1.1.Absorption,Adsorption,Haemosorption(Definitions)

Part 15.Surface phenomena. Adsorption 1

PART15
SURFACEPHENOMENA.ADSORPTION

1.GENERALUNDERSTANDINGSABOUTSORPTIONPROCESSES.

1.1.ABSORPTION,ADSORPTION,HAEMOSORPTION(DEFINITIONS).

Sorptionprocessesincludealltheprocesses,inwhichonesubstanceisboundtothesurfaceorinthevolumeofanothersubstance.

Sorptionprocessesareclassifiedasfollows:

1.Absorptionisaprocessinwhichonesubstancebindsanotherinitsvolume.

Forinstance,whengaseousNH3isbubbledthroughwater,itisabsorbedanddistributedevenlyinthewholevolumeofwater.

2.Adsorptionisaprocess,inwhichonesubstancebecomesboundtothesurfaceofanother.

Forinstance,adsorbsaceticacidformitswatersolutionandthemoleculesofaceticacidareattachedtothesurfaceofcoalparticles.

3.Haemosorptionisasorptionprocess,inwhichonesubstancebindsanotherbymeansofchemicalinteraction.

Inthiscaseitisdifficulttoclassifybetweenabsorptionandadsorption,becausetheprocessesofchemicalsorptionusuallybeginonthesurfaceandthencontinueinthevolume.

Inthefurthertextletusunderstandtheterm“adsorption”asjustaphysicalprocesswithnochemicalinteraction.

Thedifferencebetweenthephysicaladsorptionandhaemosorptioncanbeseenexperimentally,whentheinfluenceoftemperatureontheprocessisstudied.Growthoftemperaturealwayscausesadecreaseofphysicaladsorption,astheintensityofthethermalmotionofmoleculesincreaseswiththegrowthoftemperatureandthereforetheweakphysicalinteractionforces(VanderVaals’sforces)betweenthemoleculeandthesurfacecannotkeepthemoleculeonthesurface.

Theinfluenceoftemperatureonhaemosorptionhasjustoppositecharacter-ifachemicalreactionbetweenbothmoleculeandsurfaceoccurs,thereactionrategrowsbiggerwiththeincreaseoftemperature.

POSITIVEANDNEGATIVEADSORPTION.

Thatsubstance,whichbindsmoleculesofanothersubstancetoitssurfacewillbefurthercalledadsorbent.

Thatsubstance,themoleculesofwhichareboundtothesurfaceofanothersubstance,willbefurthercalledadsorbate.

Forinstance,ifmoleculesofCO2areboundtothesurfaceofsolidcarbon,carbonactsasanadsorbentandCO2asanadsorbate.

Infact,adsorptionisnotjustbindingofamoleculetothesurfaceofanothersubstance.Amoreprecisedefinitionofadsorptionisasfollows:

Adsorptionisachangeofconcentrationofonecompoundnearthesurfaceofanothercompound.

Adsorptioncanoccuratsurfacesofbothsolidandliquidsubstances.Ifadsorptionoccursonasurfaceofasolidsubstance,adsorptionisusuallyunderstoodasbindingofmoleculesofgas(orsolute,ifsolidsubstanceisincontactwithasolution)tothesurfaceofsolidsubstance.

Ifadsorptionoccursatthesurfaceofliquid,itisusuallyunderstoodasachangeofsolute’sconcentrationinthesurfacelayerofsolution.Forinstance,ifwearedealingwithawatersolutionofaceticacid,adsorptionisunderstoodasanincreaseofCH3COOHconcentrationinthesurfacelayerofthesolution,seefig.15.1.a.

Ifweconsideradsorptionaschangeofonecompound’sconcentrationneartosurfaceofothercompound,adsorptioncanbebothpositiveornegative,seefig.15.1.

Fig.15.1.Positive(a)andnegative(b)adsorption.

Letusconsideravesselwithsolution.Onecantakesamplesofsolutionfromthevolumeofsolutionandfromthesurfacelayerofthesolution.TheconcentrationsofsoluteinthesesamplesconcentrationsofsoluteinbothsamplesCS(fromthesurfacelayer)andCV(fromthevolumeofsolution)willbedifferent.

IfCSCV(theconcentrationinthesurfacelayerisgreater,thaninthevolume,or,inotherwords,thesolutemoreeagerlyresidesinthesurfacelayer,thaninthevolume),adsorptionispositive.

If,incontrary,CSCV(or,inotherwords,soluteresidesmoreeagerlyinthevolumeofsolutionandescapesfromthesurfacelayer),adsorptionisnegative.

1.3.MEASURINGUNITSOFADSORPTION.

Differentmeasuringunitsareusedforadsorption,butallthesedifferentkindsofmeasuringunitsexpresstheamountofadsorbate,boundonagivenamountofadsorbent.

Ifitisreallypossibletodeterminethesurfaceareaoftheadsorbent(forinstance,ifadsorptionoccursonasurfaceofaliquid),adsorptionismeasuredintheunitsmoles/m2org/m2,showingthenumberofmolesorthenumberofgramsofadsorbate,thatisadsorbedon1m2ofadsorbent’ssurface.

Averycommoncaseis,thatthesurfaceareaofadsorbentisunknown.Thishappensinthecaseofsolidadsorbents,becausethedeterminationofsurfaceareaofsolidsisadifficultandcomplicatedtask.Insuchcasesadsorptionisexpressedinunitsmoles/gorg/g,expressingthenumberofmolesorthenumberofgramsofadsorbate,thatareadsorbedonthesurfaceof1gofadsorbent.

Thus,all-in-all,adsorptioncanbeexpressedinunitsmoles/m2,g/m2,moles/gorg/g.

1.4.TYPESOFADSORPTIONANDTHEIRGENERALPROPERTIES

First,adsorptionisdifferent,ifasubstanceisadsorbedinmolecularorionicform.Adsorptionofionshasitsownspecificproperties,thatwillbediscussedinchapter5.

