2 Surface Tension and Its Measurement

212SurfaceTensionandItsMeasurement2.1IntroductionAdhesionisaninterfacialphenomenonthatoccursattheinterfacesofadherendsandadhesives.Thisisthefactunderlyingthemacroscopicpro-cessofjoiningpartsusingadhesives.Anunderstandingoftheforcesthatdevelopattheinterfacesishelpfulintheselectionoftherightadhesive,propersurfacetreatmentofadherends,andeffectiveandeconomicalprocessestoformbonds.Thischapterisdevotedtothediscussionofthethermodynamicprinciplesandtheworkofadhesionthatquantitativelycharacterizethesurfacesofmaterials.Laboratorytechniquesforsurfacecharacterizationhavebeendescribedwhichallowanunderstandingofthechemicalandphysicalpropertiesofmaterialsurfaces.2.2WhatisanInterface?Twosolidorliquidphasesincontacthaveatoms/moleculesonbothsidesofanimaginaryplanecalledtheinterface.Theinterfacialparticlesdifferenergeticallyfromthoseinthebulkofeachphaseastheyarepresentontheboundaryoftherespectivephaseandinteractwiththeparticlesoftheotherphase.Thecompositionandenergyvarycontinuouslyfromonephasetotheotherthroughtheinterface.Thisregionhasafinitethickness,usually0.1μm.[1,2]2.3SurfaceTensionThemoleculesofaliquidareheldtogetherbyattractionforces.Thesumofallattractiveforcesonanymoleculepresentinthebulkofaliquidaver-ageszero.Thenetforce(alsoknownascohesionforce)onasurfacemol-eculeisanon-zeroquantityinthedirectiontowardsthebulk(Figure2.1).Thisistheforcethatmustbecounteractedtoincreasethesurfacearea;theenergyconsumedbythisprocessiscalledsurfaceenergy.Theunbalancedforcesontheinterfacecauseittocontracttotheminimum.Waterdropletsaresphericalbecauseaspherehasminimumsurfaceareaforagivenvol-umeamongallgeometricshapes.Surfacetensionandsurfacefreeenergyofaliquidareequalwhilethesameisnottrueforasolidsurface.Ebnesajjad_Ch02.indd216/5/200810:15:59AM22AdhesivesTechnologyHandbookSurfacetensionisdefinedastheworkrequiredtoincreasetheareaofasurfaceisothermallyandreversiblybyunitamount.Surfacetension(g)isexpressedasthesurfaceenergyperunitareaandalternativelyastheforceperunitlength.Surfacetensionofliquidscanbemeasureddirectlyandexpressedintheunitsofforceperunitarea(dyn/cm),whichisthesimpli-fiedformofenergyperunitsurfacearea(erg/cm2=dyncm/cm2=dyn/cm).Thechallengehasbeentofindmethodstodeterminethesurfacetensionofsolidssurfaces.Surfacetensionofpolymerscanbedividedintotwocomponents—polar(gp)anddispersion(gd)—toaccountforthetypeofattractionforcesattheinterfaces.[3]Thechemicalconstitutionofthesurfacedeterminestherela-tivecontributionofeachcomponenttothesurfacetension.Thepolarcom-ponentiscomposedofvariouspolarmolecularinteractionsincludinghydrogenbonding,dipoleenergy,andinductionenergy,whilethedisper-sioncomponentarisesfromLondondispersionattractions.Theattractiveforces(vanderWaalsandLondondispersion)areadditive,whichresultsinthesurfacetensioncomponentstobeadditive:g=gp+gd.Figure2.1Liquid–liquidinterfaceandbalanceofforcesonmoleculesofliquids.Liquid2Resultantforce2Liquid1Resultantforce1Ebnesajjad_Ch02.indd226/5/200810:15:59AM2:SurfaceTensionandItsMeasurement232.4SurfaceFreeEnergyAhypotheticalexampleisusedtodescribetheconceptofsurfacefreeenergy.Supposeaboxisfilledwithaslidingcover(Figure2.2).Theslid-ingcoverisassumednottohaveanyinterfacialtensionwiththeliquid.IfthecoverisslidbacktouncoverasurfaceareaofdA,thenecessaryrevers-ibleworkwillbe(gdA).Forapuresubstance,theincreaseinthefreeenergyofthesystematconstanttemperatureandpressureisdefinedbyEq.(2.1).gdGdA(2.1)Thetotalfreeenergyofthesystemiscomposedoftheenergyofthebulkliquidandthesurfaceliquid.Thelatterisequaltothesurfacefreeenergyperunitarea(GS)multipliedbythesurfaceareaasshowninEq.(2.2).CombiningEqs.(2.1)and(2.2)resultsinEq.(2.3),whichillustratesthatthefreesurfaceenergyofapuresubstanceisequaltoitssurfacetension.SdGGdA(2.2)g⎡⎤⎢⎥⎣⎦S,TPdGGdA(2.3)2.4.1SurfaceEnergyofSolidsAsolidisdefinedasamaterialthatisrigidandresistsstress.Asolidsur-facemaybecharacterizedbyitssurfacefreeenergyandsurfaceenergy.Thesurfaceenergy(tension)ofasolidcannotbemeasuredinasimilarmannertothatofaliquid,duetothedifficultycausedbythereversibleformationofitssurface.Themethodsforthedeterminationofsurfaceenergyofsolidsaredescribedinthischapter.Figure2.2Anidealliquidbox.dAEbnesajjad_Ch02.indd236/5/200810:16:00AM24AdhesivesTechnologyHandbookSolidmaterialsurfacescanbedividedintotwocategories,thosewithhighandlowsurfaceenergy.[4]High-surfaceenergymaterialsincludemetalsandinorganiccompoundssuchasoxides,silicates,silica,diamond,andnitrides.Thesurfacetensionofhigh-energymaterialsisintherange200–500dyn/cm.Low-energymaterialsaremainlycomposedoforganiccompoundsincludingpolymerswithcriticalsurfacetension100dyn.Polymersurfaceshavebeenfurtherclassified[5,6]aslow,medium,andhighsurfaceenergy.Low-surfaceenergysolidshavecriticalsurfacetensionintherangeof10–30dyn/cm,mediumenergyfrom30to40dyn/cm,andhighenergy40dyn/cm.Low-surfaceenergymaterialssuchasoilsarespontaneouslyabsorbedbythehigh-energysurfacesbecauseofthereductioninthefreesurfaceenergyofthesystem.Thismeansthataclean,high-energysurfaceexposedtothenormalambientenvironmentwillnotremaincleanforlongbecauseoftheabsorption