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1、4Liquid-gasandliquid-liquidinterfacesStateofliquidsurfaceMolecules in the interior experiencean attractive force from neighboringmolecules which surround on allsidesMolecules on the surface haveneighboring molecules only on oneside(thesidefacingtheinterior)andthus experience an attractive forcewhich
2、 tends to pull them into theinteriorTheoverallresultofthisasymmetricforceonsurfacemoleculesisthat:ThesurfaceoftheliquidwillrearrangeuntiltheleastnumberofmoleculesarepresentonthesurfaceInotherwordsthesurfaceareawillbeminimizedAspherehasthesmallestsurfaceareatovolumeratioThesurfacemoleculeswillpacksom
3、ewhatclosertogetherthantherestofthemoleculesintheliquidThesurfacemoleculeswillbesomewhatmoreorderedandresistanttomoleculardisruptionsThus,thesurfacewillseemtohaveaskinTheinwardmolecularattractionforces,whichmustbeovercometoincreasethesurfacearea,aretermedthesurface tensionSurfacetension/surfacefreee
4、nergy():theworkrequiredtoincreasetheareaofasurfaceisothermallyandreversiblybyunitamount.Influencingfactorsonsurfacetension1.Intrinsicpropertyofthematterliquid(mNm-1)WaterFluorocarbonsHydrocarbonsPolarityorganicsubstancesDetergent-watersolutionMeltedglassMeltedmetal72.88151830225024402004003501800Sur
5、facetensionfordifferentliquids2.Temperature3.Thesurfacetensionofmostliquidsdecreaseswithincreasingtemperatureinanearlylinearfashionandbecomesverysmallintheregionofthecriticaltemperature.Ramsay-ShieldsequationWhereisthemolarvolumeoftheliquid,kisaconstantandTcisthecriticaltemperature3.PressureTheinflu
6、enceofpressureonsurfacetensioniscomplex.Commonly,thesurfacetensiondecreaseswithincreasingpressure.2.RingmethodRingmethodwasdevelopedbyLecomteduNouy,aFrenchPhysicist.Thismethodisameasuringmethodofsurfacetensionbymeansofaprecisionbalancewithhangingprobeassembly,aplatinumringwithdefinedgeometricaldimen
7、sionsandaprecisionliftmechanismtoverticallymovesampleliquidinaglassbeaker.1.Theringhangingfromthebalancehookisfirstimmersedintotheliquid2.Theringiscarefullypulledup,byloweringthesamplecup,throughthesurfaceofliquid.3.Thegravimetricassemblyisrecordingthetorquewhilepullingthroughthesurfaceortheinterfac
8、e.Thesurfacetensionorinterfacialtensionisthemaximumforceneededtodetachtheringfromtheaqueousliquidsurface.Wtot=Wring+4pRWhereWringistheweightofthering,Ristheradiusofthering,andthesurfacetension.However,theshapeoftheliquidsupportedbytheringiscomplexandthedirectionoftensionforcesarenon-vertical.Thecorr
9、ectionfactorshouldbeintroduced.F2R3.WihelmyplatemethodsDetachmentmethodMica（云母）plateormicroscopeslidepoisecontainerxyGraduallylowerthepoisetothepointofdetachmentAssumingzerocontactangle,thenthesurfacetensioncanbeexpressedasWhereWpistheweightofpoiseandWistheweightoftheplate4.Drop-volumeanddrop-weight
10、methodswhere,isthecorrectionfactor,dependingontheratior/V1/3.(seepage75)Note:Asuitabletipshouldbecarefullyselectedandthelastpercentofthedropshouldbeformedveryslowly2r5.OscillatingjetmethodOscillatingjetset-up(Thomans&Potter,1975)6.Maximumbubblepressuremethod(a)(b)(c)Themaximumbubblepressureisreached
11、atR=r.Thesurfacetensioncanbecalculatedbythefollowingequation:=PR/22rRPMaximumbubblepressureexperimentalset-up(Sugden,1922)mercuryPressuregaugeCapillariesMaximumbubblepressureexperimentalset-up(Bendure,1971)Twocapillaryunitdesign(Razouk&Walmsley,1974)Aftercorrection7.OthermethodsSurfacepotentialmetho
