Physical adsorption, chemisorption, Freundlich’s expression, Langmuir adsorption isotherm

Surface chemistry note. Isotherms

1. INTRODUCTION
Surface chemistry is the study of processes that occur at
the interface of two bulk phases.
The bulk phases can be of the type:
Liquid -liquid

2. TYPES
•ADSORPTION:istheadhesionofatoms,ions,
biomoleculesormoleculesofgas,liquid,or
dissolvedsolidstoasurface.
•ABSORPTION:isaphysicalorchemical
phenomenonoraprocessinwhichatoms,
molecules,orionsentersomebulkphase-
gas,liquid,orsolidmaterial.

3. Adsorption onactivated charcoal

4. Absorption through a membrane

5. Adsorption
Absorption(“partitioning”)
PHASE2
Adsorbate
Adsorbent
PHASE I

6. Adsorption
Physicaladsorption:VanderWaalsattractionbetween
adsorbateandadsorbent.Theattractionisnotfixedto
aspecificsiteandtheadsorbateisrelativelyfreeto
moveonthesurface.Thisisrelativelyweak,reversible,
adsorptioncapableofmultilayer
Chemicaladsorption:Somedegreeofchemical
bondingbetweenadsorbateand adsorbent
characterizedbystrongattractiveness.Adsorbed
moleculesarenotfreetomoveonthesurface.Thereis
ahighdegreeofspecificityandtypicallyamonolayeris
formed.Theprocessisseldomreversible.
.

7. ADSORPTION EQUILIBRIA:
Iftheadsorbentandadsorbatearecontacted
longenoughanequilibriumwillbeestablished
betweentheamountofadsorbateadsorbedand
theamountofadsorbateinsolution.The
equilibriumrelationshipisdescribedby
ISOTHERMS

8. Causes of
Adsorption
•DislikeofWaterPhase–‘Hydrophobicity’
•AttractiontotheSorbentSurface
•vanderWaalsforces:physicalattraction
•electrostaticforces(surfacecharge
interaction)
•chemicalforces(e.g.,-andhydrogen
bonding)

10. Types ofAdsorption
Dependingonthenatureofattractiveforcesexistingbetween
theadsorbateandadsorbent,adsorptioncanbeclassifiedas:
Physical
Adsorption
Chemical
Adsorption

11. •Forcesofattractionarevander
Waals’forces
•Lowenthalpyofadsorption(20-
40k.J/mole)
Forcesofattractionarechemicalbond
forces
Highenthapyofadsorption(200-40
k.J/mole)
•This process is observed under
conditions of lowtemperature
This process takes place at high temperatures
•It is notspecific It is highlyspecific
•Multi-molecular layers may be
formed
Generally, monomolecular layer is formed
•This process isreversible This process isirreversible
Physisorption
Differencebetween
Chemisorption

12. •Physisorption isa general
phenomenon andoccursinany
solid/fluidorsolid/gassystem.
•In physiorption, perturbation of the
electronic states of adsorbent and
adsorbate isminimal
•Typical binding energy of
Physisorption is about 10–100meV.
•The elementary step in
Physisorptionfromagasphase
doesnotinvolveanactivation
energy.
•For Physisorption, under appropriate
conditions, gas phase molecules can
form multilayer adsorption.
•Chemisorptions is characterized
by chemicalspecificity.
•Forchemisorption,changesin
theelectronicstatesmaybe
detectablebysuitablephysical
means.
•Chemisorption usually forms
bonding with energy of 1–10
eV.
•Chemisorption often involves
an activation energy.
•In chemisorption, molecules
are adsorbed on the surface by
valence bonds and only form
monolayeradsorption.

13. •Adsorptionisothermdescribestheequilibriumofthesorptionofa
materialatasurfaceatconstanttemperature.
•Itrepresentstheamountofmaterialboundatthesurfaceasa
functionofthematerialpresentinthegasphaseand/orinthe
solution.
•Sorptionisothermsareoftenusedasempiricalmodels,whichdonot
makestatementsabouttheunderlyingmechanismsandmeasured
variables.
•Theyareobtainedfrommeasureddatabymeansofregression
analysis.Themostfrequentlyusedisothermsarethelinearisotherm,
Freundlichisotherm,theLangmuirisotherm,andtheBETmodel.
ADSORPTION ISOTHERMS