Ifasubstanceisadsorbedinamolecularform(molecularadsorption),thenadsorptiondependsontheaggregatestateoftheadsorbentandtheadsorbate:

1)adsorptioninthefrontierliquid/gas.Inthiscaseinthefrontierbetweenliquidandgasthereareadsorbedmoleculesoftheso-calledsurfaceactivecompounds(seeexplanationinthefurthertext),thataredissolvedintheliquid.

2)adsorptioninthefrontierliquid/liquid.Inthiscasethemoleculesofsurfaceactivecompounds,thatwereinitiallydissolvedinoneorbothliquids,areadsorbedonthefrontieroftwoliquids,whichareinsolubleineachother(forexamplewaterandbenzene).

3)adsorptioninthefrontiersolid/gas.Inthiscasethemoleculesofgasareadsorbedonthesurfaceofasolidsubstance.

4)adsorptioninthefrontiersolid/liquid.Inthiscasethemoleculesofsolutefromtheliquidphaseareadsorbedonthesurfaceofthesolidsubstance.

1.5.ADSORPTIONEQUILIBRIUM

Whenaprocessofadsorption(bindingofthemoleculestothesurfaceofanothersubstance),begins,thecontraryprocess-desorptionorliberationofadsorbedmoleculesfromthesurfacebegins,too.Aftersometime,anequilibriumisreached-thenumberofmolecules,thatareadsorbedonthesurfaceataunitoftimeisequaltothenumberofmolecules,thataredesorbed(leavethesurface)andtheamountofadsorbedsubstancedoesn’tchangeintimeanymore.

Thisadsorptionequilibriumcanbeexpressedas:A+SA(S),where

Aisamoleculeofadsorbate,

Sisafreesite(place)inthesurfaceofadsorbent,

A(S)isamolecule,adsorbed(bound)tothesurfacesite.

Atdifferentconcentrationsofadsorbatedifferentadsorptionequilibriumsarereached-thegreateristheconcentrationofadsorbate,thegreaterwillbethenumberofmoleculesofadsorbate,thatareboundtothesurfaceofadsorbentattheequilibriumstate.

II.ADSORPTIONINTHEFRONTIERLIQUID/GAS

Solutescanbeadsorbedonthesurfaceofsolution(atthefrontierbetweenthesolutionandair),iftheyaresurfaceactive(seechapterII.2.)

Theabilityofaliquidsurfacetoadsorbmoleculesofsoluteiscausedbythefreeenergyoftheliquidsurface.

II.1.FREEENERGYOFLIQUIDSURFACE.SURFACETENSION.

Letusconsiderapuresolventandletusdiscussthedifferencebetweenstateofasolvent’smoleculeinthevolumeofsolventandinthesurfacelayerofit,seefig.15.2.Theattractionforcesofthesurroundingmolecules(shownbyarrowsinfig15.2.)actoneverymoleculeofsolvent.

Forasolventmolecule,situatedinthevolume,theseforcesactequallyinallthespatialdirectionsandthereforetheycompensateeachother.

Thestateofamoleculeinthesurfacelayerisdifferent-practicallynoattractionforcesactatthemoleculefromthesideofgas(gasismuchmorerarefiedwhencomparedtoliquid)andthereforethecomponentattractionforces,thatisorientedtowardstheinsideofliquid(arrowdowninfig.15.2)isnotcompensated,thereforearesultingforce,actinginthedirectiontowardsinsideofliquidremains.

Fig.15.2.Stateofsolvent’smoleculeinthevolume
ofsolventandinthesurfacelayerofit.

Thenon-compensatedattractionforces,thatactinthedirectiontowardsinsideofliquid,causetheso-calledfreesurfaceenergyofliquidES.

Thesurfacefreeenergyisproportionaltothesurfaceareaofliquidandthisproportionalitycanbeexpressedas:

ES=S,(15.1)

whereSistheliquidsurface,m2

Theproportionalitycoefficientsiscalledthesurfacetensionoftheliquid.Thevalueofsurfacetensionshows,howgreatisthesurfacefreeenergyof1mofliquidsurface.Themeasuringunitofis

Ontheotherhand,as1J=1N1m,theunitofscanbealsodefinedas

(15.3)

From(15.3)onecansee,that:

Surfacetensioncanbealsodefinedasaforce,withwhichan1mbroadsurfacetendstocontract.

II.2.SURFACEACTIVITYANDSURFACEACTIVECOMPOUNDS

Asitisknownfromthermodynamics,aprocess,inwhichthefreeenergyofasystemisdecreasing,occursspontaneously.Forthisreasonacompoundcanbeadsorbedonthesurfaceofliquid,ifthepresenceofthiscompoundinthesurfaceofliquiddecreasesthesurfacefreeenergy.

Compounds,thatareadsorbedatsurfacesofliquidsandcauseadecreaseofthesurfacefreeenergy(surfacetension)ofliquid,arecalledsurfaceactivecompounds(anabbreviationSACwillbeusedforsurfaceactivecompoundsinfurthertext)

II.2.1.STRUCTUREOFSACMOLECULES

Tobesurfaceactive,amoleculehastohaveaspecialsortofstructure.Bythemost,SACareorganiccompoundswithabiphilestructure-themoleculeconsistsoftwoparts,oneofwhichisahydrophobichydrocarbonradicalandtheotherisapolarfunctionalgroup,whichishydrophilic.

Bythemost,themoleculeofSACisschematicallydrawnas:

wherethepolargroupisshownasa“head”ofmoleculeandthenon-polarhydrocarbonradicalisshownasa“tail”ofmolecule.

Suchakindofmolecularbuildupischaracteristicfor:

organicacidsR-COOH(R-hydrocarbonradical)

alcoholsR-OH

aminesR-NH2

andsomeotherclassesoforganiccompounds.