12、dFallingmeniscusmethodInthismethod,atubeisloweredintobulksolutionuntilameniscusisformedatthesmallcapillaryorificeatthetopofthetube.Thebulksolutionisthenlowereduntilacriticalheight,h0,markedbytheruptureofthemeniscus.TherelationshiptosurfacetensionisWhere:risthecapillaryradiusandh0istheheightofmeniscu
13、sbreakabovebulksolutionSummaryCapillaryrisemethodAcommonmethodtodeterminesurfacetensionofpureliquid.Aftercorrection,theaccuracyofthismethodreaches0.05%.RingmethodTheoperationofthismethodisverysimple.However,theinfluencefactorsarecomplex,forexample,contactangle,sizeofthecontainerandhorizontalityofthe
14、ring.WihelmyplatemethodsAcommonmethodtodeterminesurfacetension.Itdoesnotneedthedataofdensityandcorrection.Theuniquerequirementisthattheliquidmustwettheplatewell.Drop-volumeanddrop-weightmethodsTherelativedeviationiswithin0.1%,dependingontheHarkins&Brownscorrection.OscillatingjetmethodThemainadvantag
15、einthismethodliesintheaccuracyinwhichthesurfaceagecanbecalculated.AdsorptionandorientationatinterfacesSurfaceactivityThestrongadsorptionofsuchmaterialsatsurfacesorinterfacesintheformofanorientatedmonomolecularlayeristermedsurfaceactivity.SurfactantAsurfactantisbrieflydefinedasamaterialthatcangreatly
16、reducethesurfacetensionofwaterwhenusedinverylowconcentrations.Aparticulartypeofmolecularstructureperformsasasurfactant.Thismoleculeismadeupofawatersoluble(hydrophilic)andawaterinsoluble(hydrophobic)componentHydrophobeHydrophileSchematicSketchofSurfactantMoleculesinWaterThehydrophilicendofthesurfacta
17、ntisstronglyattractedtothewatermoleculesandtheforceofattractionbetweenthehydrophobeandwaterisonlyslight.Asaresult,thesurfactantmoleculesalignthemselvesatthesurfaceandinternallysothatthehydrophileendistowardthewaterandthehydrophobeissqueezedawayfromthewater.Thechainswilltendtoliehorizontallyontopofth
18、ewatersurfaceatlowcoverage,butwilltendtoassumemoreuprightconfigurationsathighcovergage.ClassificationofsurfactantsTheprimaryclassificationisdependonthetypeofthehydrophilicgroups.Accordingtothechargecarriedbythesurfaceactivepartofthemolecule,surfactantsareclassifedasanionic,cationic,non-ionic or amph
19、olytic.Anotherclassificationisrelatedtotheirtechnologicalapplication.Surfactantsarenamedasdetergent,wetting agent,emulsifier and dispersant.Surface-activeagentsAnionicCarboxylatesRCOOMSulphatesR-O-SO3MSulphonatesR-SO3MPhosphatesR-O-PO3MCationicammoniumsR-NH3AAmineRnNHmANon-ionicPolyethyleneoxidesR-O
20、(CH2CH2O)nHSpansTweensAmpholyticDodecylbetaineGibbsadsorptionequationAdsorption:Theaccumulationofsubstancesontheinterface.Surfaceexcessconcentration:Thedifferenceinconcentrationbetweensurfaceandinnerduetotheadsorptionistermed“surfaceexcess”.Surfaceexcessconcentrationisquantitydefinedbywhereniisthesu
21、rfaceexcess(oradsorption)ofcomponentiandAistheareaofthedividingsurfaceorinterfaceSSSSABABInterfaceThechangeofsurfaceenergycanbeexpressedbyWhereiissurfaceexcessconcentrationof“i”anddithechangeinchemicalpotentialof“i”.Atequilibrium:di=RTdlnai,whereaiistherelativeactivityof“i”.Fortwo-componentsolutionT
22、hereforeIfthesurfaceatwhichthesurfaceexcessconcentrationofthesolvent(A)iszeroisselectedasthedividingsurface,theaboveexpressionthensimplifiestoFordilutesolutionCBaBSurfaceexcessgivenbyslopeofplotofvslnCKnowing,areapermoleculeattheinterfacecanbecalculated.ExerciseThefollowingsurfacetensionsweremeasuredforaqueoussolutionsofn-pentanolat20C:C(moldm-3)00.010.020.030.040.050.060.080.10(mNm-1)72.664.660.056.854.351.949.846.043.0Calculatesurfaceexcessconcentrationsandtheaverageareaoccupiedbyeachadsorbedmoleculeforbulkconcentrationsof0.01,0.02,0.04and0.08moldm-3.