14. Commonly Reported AdsorptionIsotherms

16. Langmuir Isotherm:
This model assumes monolayer coverage and constant binding energy
between surface and adsorbate. The model is:
K Q
0
C
q
e
ae
1K C
e
C
ehas units ofmg/L.
Khas units ofL/mg
arepresents the maximum adsorption capacity (monolayer
coverage) (g solute/g adsorbent).
Q
0

17. For the Langmuirmodellinearizationgives:
aae
Q
0
q
C
e
K Q
0

 1

C
e
A plot of C
e/q
e versus C
e should give a straight line with intercept:
a
1
K Q
0
andslope:
1
Q
0
a
eaae
CQ
0
q
1

1
K Q
0

1

1

18. Here a plot of1/q
e
versus1/C
e
should givea
straight line withintercept 1/Qandslope
0
a
1
KQ
For the Freundlich isotherm use the log-log version :
n
Fe
logqlogK
1
logC
Alog-logplotshould yield an interceptoflog
K
F
and a slope of 1/n.

19. FreundlichIsotherm:
nand K
F are system specificconstants.
Forthespecialcaseofheterogeneoussurfaceenergies
inwhichtheenergyterm,“K
F”,variesasafunctionof
surfacecoverageweusetheFreundlichmodel.
1
q
eK
FC
e
n

20. Factorswhichaffectadsorptionextent
Solubility:
•Ingeneral,assolubilityofsoluteincreasesthe
extentofadsorptiondecreases.Thisisknownas
the“Lundelius’Rule”.
•Factorswhichaffectsolubilityincludemolecular
size(highMW-lowsolubility),ionization
(solubilityisminimumwhencompoundsare
uncharged),polarity(aspolarityincreasesget
highersolubilitybecausewaterisapolarsolvent).

21. pH:
pHoftenaffectsthesurfacechargeontheadsorbent
aswellasthechargeonthesolute.Generally,for
organicmaterialaspHgoesdownadsorptiongoesup.
Temperature
Adsorptionreactionsaretypicallyexothermic
i.e.,H
rxnisgenerallynegative.Hereheatis
givenoffbythereactionthereforeasTemp.
increasesextentofadsorptiondecreases.

22. Presence of othersolutes:
Ingeneral,getcompetitionforalimited
numberofsitesthereforegetreduced
extentofadsorptionoraspecificmaterial.

23. Inpreservingvacuum:In
Dewarflasksactivated
charcoalisplacedbetween
thewallsoftheflasksothat
anygaswhichentersinto
theannularspaceeither
duetoglassimperfectionor
diffusionthoughglassis
adsorbed.
Applications ofAdsorption
Adsorptionfindsextensiveapplicationsbothinresearch
laboratoryandinindustry.Afewapplicationsare
discussedbelow/;

24. Ingasmasks:
Allgasmasksaredevices
containingsuitableadsorbentso
thatthepoisonousgasespresent
in the atmosphere are
preferentiallyadsorbedandthe
airforbreathingispurified.
In clarification of sugar:
Sugarisdecolorizedbytreating
sugarsolutionwithcharcoal
powder.Thelatteradsorbsthe
undesirablecolorspresent.

25. Thepaintshouldnotcontaindissolvedgasesasotherwise
thepaintdoesnotadherewelltothesurfacetobepainted
andthuswillhaveapoorcoveringpower.Thedissolved
gasesaretherefore,removedbysuitableadsorbents
duringmanufacture.Further,allsurfacesarecoveredwith
layersofgaseous,liquidorsolidfilms.Thesehavetobe
removedbeforethepaintisapplied.Thisisdoneby
suitableliquidswhichadsorbsthesefilms.Suchliquidsare
calledwettingagents.Theuseofspiritaswettingagentin
furniturepaintingiswellknown.
Inpaintindustry:

26. Surface tension
consideramolecule,Aintheinteriorofaliquid.Itisbeing
attractedbyneighboringmoleculesequallyinalldirection
andthustheresultantforceonitiszero.Nowconsider
anothermolecule,Batthesurface.Itisexperiencinga
resultantforcedownwardbecauseofgreaterattraction
formoleculesintheliquidthanmoleculesinthevapors
abovetheliquid.Thisisthecasewiththemoleculesat
thesurface.Thesurfaceoftheliquidtendstocontractto
havetheminimumsurfaceareapossible.Thisforceinthe
surfacecalledsurfacetension.