Emulsion
Contributedby:LaurierL.Schramm Publicationyear:2014
Asystemofsmallliquiddropletsdispersedinasecond,immiscibleliquid.Emulsionsareatypeof colloidaldispersion.Byclassicaldefinition,emulsiondropletshavediametersbetween1nanometerand1 micrometer.However,forpracticalpurposes,theprinciplesofcolloidsciencecanbeusefullyappliedto emulsionswhosedropletsareaslargeastensorevenhundredsofmicrometers.Thedropletsinanemulsionare largeenoughthattheydonotbehaveliketheatomsandmoleculesofclassicalchemistry.Forexample,
emulsionsdonotgenerallybehaveliketruesolutionsandmayhaveundetectablefreezing-pointdepressions.On theotherhand,thedropletsaresmallenoughthattheydonotbehavelikethemacroscopicparticlesofclassical physics.Forexample,emulsiondropletsmayrise(cream)orfall(sediment)extremelyslowlyinapparent violationofStokes’law. Seealso:COLLOID;SEDIMENTATION(INDUSTRY).
Inmostemulsions,oneoftheliquidsisaqueous(thatis,watercontainingdissolvedsubstances),whiletheother isanoil(usuallyahydrocarbonliquid).Thetwosimplestkindsofemulsionarethoseinwhichoildropletsare dispersedinwater(denotedoil-in-water,orO∕W)andthoseinwhichwaterdropletsaredispersedinoil(denoted water-in-oil,orW∕O).Inpractice,emulsionscanbemuchmorecomplexandcanconsistofdropletswithin droplets.Adoubleemulsionmightcontainaqueousdropletsdispersedinoildropletsthatareinturndispersedin acontinuousaqueousphase.Suchanemulsionistermedwater-in-oil-in-water,orW∕O∕W.Theoppositekindof doubleemulsionisO∕W∕O.Doubleemulsiondropletscanbequitelarge(tensofmicrometers),andeachcan containmanydropletswithinit.
Preparation
suchasviscosity,color,andappearance.Properlyformulatedemulsionscanremainstableformonthsoreven years. See also: SURFACTANT.
Emulsionsarepreparedbythebreaking-upofoneliquidphaseintodropletsandthenthefurtherbreaking-up,or comminution,ofthesedropletsintosmallerdroplets.Typically,theemulsifyingagentisdissolvedintothephase whereitismostsoluble,afterwhichthesecondphaseisaddedandshearisappliedtothemixtureusingeither high-speedmixingorvigorousagitation.Avarietyofhigh-shearmixing∕agitatingdevicesareavailableforthis purpose,includingpaddle,propeller,orturbinemixers,rotor-statorhomogenizers,andultrasoundgenerators. Animportantaspectofemulsionpreparationisselectinganappropriateemulsifyingagent.Althoughsome experimentationwillalwaysberequired,therearesomerulesofthumbthatprovideusefulstartingpointsfor predictingthetypeofemulsionthatwillformunderagivensetofcircumstances.Forexample,Bancroft’srule predictsthatifanemulsifyingagentispreferentiallydissolvedin,orwettedby,oneoftheliquidphases,that phasewilllikelyformtheexternal(continuous)phase.Arelatedruleofthumb,theorientedwedgetheory, predictsthatsoapsofmonovalentmetalcations,suchassodium,tendtoproduceO∕Wemulsionswhilethoseof polyvalentmetalcations,suchascalcium,willtendtoproduceW∕Oemulsions.Thesamebasicconcepts underliethedimensionlesshydrophile–lipophilebalance(HLB)scale,inwhichtherelativetendencyofan emulsifyingagenttoprefertodissolveinoilversuswaterisquantified.OntheHLBscale,oil-soluble(lipophilic) emulsifiershavelowvalues,usuallylessthan9,whilewater-soluble(hydrophilic)emulsifiershavehighvalues, usuallygreaterthan11.AverypolarnonionicsurfactantmighthaveanHLBvalueofcloseto20,whileanionic surfactant,suchassodiumdodecylsulfate,hasanHLBofabout40.GoodemulsifiersforW∕Oemulsionstendto haveHLBvaluesofabout3to8,whilegoodemulsifiersforO∕WemulsionstendtohaveHLBvaluesofabout8to 18.Theserulesofthumbprovideindicationsofthekindofemulsionapotentialemulsifyingagentmaypromote, butnotitsefficiency.Inaddition,mixturesofemulsifierstendtobemoreefficientatemulsifyingthanpure compoundshavingthesameHLB.Laboratoryemulsificationtestsarevirtuallyalwaysrequiredinordertoselect anappropriateemulsifyingagent(ormixture)andtodeterminetheoptimumamountofemulsifiertouse.