Inordertounderstand,whythemoleculeshavingabiphilestructurearesurfaceactive,letusthink,whathappens,ifsuchamoleculeisdissolvedinwater.Waterisapolarcompound,thereforestrongintramolecularinteractionforcesactbothbetweeneachtwoneighboringwatermoleculesandbetweenwatermoleculesandpolargroupsofSAC.

Ifanonpolar“tail”ofSACmoleculeisplacedbetweentwoneighboringwatermolecules,itinterferestheattractionforcesbetweenwatermoleculesandthereforelocationof“tails”ofSACmoleculesinthevolumeofwaterisenergeticallyinconvenient.Forthisreason,the“tails”ofSACmoleculesarepushedoutofthevolumeofsolutiontoitssurface,seefig15.3a.

Astheresult,theconcentrationofSACmoleculesinthesurfacelayerofsolutionisgrowing-apositiveadsorptiontakesplace.ThemoleculesofSACinthesurfaceofwaterareorientedso,thattheirhydrophilic“heads”(polargroups)areinsidewater(becauseattractionforcesactbetweenthesepolargroupsandwatermolecules),butthenon-polargroups(“tails”)areoutsidewater.

Ifwechooseanon-polarsolvent(benzene),thesituationisviceversa-nowthepositioningofpolargroupsinnon-polarmediaisenergeticallyinconvenient.Astheresult,moleculesofSACarealsolocatedmoreinsurfacelayer,thaninthevolumeofsolution(apositiveadsorptiontakesplace),butnowthenon-polargroups(“tails”)areorientedtowardsvolumeofsolutionandpolargroupsareorientedtowardsoutsideofsolution,seeFig.15.3,b.

ThelocationofSACmoleculesinthesurfacelayerofsolutiondecreasesthenumberofsolventmoleculesinthesurfacearea,thusdecreasingthesurfacetension(and,consequently,thesurfacefreeenergy)ofthesolvent.

Fig.15.3.OrientationofSACmoleculesinthesurfaceof
a)polarsolvent(water)b)nonpolarsolvent(benzene)

Thesurfaceactivity,G,isfoundas:

anditisdefinedasthechangeofsurfacetensionofthesolution,causedbyanincreaseofSACconcentrationfor1mole/l.(Occasionally,thesamesymbolisusedforbothsurfaceactivityandGibbs’senergy.Note,thatthesetwounderstandingsarecompletelydifferent!)

II.2.2.COMPARISONOFTHESURFACEACTIVITYOFDIFFERENTSAC.TRAUBE’SRULE.

ThesurfaceactivityincreaseswiththeincreaseofthelengthofhydrocarbonradicalinthemoleculeofSAC.Itiseasytounderstandthisphenomenon,ifweremember,thatalongernon-polarradicaldisturbsthemutualinteractionofthepolarwatermoleculesmoreefficientlythanashorterone.Forthisreason,thelongeristhehydrocarbonradicalofaSAC,themoreintensivelyitwillbepushedoutfromthesolutionbywatermolecules.

Wheninvestigatingthesurfaceactivityofunivalentorganicacids,Traubefoundout,that:

Forunivalentorganicacidsanincreaseofthelengthofhydrocarbonradicalbyone-CH2-groupcausesanincreaseofsurfaceactivity3-5times.(Traube’srule).

ThisruleisvalidforotherclassesofSAC,aswell.TocomparethesurfaceactivityofSACfromdifferentclassesoforganiccompounds,onecanusethefollowingcriterion:

TheloweristhesolubilityofgivenSACinthegivensolvent,thegreaterisitssurfaceactivity.

II.3.CALCULATIONOFADSORPTIONVALUEINTHEFRONTIERLIQUID/GAS.

ThevalueofadsorptionofaSACinthesurfaceofsolution(inthefrontierbetweensolutionandair)isfound,usingGibbs’sequation:

,where:(15.5)

isthevalueofadsorption,moles/m2;

isthesurfacetensionofthesolution,J/m2;

CistheconcentrationofSAC,moles/l;

Risuniversalgasconstant;

Tisabsolutetemperature,K

Inapproximatecalculationstheequation(15.5a)isused:

(15.5a)

Thensurfacetensions1and2areexperimentallymeasuredattwodifferentconcentrationsofSACC1andC2,andtheirdifference

∆=1-2

isfound.Theotherparametersintheequation(15.5a)are:

∆C=C1-C2and

Theminus(“-”)signintheequations(15.5)and(15.5a)istheresultofthefact,thatthesurfacetensiondecreasesatpositiveadsorption.

Ifacompoundissurfaceinactive(likemostinorganiccompounds,e.g.NaCl),anincreaseofitsconcentrationcausesanincreaseofthesurfacetensionand,astheresult,adsorptionisnegative-theconcentrationofcompoundinthesurfacelayerissmaller,thaninthevolumeofsolution.

AdsorptionofSACinthefrontierbetweenliquidandgas,asitcanbeseenfromGibbs’sequation,dependsonthefollowingfactors:

1)ConcentrationofSAC-thegreateritis,thegreaterisadsorption,

2)temperature-thegreateritis,thesmallerbecomesadsorption,

3)surfaceactivityofsolute(-d/dC)-thegreateritis,thegreaterisadsorption.

II.4.DEPENDENCEOFADSORPTIONONTHECONCENTRATIONOFSAC(ADSORPTIONISOTHERM).

Adsorptionisothermisacurve,thatshowsthedependenceofadsorptionontheconcentrationofSACatagiventemperature,seefig.15.4.

Asonecansee,theadsorptionisothermhas3areas(seetheorientationofSACmoleculesinthese3areasinfig.15.5):

1starea-whileconcentrationofSACisverysmall,allthesurfaceofliquidisfreeandamoleculeofSACimmediatelyfindsafreeplaceinthesurface,thereforeisproportionaltoC.AtthissituationamoleculeofSAC“lies”onthesurfaceofliquid.

