Surfactant

SurfactantTheUbiquitousAmphiphilesWhatareSurfactants?ThenamesurfactantstandsforSURFaceACTiveAgeNT.Asurfactantisbrieflydefinedasamaterialthatcangreatlyreducethesurfacetensionofwaterwhenusedinverylowconcentrations.Table1PercentConcentrationrequiredtoreducethesurfacetensionofwatertoindicatedvaluesSurfacetension,dynes/cm7350403022Softanol.仲醇聚氧乙烯9000.0030.0080.005---Ethanol091840100IUPACdefinitionsofsurfactantisasubstancewhichlowersthesurfacetensionofthemediuminwhichitisdissolved,and/ortheinterfacialtensionwithotherphases,and,accordingly,ispositivelyadsorbedattheliquid/vapourand/oratotherinterfaces.Thetermsurfactantisalsoappliedcorrectlytosparinglysolublesubstances,whichlowerthesurfacetensionofaliquidbyspreadingspontaneouslyoveritssurface.Mostfamiliarofallsurfactantsissoap,asimplesubstancewhich,inwater,clearlydemonstratestwoeffects.Itproducesfoamduetoitsactionattheair–waterinterfaceItmakesthegreasetransferfromgrubbyhandsintothesoapywaterasaresultofitsactivityatthewater–oil(grease)interface.However,soapwasprobablynotthefirstsurfactantintheserviceofhumankind.Manyplantsproducesignificantquantitiesofsaponins(皂角苷)whichhavesurfactantproperties.OnesuchplantisthesoapwortSaponariaofficinaliswhosefoliageyieldsaglycoside(苷)capableofwetting,foamingandgreasedispersion–theveryqualitiesthatwerecogniseinamoderndetergent.Itislikelythatthesaponinsprovidedourancestorswithourfirstusefulsurfactants.Thesenaturalglycosidesarestillinusetodayforspecialisedprocessessuchaswashingdelicatefabrics.Modernsurfactants,however,areofmanydifferentchemicaltypesanddofarmorethanproducefoamsanddispersegrease.Surfactantscanbebroadlydefinedascompoundswhich,whendissolvedinwater,concentrateatsurfaces(interfaces)suchaswater–airorwater–oil.Theinterfacialactivityofthesesubstances,whichcanbeexplainedintermsoftheirmolecularstructure,givesrisetoawiderangeofsurfacechemistryfunctions:wetting,emulsifying,solubilising,foaming/defoaming,rheology-modifying,antistatic,‘glossing’,lubricityandsurfaceconditioning.Fig1.TheglobalsurfactantmarketTable2.ThemajorsurfactantmarketsValue(£m)Quantity(kt)Householddetergents28004000Industrialandinstitutionalcleaning420530Personalcare940860Cropprotection290200Oilfield390440Paintsandcoatings140160Textilespinfinish200160Textileauxiliaries450500Construction190470Emulsionpolymerisation240290Food190200Leather3060ORE/mineral60150Plasticadditives6040Pulpandpaper100120Explosives1010Other630380Total71408570Seldomaresurfactantsontheirownputdirectlyintouse.Intheareaofhouseholdcleaningpreparations,thesurfactantisnormallyblendedwitharangeofingredientssuchasothersurfactants,thickeners,foamingordefoamingagents,alkalis/salts,chelatingagentsandsoon.ThisistheprovinceofspecialistformulatorssuchasColgatePalmolive,Unilever,andProcter&Gamble.Generally,thesemodernformulatedproductsrelyonman-madesurfactantsbutthereareexceptions.Certainfoods,inparticular,includenaturalsurfactantssuchaslecithin(卵磷脂),anemulsifierinchocolateandicecreammanufacture.Naturealsoreliesheavilyonsurfactantchemistry:ourliverproducessurfactants,thebileacids;ourlungsusesurfactantstomaximisetheefficiencyofgasexchangeacrosstheair–waterinterface;andineverycellineveryorganinourbodiesthereisacomplexmembranethatfunctionsduetosurfactantchemistry.PropertiesofSurfactantsSurfactantsarecharacterisedbytheessentialfeaturesthatwesawinsodiumoleate(油酸盐).Asurfactantmoleculehasahydrophilic(water-loving)headandalonghydrophobic(water-hatingoroil-loving)tail.Forthisreason,weoftendescribesurfactantsisbeingamphiphilicmolecules-theyloveeverything.Surfactantsaresaidtohaveaheadandatail.Theheadishydrophilicwhichmeansthatitiswaterloving,anditisgenerallydepictedasacircle.Thetailisgenerallyalonghydrocarbonchainandishydrophobic,whichmeanswater-hating(thereforeoil-loving).Thetailmaybedepictedeitherasastraightlineorawavytail.Themolecularstructureofsurfactantsmeansthattheyhaveunusualproperties,leadingtobothwidespreadandhighlyspecialisedapplications.Thepropertiesofsurfactantsfallintotwobroadcategories:adsorptionself-assembly.AdsorptionAdsorptionisthetendencyforasurfactantmoleculetocollectasaninterface.Adsorptionisverydifferenttoabsorption(don'tgetthemconfused!):Theadsorptionpropertiesofsurfactantsmeanthatsurfactantmoleculesareusuallyfoundattheinterfacebetweenanoilphaseandawaterphaseorawaterphaseandanairphase.Thismolecularpropertyleadstothemacroscopicpropertiesofwetting,foaming,detergencyandemulsionformation.Surfactantmoleculestendtoadsorbtothesurfaceofoildroplets.Thehydrophilicheadsstickoutintothewaterphase,whilethehydrophobictailshappilystickintotheoilphase.Self-AssemblySelf-assemblyisthetendencyforsurfactantmoleculestoorganisethemselvesintoextendedstructuresinwater.Thisincludestheformationofmicelles,bilayersandliquidcrystals.Thesestructuresareformedbywhenthehydrophobictailsofthesurfactantsclustertogether,formingsmallaggregatessuchasmicelles,orlargelayerstructures(bilayers)whicharesimilartoacellwall.Thesepropertiesmakesurfactantsveryinterestingstudy,andareareasofcurrentresearch.BasicSurfact