Properties
Althoughemulsionscanbepreparedthatwillremainstableforweeks,months,orevenyearsinsomecases,most emulsionsarenottrulystable.Eventually,thedisperseddropletswillfallorriseduetogravity
(sedimentation∕creaming).Astheycomeintocontactwitheachother,somewillbegintosticktogether (aggregation),andsomeofthosedropletsthatsticktogetherwilleventuallyfuseintolargerdroplets
sothatthedropletsbehavealmostasthoughtheyweretrulydissolved.Thedropletsinamicroemulsioncanbe sosmallthattheyscatterverylittlelightandappeartobetransparent.
Emulsionshavesomespecialproperties.Havingasystemofsmallliquiddropletsdispersedinasecond, immiscibleliquidmeansthatemulsiondropletshavehighsurfacearea–to–volumeratios.Thismeansthat,by beingpresentinanemulsion,averysmallvolumeof(dispersed)liquidcanbemadetopresentaverylarge amountofsurfacearea,suchasforreactionsbetweenspeciesatthesurfaceandspeciesinsolution.
Anotherconsequenceofhavingadispersionofsmalldropletsisanincreasedresistancetoflow,orviscosity.An emulsionwithasmallvolumeofliquid(lessthanabout10%byvolume)makingupthedisperseddropletswill have,essentially,asmallnumberofsmalldropletssothatindividualdropletswillnotencountereachothervery oftenandwillhavelittleeffectonviscosity.However,asthevolumeofliquidmakingupthedisperseddroplets increasesaboveabout10%byvolume,thenumberofdropletsincreasesandsodoesthefrequencyof
droplet-dropletencounters.Asdropletsencounterandinteractwitheachothermoreoften,thereisa“crowding” effect,anditbecomesmoredifficulttomaketheemulsionflow.Asthetotalvolumeoccupiedbythedroplets increases,sodoestheemulsionviscosity.Insomeemulsionsthesurfacesofthedisperseddropletscarryelectric charges,whileinothersthedropletsurfaceshavelongsurfactantorpolymermoleculesextendingoutwardfrom them.Inthesecases,theelectricfieldsandextendedmoleculescauseadditionalforcesbetweendropletswhen theyencountereachother,whichfurtheractstoincreaseemulsionviscosity. See also: VISCOSITY.
Intheory,themaximumvolumepossiblefortheliquidmakingupthedisperseddropletsis74%—themaximum volumeforclose-packedrigidspheres.Inspecialcases,itispossibletomakeemulsionsinwhichthedroplets distortandoccupyagreatervolume.Generallywhenthishappens,theemulsioninverts.Forexample,anO∕W emulsionpushedtohaveatotaloildropletvolumeof80%byvolumewillusuallyquicklyinverttobecomea W∕Oemulsionhavinga20%dispersedwaterdropletvolume.
Characterization
Emulsionsmaybecharacterizedinanumberofways.
1.ThenatureoftheemulsioncanbecharacterizedasO∕W,W∕O,O∕W∕O,W∕O∕W,orO∕W∕O∕W.Thenature ofthedispersedphasecanoftenbedeterminedusingdyesand∕ormicroscopy,whereasthenatureofthe externalphasecanbedeterminedbyelectricalconductivity.Theexistenceofmultipledispersedphasescan usuallybedeterminedusingopticalorelectronmicroscopy.