2ndarea-whentheconcentrationofSACgrowsbigger,notallthesurfaceofliquidisfreeanymoreandthemoleculesofSAChaveto“seek”forafreeplaceinthesurface,thereforethereisanegativedeviationfromthedirectproportionalitybetweentheadsorptionandconcentrationofSAC.

3rdarea-AtverygreatconcentrationsofSACallthesurfaceofliquidisalreadyoccupiedbySACmoleculesandafurtherincreaseofSACconcentrationdoesn’tcauseanyincreaseofadsorption-themaximaladsorptionisalreadyreached.MoleculesofSACinthisareastandvertically,formingtheso-calledLangmuir’sfence.

Asonecansee,theconsiderationsaboutSACconcentrationandvalueofadsorptionareverymuchaliketothoseabouttheconcentrationofsubstrateandtherateofenzymecatalyzedreaction.

Fig.15.4.Adsorptionisotherm Fig.15.5.OrientationofSAC. moleculesatdifferent regionsofisotherm.

II.5.THECONNECTIONBETWEENISOTHERMSOFSURFACETENSIONANDADSORPTION.

BothisothermofSACadsorptionandofsurfacetensionofaSACsolutionareshowninfig.15.6.WhentheconcentrationofSACtendsto0,tendsto0aswell,butthesurfacetensiontendstotheoneofpuresolvento.

WhentheconcentrationofSACgrows,thesurfacetensionisdecreasing,butadsorptionisincreasing.Whenadsorptionreachesitsmaximalvaluemax(amonomolecularlayerofSAConthesurfaceofsolutionisformed)thesurfacetensionhasreacheditsminimalvalue,whichcorrespondstothesurfacetensionofpureSAC(ifSACisaliquidcompoundatthistemperature).

Fig.15.6.IsothermsofadsorptionandsurfacetensionofaSACsolution

II.6.USEOFTHEMAXIMALADSORPTIONFORFINDINGOFLENGTHANDTHECROSS-SECTIONAREAOFSACMOLECULE.

Whenadsorptionhasreacheditsmaximalvaluemax,allthesurfaceofliquidiscoveredbyamonomolecularlayerofSACmolecules,whichstandvertically.Knowingthevalueofmax,onecancalculatethenumberofmolecules,thatoccupyagivensurfaceareaSoftheliquid.

Fig.15.7.Situationonthesurfaceofliquidatmax.

Asmax isthemaximalnumberofSACmoles,thatcanbeplacedona1m2areaofsolutionsurface,

,where(15.6)

nisthenumberofmolesofSACand
Sisthesurfacearea.

Onecanfindthenumberofmoles,locatedonasurfaceareaSas

n=maxS(15.6a)

andthenumberofmolecules,thatarelocatedonthissurfaceareais:

N=nNA=maxSNAmolecules/m2(15.6b)

(NA-Avogadro’snumber).

Now,whenweknow,howmanymoleculesarelocatedon1m2ofliquidsurface,wecanfindthearea,correspondingto1molecule:

(15.7)

Next,letusderiveanequationforcalculationofthelengthofaSACmolecule.ForthesepurposeswehavetoexpressthemassofadsorbedSACintwodifferentways:

First,themassofadsorbedSACcanbefoundasitsvolume,multipliedbyitsdensity().VolumeoftheadsorbedlayerofSACis

V=Sl,where

Sisthesurfaceareaand

listhelengthofmolecule

(asitisamonomolecularlayerofSAC,thethicknessofthelayerlisequaltothelengthofSACmolecules,standingvertically).SothemassoftheSAClayeris

m=VSAC=SlSAC(15.8)

Ontheotherhand,themassofacompoundcanbeexpressedasthenumberofmoles,multipliedbythemolarmassofcompound:

m=nM,

butasthenumberofmolescanbefoundfromtheexpression(15.6a),massofSACis:

m=maxSMSAC(15.9)

WehavefoundthesamemassofSACby2differentmethods,thereforetheleftsidesofequations(8)and(9)mustbeequal.

Iftheleftsidesof2equationsareequal,rightsidesareequal,too:

SACSl=maxSMSACor

III.ADSORPTIONINTHEINTERFACELIQUID/LIQUID

Inthiscaseofinterface,aswellasintheliquid/airinterface,themoleculesofSAC,dissolvedinoneorbothliquidscanbeadsorbed.

MoleculesofSACinliquid/liquidinterfacewillbeorientedso,thatthehydrophilicpartsareturnedtowardsthemorepolarliquidandthehydrophobicparts-towardsthelesspolarliquid.Ifwechoose,forinstance,waterandbenzene(theyareinsolubleineachother),benzenewillbeintheupperlayer(becauseitsdensityissmaller)andwaterwillbesituatedinthelowerlayerandthemoleculesofSACwillbeorientedasshowninfig.15.8.

Fig.15.8.OrientationofSACmoleculesintheinterfacebetweenwaterandbenzene.

IV.MOLECULARADSORPTIONATSOLIDSURFACES.

Inthischapterwewilldealwiththeadsorptiononsolidsurfaces.Adsorptiononsolidsurfaceshas2maincases:

1)adsorptionintheinterfacebetweenasolidandgas(seechapterIV.2)and

2)adsorptionintheinterfacebetweenasolidandsolution(chapterIV.3).

InbothcasestheadsorptionisothermissimilarandisdescribedbyFreundlich’s(chap.IV.4.1)orLangmuir’s(chap.IV.4.2.)equations.

Adsorptiononsolidsurfacesisstronglydependentonthespecificsurfaceofsolid(chap.IV.1)andonehastonote,thattheadsorptiondoesn’ttakeplaceonallthesurfaceofsolidadsorbent,butonlyontheso-calledactivesitesinthesurface-places,wheresurfaceisuneven,vacancies(lackofatominitsnormalposition)orplaces,whereanatomislocatedonthesurfaceinthenextatomplane.

