivity,viscosityandsurfacetensionofthecarbohydrate‐surfactantsolutions.
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Surfactant Conductivity Surfactant concentration (µS) (%) 0 17±1 ‐ 5 33±1 Span20 30 73±2 Resistant 5 35±2 starch Tween20 30 136±2 5 209±3 Lecithin 30 862±6 0 798±1 ‐ 5 790±1 Span20 30 786±2 Maltodextrin 5 802±3 Tween20 30 843±7 5 928±6 Lecithin 30 1776±8
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2.0±0.5 2.3±0.1 2.5±0.7 2.2±0.6 2.8±0.1 2.2±0.1 5.4±0.9 2.2±0.2 2.2±0.1 2.4±0.1 2.2±0.5 2.3±0.2 2.8±0.2 5.3±0.6
Surface Tension (mN/m) 56.1±1.6 26.1±0.8 25.9±0.5 31.0±0.1 35.4±0.9 29.9±0.3 27.5±2.3 52.7±4.1 25.3±0.8 24.7±0.5 35.1±0.4 35.0±3.5 32.5±1.3 26.2±0.3
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Viscosity (cP)
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Highlights
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‐ElectrosprayingwasusedtodeveloplowMwcarbohydrate‐basedcapsules
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‐SurfactantadditionabovetheCMCallowedcapsuleformationfromaqueoussolutions
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‐Surfactanttypeandconcentrationinfluencedcapsulesizeandmorphology
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‐Changesincapsulesizeuponsurfactantadditionwererelatedtosolutionproperties
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‐Smallerandmorehom*ogeneouscapsulesobtainedincreasingsurfactantconcentration
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Figure 2_reviewed
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Figure 4
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