3.Compositionisusuallydeterminedbyatitrationmethodforsimpleemulsions.Formorecomplexemulsions, compositionisusuallydeterminedbyseparatingtheoilandwater,andanygasorsolids,andmeasuringtheir respectivequantities.
4.Stabilityoftheemulsionagainstbreakingisusuallydeterminedusingstandardizedbottleorjartests,inwhich emulsionsamplesareallowedtostandforaspecifiedperiodoftime,withorwithoutcentrifuging,andthen examined. Seealso:ELECTRONMICROSCOPE;ELECTROLYTICCONDUCTANCE;LIGHT-SCATTERINGTECHNIQUES.
Occurrence
Systemscontainingcolloidal-sizedparticles,droplets,orbubblesareimportantbecausetheyfeatureprominently, inbothdesirableandundesirablecontexts,inawidevarietyofpracticaldisciplines,products,andindustrial processes(seetable).Theproblemsassociatedwithcolloidsareusuallyinterdisciplinaryinnature,andabroad scientificbaseisrequiredtounderstandthemcompletely.
Emulsionsarecommonlyusedinmanyindustries.Emulsionproductsincludesomefoods,insecticidesand herbicides,polishes,drugs,biologicalsystems,personal-carecreams,ointments,andlotions,greases,inks,paints, andvarnishes.Doubleemulsionshaveapplicationsincosmetics,agriculture,food,photography,leather,and drugdelivery.Theuseofemulsionstoprovidepermanentandtransientantifoamsisanapplicationthatis importanttoanevenbroadervarietyofproductsandprocesses,includingfoods,cosmetics,pharmaceuticals, pulpandpaper,watertreatment,andmineralsbeneficiation.Emulsionsmaybeencounteredinvirtuallyallofthe processindustries,includingmineralprocessing,petroleumproductionandrefining,asphaltpaving,andpaper de-inking.Emulsionsalsooccurintheenvironment.Examplesincludethedifficult-to-treat“chocolatemousse” typeofemulsionthatmaybeformedwhencrudeoilisspilledontheocean,andsoilremediation,inwhichsoil contaminantsareremovedintheformofanemulsionduringasurfactantflushingprocess.Acurrentlyemerging areaisthatofnanoemulsions,havingdropletsizesofabout1nmorless,whichareofinterestinmedical
diagnosticsanddrugtreatmentsbecauseoftheirpotentialabilitytocrosstheendothelialcellsthatprotectthe brain.
Demulsification
Someexamplesofemulsionoccurrence
Applicationarea, Emulsionexamples, Emulsiontypes,∗,
Environment, Insecticideandherbicide formulations,
O∕W,
blank Waterandsewagetreatment
emulsions,
O∕W,
blank Oilspillemulsions, W∕O,O∕W,
blank Sulfideliquidsinmagmaorlava, L∕L,
Mineralprocessing, Emulsionflotationmedia, O∕W,
blank Oilflotationfroth, O∕W,
Petroleumproduction, Oilwellstimulationemulsion, O∕W,W∕O,
blank Emulsiondrillingfluid, O∕W,W∕O,
blank Produced(well-head)emulsions, W∕O,
blank Enhancedoilrecoveryinsitu
emulsions,
O∕W,
Petroleumtransportation, Transportationfuelemulsion(70% heavyoil),
O∕W,
blank Heavyoilpipelineemulsions, O∕W,
blank Oilspillmousseemulsions, W∕O,
blank Tankerbilgeemulsions, O∕W,
Manufacturingandmaterials, Asphalt(paving)emulsion, O∕W,
blank Aqueouspolymer-typepaints, O∕W,
blank Microemulsionmetalworkingoil, O∕W,
Foodproducts, Milk,icecream,creams,whippable toppings,
O∕W,
blank Mayonnaise,hollandaise,b´earnaise sauces,
O∕W,
blank Butter,margarine,spreads,
processedcheese,
W∕O,
blank Saladdressings, O∕W,W∕O,
Agriculture, Pesticide,herbicideandfungicides, O∕W,W∕O∕W,
Biologyandmedicine, Blood, O∕W,
blank Vesicles,vacuoles, W∕W,W∕W∕W,
blank Emulsionencapsulateddrugs, O∕W,W∕O,W∕O∕W,
Personalcare, Cosmeticandskincarecreams, W∕O,O∕W,W∕O∕W,
blank Microemulsionhairdyes, W∕O,
,∗Liquid(L),water(W),oil(O).