IV.1.SPECIFICSURFACEOFSOLIDS

Theratiobetweenthesurfaceareaandvolumeofasolidiscalledspecificsurface:

anditsmeasuringunitinSIsystemis

Forasolidcompound,consistingofcubicparticles,thespecificsurfacecanbefoundasaratiobetweenthesurfaceareaofcubeandthevolumeofcube.AsthesurfaceareaofcubeisS=6a2,whereaisthelengthofthecubeedgeandthevolumeofcubeisV=a3,thespecificsurfaceforcubicparticlesisfoundas:

Ifthesolidcompoundconsistsofsphericparticles,thesurfaceareaofsphereisS=4r2andthevolumeofsphereis,thereforethespecificsurfaceforsphericparticlesis:

where

disthediameterofthesphere.

Asonecansee,thespecificsurfaceinbothcasesisfoundas6,dividedbythelinearsizeofparticle

Thespecificsurfaceincreasesquickly,ifthesolidcompoundisgrindtosmallerparticlesandinmanycasesthespecificsurfacecanreachverylargevalues.

Intable15.1.thereareshownthesurfaceareasof1cm3ofsolidcompoundsatdifferentlinearsizesofparticles.Inthecases,whenthesolidisgroundtoaparticlesize,thatcorrespondstothesizeofcolloidalparticles,thesurfaceof1cm3becomesreallyveryhuge.

Table1.Surfaceareaof1cm3ofsolidatdifferentparticlesizes.

Linear
sizeof
particles
a(m) / Systems,inwhichsuchaparticlesizeisobserved / Totalsurfaceareaof1cm3ofsolidsubstance / Comparison
10-2 / pebbles / 6cm2 / surfaceofacoin
10-3 / gravel / 60cm2 / anote-book
10-5 / suspensionparticles / 600cm2 / surfaceofalittletable
10-7 / greatestcolloidalparticles / 60m2 / floorofabigroom
10-9 / smallestcolloidalparticles / 6000m2 / 1/2ofafootballsquare

IV2.ADSORPTIONINTHEINTERFACESOLID/GAS.

Intheinterfacebetweenasolidandgasadsorptionofgasmoleculestakesplace.Thevalueofadsorptionisdeterminedby:

1)thespecificsurfaceofadsorbent(seepreviouschapter)-thegreateritis,thegreaterisadsorption.Forthisreasontheadsorbenthastobegroundtoasmallparticlesize.

2)temperature-thegreateritis,themoreintensiveisthethermalmotionofgasmoleculesandthesmallerbecomesadsorption.

3)pressure(concentration)ofgas-thegreateritis,thegreaterisadsorption.

4)natureofgasandadsorbent(solid)-polaradsorbents(forinstance,silicagelSiO2)betteradsorbpolargases,non-polaradsorbents(e.g.)betteradsorbmoleculesofnon-polargases.

5)whenadsorptionofdifferentgasesonthesamesolidadsorbentiscompared,itisfoundout,thatadsorptionisgreaterforthegases,thatcanbeliquefiedmooreeasily.

ThereasonforadsorptionofgasesonsolidsurfacesistheexistenceofVanderVaals’sforces(induction,orientationanddispersionforces)andtheadsorptiontakesplaceontheactivesitesinthesurfaceofsolid(andnotatallsurfaceofadsorbent).

Theadsorptionisothermandequations,describingit,arediscussedinchapterIV.4.

IV.3.MOLECULARADSORPTIONATTHEINTERFACEBETWEENASOLIDADSORBENTANDSOLUTION

IV.3.1.AFFECTINGFACTORS

Tounderstandtheadsorptionattheinterfacebetweenasolidadsorbentandsolution,onehastotakeintoaccount,thatherethreesubstancesareincontactatthesametime-theadsorbentisincontactwithbothsolventandsolute.

Ofcourse,adsorptionofsolute,andnotofthesolventisrequired,becauseitallowstoextractthesolutefromsolution.Aswell,insomecasestheadsorptionofsolutefromsolutionprovidesapossibilitytogetridofsomeadmixtures,thatarepresentinsolution(forinstance,togetridofaldehydesandhigheralcoholsintheindustryofalcoholicdrinks),inothercasesitallowstoseparateanecessarycompoundfromsolvent.

Inordertocarryouttheadsorptionofsolute,onehastobesure,thatthesolventisnotadsorbed-assoonasthesolventcanbeadsorbedonthegivenadsorbent,itwillimmediatelycoveralltheadsorbent’ssurface(asthereisgreatammountofsolvent)andnoplacewillremainforthesolute.

Practicallyallthismeans,thatifthesoluteisapolarcompound,apolar(hydrophilic)adsorbenthastobechosen(polarcompoundsareadsorbedwellonpolarsurfaces),butthesolventmustbeanon-polarcompound.

Inthecontrarycase,ifthesolute,thathastobeadsorbed,isnon-polar,anon-polar(hydrophobic)adsorbenthastobeused,butthesolventhastobepolar(thenitwillnotbeadsorbed).

Ifthesolute,thathastobeadsorbed,isaSAC,itcanbeadsorbedonbothpolarandnon-polaradsorbents(remember,thatmoleculesofSACarebiphile).Insuchacasejustoppositecharactersofsolventandadsorbenthavetobechosen.

Letusdiscussarealexample.Ifaceticacid(itisaSAC)hastobeadsorbedfromitswatersolution,onehastochooseahydrophobicadsorbent,forinstance,.Thenaceticacidwillbeadsorbed,butwaternot.

Ifthesameaceticacidhastobeadsorbedfromitsbenzene(anon-polarsolvent)solution,theadsorbenthastobechosenhydrophilic(polar),forinstance,itcouldbesilicagel.AceticacidasaSACcanbeadsorbedonsilicagel,butbenzenecannot.

All-in-all,adsorptiononsolidadsorbentsfromsolutionsisaffectedbythefollowingfactors:

1)t°-thehigheristemperature,thelowerisadsorption,

2)concentrationofsolute-thegreateritis,thegreaterisadsorption,

3)natureofadsorbentandsolvent(seediscussionabove),

4)natureofsolute(seediscussionabove),

5)therelationsbetweensoluteandsolvent-thelowerissolubilityofthegivensoluteinthegivensolvent,thegreaterwillbeadsorption.

IV.3.2.DETERMINATIONOFADSORBENT’SNATURE.

Asonecouldseefromthepreviouschapter,foracorrectchoiceofadsorbentitisnecessarytoknow,whethertheadsorbentispolar(hydrophilic)ornon-polar(hydrophobic).Itisnotalwaysclearlyknownbeforeexperiment.Thenitisnecessarytodeterminethenatureofadsorbent.Itisdoneasfollows:

Agivenamountofadsorbent(usually1g)ismoistenedbywaterandanother1gofadsorbentbybenzene(non-polarsolvent)andtheamountofheat,liberatedintheseprocessesismeasured.Theheat,liberatedinthisprocessiscalledmoisteningheat(Qmoistening)oftheadsorbent.

Ifthemoisteningheatbywaterisgreater,thanbybenzene,theadsorbentiscalledhydrophilic,ifmoisteningheatbybenzeneisgreater,thanbywater,adsorbentishydrophobic:

Qmoist.,H2OQmoist.,C6H6=>hydrophilicadsorbent

Qmoist.,C6H6Qmoist.,H2O=>hydrophobicadsorbent

IV.3.3.CHANGEOFADSORBENT’SNATUREINADSORPTIONPROCESS

IfaSACisadsorbedonthesurfaceofadsorbent,thepropertiesofsurfacechangetotheoppositeones.Forinstance,thesurfaceofcoal(ahydrophobicadsorbent)becomeshydrophilicafteradsorptionofSACmoleculesbecomeshydrophilic,butthesurfaceofsilicagel(ahydrophilicadsorbent)-becomeshydrophobic,whenmoleculesofSACareadsorbedtoit,seefig.15.9.

Fig.15.9SurfacesofadsorbentsafteradsorptionofSACmolecules:a-surfaceofcoal,b-surfaceofsilicagel.

Atthesurfaceofcoal,thatishydrophobic(non-polar)themoleculesofSACarebound,usingtheirnon-polarhydrocarbonradicals(fig.15.9a),thereforethepolargroupsofSACmoleculesareturnedtowardsoutside.WhenallthesurfaceiscoveredbySACmolecules,ithasbecomehydrophilic,becauseitspropertiesarenowdeterminedbythepolargroupsofSACmolecules.

Atthesurfaceofsilicagel(fig.15.9b),whichispolar,themoleculesofSACarebound,usingtheirpolargroupsandinthiscasethenon-polarhydrocarbonradicalsaretheones,thatareturnedtowardsoutside.WhenallthesurfaceofsilicageliscoveredbymoleculesofSAC,ithasbecomehydrophobic(non-polar).

IV.4.ADSORPTIONISOTHERMSONSOLIDADSORBENTSFREUNDLICH’SANDLANGMUIR’SEQUATIONS

Theformoftheadsorptionisothermatthesurfaceofasolidadsorbentissimilarbothforadsorptionofgasesandforadsorptionofsolutefromsolution,seefig.15.10.

Theonlydifferencebetweenthesetwoisothermsis,thatinthecaseofadsorptionofthemoleculesofagasonasolidsurface,thevalueadsorptionisplottedversuspressureofgas,butinthecaseofadsorptionofsoluteonthesurfaceofasolidadsorbent(atthesolid/solutioninterface)adsorptionisplottedversusconcentrationofsolute.

Fig.15.10.Adsorptionisotherms:
a-attheinterfacesolid/gas, b-attheinterfacesolid/solution.

Adsorptionisothermcanbedividedinto3areas:

1)ionthefirstarea,whiletheconcentration(orpressureionthecaseofagaseousadsorbate)ofadsorbateislittle,thesurfaceofadsorbentispracticallyfree,thereforethemoleculesofadsorbateeasilyfindnon-occupiedplacesinthesurfaceofadsorbent.Forthisreasonanincreaseofgaspressureorsolute’sconcentrationcausesaproportionalincreaseofadsorption.

2)inthesecondpartoftheisothermagreatpartofadsorbent’ssurfaceisalreadyoccupiedbythemoleculesofadsorbate.Whenconcentration(pressure)ofadsorbateincreases,thenewmoleculesofadsorbatehavetoseekforafreeplaceinthesurface,thereforeadsorptiongrowslessthenproportionallytotheconcentration(pressure)ofadsorbate.

3)inthethirdareaoftheadsorptionisothermallthesurfaceofadsorbentisalreadycoveredbythemoleculesofadsorbate,nomoremoleculescanbeadsorbedatanincreaseofconcentration(pressure)ofadsorbate,thereforeatafurtherincreaseoftheamountofadsorbatethevalueofadsorptionremainsconstant-amaximalvalueofadsorptionmaxisreached.

IV.4.1.FREUNDLICH’SEQUATION

Aswewillseeionthefurthertext,twodifferentequationsareusedtodescribemathematicallytheadsorptiononsolidadsorbents.TheyarenamedFreundlich’sequationandLangmuir’sequation.

Langmuir’sequationisusedtodescribealltheadsorptionisotherm,(seenextchapter).Langmuir’sequationworkswellinthe1stand3rdareasoftheisotherm,butitisveryunpreciseinthe2ndarea.