treaters,desalters,andfree-waterknockouts.Allaredesignedtoinducedropletaggregationandcoalescence.In mostcases,achemicaldemulsifierisalsoneeded.
Chemicaldemulsifiersgenerallyactbyneutralizingthestabilizingeffectoftheoriginalemulsifier.Forexample,a W∕Oemulsioncansometimesbebrokenbyaddingjusttherightamountofasurfactantthatwouldnormally promoteanO∕Wemulsion.Dosageiscriticallyimportant.Addingtoomuchofsuchasurfactantwillusually createthereverseemulsion,replacingoneproblememulsionwithanother.Similarly,ifanemulsionisstabilized byelectricalforces,demulsificationcouldbebroughtaboutbyaddingachemicalagentthatcanovercomeor reducetheseforces.Followingadditionofanappropriatedemulsifier,mechanicalagitationisoftenallthatis neededtocausetheseparationoftheoilandwaterphases.Awiderangeofpotentialchemicaldemulsifiersis available,butselectinganappropriatedemulsifier,andtheoptimumdosage,caninvolveconsiderabletestingdue tothelargenumberoffactorsthancaninfluenceemulsionstabilityanddemulsifierperformance.
LaurierL.Schramm
Bibliography
P.Becher,Emulsions:TheoryandPractice,3ded.,AmericanChemicalSociety,2001
P.Becher(ed.),EncyclopediaofEmulsionTechnology,vols.1–3,MarcelDekker,1983,1985,1988
B.P.Binks(ed.),Modern Aspects of Emulsion Science ,RoyalSocietyofChemistry,Cambridge,1998
L.L.Schramm,Emulsions,Foams,andSuspensions,FundamentalsandApplications,Wiley-VCH,Weinheim, 2005
A.L.Smith(ed.), Theory and Practice of Emulsion Technology ,AcademicPress,1976
Additional
Readings
T.Cosgrove(eds.),ColloidScience:Principles,MethodsandApplications,2ded.,JohnWiley&Sons, Chichester,WestSussex,UK,2010
Y.Daietal.,Insituencapsulationoflaccaseinmicrofibersbyemulsionelectrospinning:Preparation, characterization,andapplication,BioresourceTechnol.,101(23):8942–8947,2010
DOI:http://doi.org/10.1016/j.biortech.2010.07.027
M.El-NokalyandD.Cornell(eds.),Microemulsions and Emulsions in Foods ,AmericanChemicalSociety,1991
P.KumarandK.L.Mittal(eds.),HandbookofMicroemulsionScienceandTechnology,MarcelDekker,1999
K.J.Lissant,Demulsification,IndustrialApplications,MarcelDekker,1983
Y.MorozumiandY.Saito,Effectofphysicalpropertiesonmicroexplosionoccurrenceinwater-in-oilemulsion droplets,Energ.Fuel.,24(3):1854–1859,2010DOI:http://doi.org/10.1021/ef9014026
L.L.Schramm(ed.),Emulsions:FundamentalsandApplicationsinthePetroleumIndustry,American ChemicalSociety,1992
D.O.Shah(ed.),Macro-andMicroemulsions:TheoryandApplications,AmericanChemicalSociety,1985
C.G.Sumner,Clayton’sTheTheoryofEmulsionsandTheirTechnicalTreatment,5thed.,Blakiston,1954