Inmanypracticalcasesonehastodealwiththe2ndareaoftheadsorptionisotherm.InthisareaadsorptionisdescribedwellbyFreundlich’sequation,themathematicalformofwhichlooksasfollows:

=kp1/n(forgasadsorption)

=kC1/n(foradsorptionfromsolutions)

where:isadsorption,moles/g

porCispressureofgasorconcentrationofsolute

kand1/nareempiricalconstants.

Thus,whenworkinginthe2ndareaofadsorptionisotherm,onehastouseFreundlich’sequation.Ifonehasstudiedthedependenceofadsorptionontheconcentration(pressure)ofadsorbate,thenextistodeterminetheconstantskand1/noftheequation-iftheyareknown,theadsorptionisothermcanberestoredatanytime.

Todeterminetheconstantskand1/n,onehastomeasureexperimentallytheadsorptionatdifferentconcentrationsofsolute(orpressuresofagaseousadsorbate),tochangetheformofFreundlich’sequationinsuchaway,thatanequationofastraightlineisobtainedandtographicallydeterminethevaluesconstants.

TheFreundlich’sequationcanbetransformedintoanequationofstraightline,ifalogistakenfrombothsidesofequation:

log=log(kC1/n)or

log=logk+logC1/nor

log=logk+logC

y = b +a x

Theobtainedequationisany=ax+btypeequationofstraightline,wherelogplaystheroleofy, logCplaystheroleofx, logkplaystheroleofthecoefficientband1/nplaystheroleofthecoefficienta.

Fig.15.11.GraphicdeterminationoftheconstantsofFreundlich’sequation

Inordertodeterminetheseconstantsgraphically,onehastoplotlogasafunctionoflogC,seefig.15.11.

Nowlogkcanbefoundasthelogarithmofadsorption(log)atlogC=0,but1/n(aswellasthecoefficientaintheordinaryequationofstraightline)canbedeterminedastangentoftheanglebetweenthegraphandx-axis.

IV.4.2.LANGMUIR’SEQUATION

IncontrarytoFreundlich’sequation,thatiswrittenempirically,Langmuir’sequationhasbeenderivedfromtheadsorptionequilibriumusingthefact,thatwhentheadsorptionequilibriumisreached,theratesofadsorptionanddesorptionareequal.Adsorptionequilibriumcanbewrittenas:

A+SASwhere

Aisamoleculeofadsorbate,

Sisafreesiteinthesurfaceofadsorbent,

kdesandkadsaretherateconstantsofadsorptionanddesorptionprocesses.

Langmuir’sequationlookslike:

(forgasadsorption)

(foradsorptionfromsolution)

where:

isthevalueofadsorption,moles/g,

maxisthemaximaladsorption,moles/g,

porcisthepressureofgasortheconcentrationofsolute,

KisLangmuir’sconstant,whichisequaltotheratiobetweentherateconstantsofadsorptionanddesorptionprocesses(kads/kdes).

Langmuir’sequationisamathematicallysimilartotheMichaelis’s-Menten’sequation,thereforeitsabilitytodescribetheadsorptionprocessissimilar,too.

1.Iftheconcentration(pressure)ofadsorbateislowandCK,onecanignoreC(orp)inthenominatoroftheequation.Thentheequationbecomes:

AsKandmax areconstantvalues,theobtainedequationisanequationofdirectproportionalityandsuitswellthe1stpartoftheadsorptionisotherm(fig.15.10).

2.Iftheconcentration(pressure)ofadsorbateisveryhigh,CKandonecanignoreKinthenominatorofLangmuir’sequationandtheequationbecomes:

Asitwassaidbefore,itisnotworthtouseLangmuir’sequationinthe2ndarea,whereitisunprecise.InthisareaitisbettertouseFreundlich’sequation.

V.ADSORPTIONOFIONS.

Inallthepreviouschaptersthemolecularadsorptionwasdiscussed-inallthecasesadsorbatewasinaformofmolecules.

Adsorptionofionshasitsownspecificproperties,differentfromtheonesofmolecularadsorption.Therearetwomaincasesofionadsorption,whichdifferverymuchfromeachother:

1)ionexchangeadsorption,inwhichtheionsfromthesolutionexchangewiththeions,thatarepresentinthesurfacesofspecificadsorbents-ionexchangeresins(alsocalledionites),seechapterV.1.,

2)selectiveadsorptionofions,whichoccursonthesurfacesofinsolublesalts,whentheinsolublesaltsareinacontactwithwatersolutionsofelectrolytes(seechapterV.2.)

V.1.IONEXCHANGEADSORPTION

V.1.1.THESTRUCTUREOFIONEXCHANGERESINS

Ionexchangeadsorptionoccursonthesurfacesofionexchangeresins(ionites).

Theionexchangeresinsarewater-insolublepolymers.Theycontainfunctionalgroups,thatareabletoexchangeionswiththesurroundingsolution.Ionexchangeresins,whichareabletoexchangecations,arecalledcationexchangeresins,butthese,whichareabletoexchangeanions,arecalledanionexchangeresins.

Incationexchangeresinsthefunctionalgroups,thatareabletoexchangecationsare,bythemost,theacidicgroups-SO3Hor-COOH.ThesegroupscanexchangetheirHionswithanyotherpositiveions.ThegeneralformofacationexchangeresinthereforeiswrittenasR-Handitsionexchangereactionas:

R-H+H+R-Me+H+

Inanionexchangeresinsthefunctionalgroups,thatareabletoexchangeanionsarebasic,thereforetheyareabletoexchangeOH-ionswithothernegativeionsfromthesurroundingsolution.

AnanionexchangeresininageneralformiswrittenasR-OHanditsionexchangereactionas:

R-OH+X-R-X+OH-

Thestructuresofcationexchangeresinsandanionexchangeresinsareshowninfig.15.1

a b

Fig.15.12.Schematicstructuresof
a-cationexchangeresin,b-anionexchangeresin

Asonecansee,inthecaseofcationexchangeresinthenegativechargesarelocatedintheporousstructureoftheionexchangeresin,butH+ionsarelocatedinthesolutioninsidetheporesofitsstructure.

Forinstance,acationexchangeresin,whichhasasulphhydrylgroup-SO3Hinitsstructurecandissociate,sendingH+ionsintotheoutersolution:

Thenegativeionremainsinthestructure,butthepositiveH+ionistransferredintothesolution,thatfillstheporesofthecationexchangeresinand,consequently,thisH+ioncanbeexchangedtoametalionfromthesolution.

Ananionexchangeresin,onitsturn,dissociatesso,thatthepositiveionremainsinitsstructure,butthenegativeOH-ionistransferredintothesolution,thatfillsthepores:

andcanthereforebeexchangedtoanothernegativeion.

V.1.2.SPECIFICPROPERTIESOFIONEXCHANGE.

Theionexchangeadsorptionhastwoveryimportantspecificproperties:

1.Ionexchangeproceedsinequivalentamounts.Thismeans,thatwhenoneequivalentofionsisboundtotheionexchangeresin,oneequivalentofH+ionsorOH-ionsistransferredintosolution.

Thus,ifacationisboundtothecationexchangeresin,thenumberofH+ions,transferredintothesolution,isequaltothechargeofthecation:

R-H+Na+R-Na+H+

Aswell,incaseofanionexchangeresins,whenanacidremainderionisboundtotheanionexchangeresin,thenumberofliberatedOH-ionsisequaltothechargeofacidremainderion:

Thismeans,thatwhenmetalionsfromthesolutionareboundtocationexchangeresin,anequivalentamountofH+ionsaretransferredintosolution,butwhenanionsareboundtoanionexchangeresin,anequivalentamountofOH-ionsaretransferredintothesolution.

Usingthisproperty,onecandeterminethetotalconcentrationofcationsinthesolutionwithoutsystematicanalysisofeachion.Itisenoughtopassthissolutionthroughacolumn,filledwithcationexchangeresin-allthecationswillbeexchangedtoH+ions.AfterthattheliberatedH+ionscanbetitratedbyastrongbase.

Aswell,todeterminethetotalconcentrationofanionsitisenoughtopassthesolutionthroughionexchangecolumn,filledwithanionexchangeresinandthentotitratetheliberatedOH-ionsbyastrongacid.

2.Ionexchangeadsorptionisreversible.Thismeans,thatthesameionexchangeresincanbeusedmanytimes.Whenacationexchangeresinissaturatedwithmetalions(alltheH+ionsintheacidicfunctionalgroupsofthecationexchangeresinarealreadyreplacedbymetalions),thecationexchangeresincanberegeneratedintoitsinitialform,iftheionexchangecolumniswashedbyasolutionofastrongacid-thentheionexchangeoccursintheoppositedirection,themetalionsarewashedoutfromthecationexchangeresinandH+ionsreplacetheminthefunctionalgroupsoftheresin,forinstance,

R-Na+HCl=>R-H+NaCl

Anionexchangeresinscanberegeneratedbywashingthemwithasolutionofstrongbase,thentheacidremainderionsarewashedoutfromtheresinsandreplacedbyOH-ionsinthefunctionalgroupsofanionexchangeresins,e.g.:

R-Cl+NaOH=>R-OH+NaCl

Aftersucharegenerationtheionexchangeresinsarereadyforuseagain.

V.1.3.APPLICATIONSOFIONEXCHANGEADSORPTION

Ionexchangeadsorptioncanbeusedfor:

1)purificationofwater(theso-calleddeionizedwaterisobtainedthen).Todothis,waterisfirstpassedthroughacolumn,filledwithacationexchangeresin,whereallthemetalionsofthesolutionarechangedtoH+ions.

Afterthat,waterispassedthroughacolumn,filledwithanionexchangeresinwherealltheacidremainderionsarechangedtoOH-ions.Finally,H+andOH-ionsreacttoformwater:

H++OH-=>H2O

andpurewaterisobtained.Thismethodforwaterpurificationiswidelyusedinsteadofdistillation.

2)Forpracticalseparationofmetalionsfromeachother.Severalrulesregulatetheadsorptiopnofionsofdifferentmetalsoncationexchangeresins:

1.Thegreateristhechargeofion,thestrongeritisboundtothesurfaceoftheionexchangeresin.Forinstance,forNa+,Ca2+andAl3+ionstheabilitytobeboundtothesurfaceofacationexchangeresingrowsinthefollowingsequence:Na+Ca2+Al3+.

2.Ifthechargeofdifferentionsisequal,theiradsorptionabilityontheionexchangeresinsisdeterminedbythesizeofions:thegreateristhesizeofion,thestrongeritisboundtotheionexchangeresin(infact,thegreateristhesizeofion,theweakeritishydratedandthereforetheeasieritisadsorbed).Thus,forinstance,K+isboundstrongerthanNa+orBr-isboundstrongerthanCl-.

/ Fig.15.13.
Separation of metal ions
in cation exchange column

If,forinstance,asolution,containingamixtureofNa+,Ca2+andAl3+ionsisbepouredintoacolumn,filledwithacationexchangeresin,andafterthatasolution,containingdilutedHClispassedthroughthecolumn,themetalionsstarttravellingdownbythecolumn.InthisexampleNa+ ionswillbethefirstonestocomeoutfromthecolumn,astheyareweakerboundtotheionexchangeresin,Ca2+ionswillcomenextandAl3+ionswillbethelastonestocomeout.

Iftheionexchangecolumnislongenough,insuchawayitwillbepossibletocompletelyseparatethemetalionsfromeachother.

V.2.SELECTIVEADSORPTIONOFIONS

Theselectiveadsorptionofionsoccursonthesurfacesofinsolublesalts.Thiscaseofadsorptionisresponsiblefortheformationofcolloidalparticlesandwillbediscussedatthetopic“Colloidalsolutions”.