View
1
Download
0
Category
Preview:
Citation preview
Langmuir's Theory of Adsorption: A Centennial Review
Authors: Hans Swenson and Nicolas P. StadieThis document is the unedited author's version of a Submitted Work that was subsequently accepted for publication in Langmuir, copyright © American Chemical Society after peer review. To access the final edited and published work, see DOI: 10.1021/acs.langmuir.9b00154.
Swenson, Hans, and Nicholas P. Stadie. "Langmuir's Theory of Adsorption: A Centennial Review." Langmuir: the ACS Journal of Surfaces and Colloids 35, no. 16 (April 2019): 5409-5426. DOI:10.1021/acs.langmuir.9b00154.
Made available through Montana State University’s ScholarWorks scholarworks.montana.edu
1
Langmuir'sTheoryofAdsorption:aCentennialReview1‐3
HansSwensonandNicholasP.Stadie*
DepartmentofChemistry&Biochemistry,MontanaStateUniversity,Bozeman,MT,59717,USA
*E‐mail:nicholas.stadie@montana.edu
The100thanniversaryofLangmuir’stheoryofadsorptionisasignificantlandmarkfor the physical chemistry and chemical engineering communities. Despite itssimplicity, the Langmuir adsorptionmodel captures the key physics ofmolecularinteractions at interfaces and laid the foundation for further progress inunderstanding interfacial phenomena, developing new adsorbent materials, anddesigningengineeringprocesses.TheLangmuirmodelhashadanexceptionalimpacton diverse fields within the chemical sciences ranging from chemical biology tomaterialsscience ,animpactthatbecameclearerwiththedevelopmentofmodifiedadsorptiontheoriesandcontinuestoberelevanttoday.
2
Introduction
IrvingLangmuirisknownforwide‐rangingcontributionsinchemistryandphysics,bothinfundamentalandappliedconcepts,andisindeliblylinkedtothebirthofthescienceofinterfaces.Histhreeseminalworksonadsorptiontheory,thefoundationforthemodernunderstandingofalltypesofchemicalinterfaces,werepublishedin1916,11917,2and1918,3thelastofwhichisnowonecenturyoldandthesubjectofcelebrationinthishistoricalarticle.The“superficial”natureofthisfieldofresearchwasplayfullyacknowledgedinthepresentationspeechoftheNobelPrizeinChemistrytoLangmuirin1932.ItwasalsoacknowledgedinSöderbaum’sverynextbreaththat“inreality,surfacechemistryhascontributedgreatlytothedeepeningofourknowledgeofmatter.”4Thisremainstruetoday,withmajorworldwideresearchefforts,e.g.,inheterogeneouscatalysis,devotedtophenomenaatinterfaces.
ThehistoryandimpactofLangmuir’sthreeseminalworks,referredtoasLangmuir1916,Langmuir1917,andLangmuir1918,arebrieflyreviewedherein.Thecontextinwhichadsorptiontheorywasbirthedwillfirstbesummarized,followedbyabriefaccountofLangmuir’searlyworkthatleddirectlytothedevelopmentofhismonolayertheory.Inadditiontothesimple,idealpictureofhomogeneousmonolayeradsorptionthathascometobeknowncolloquiallyastheLangmuiradsorptionmodel,Langmuiralsodevelopedseveralmorecomplexmodelssuchasformultilayerandheterogeneousadsorptionalreadyin1918.TheingeniousearlyexperimentscarriedoutbyLangmuirandhisassistant,Mr.S.P.Sweetser,laidthegroundworkforfuturedemonstrationsofthewidespreadapplicabilityofthissimpletheoryofsite‐specificmolecularadsorption.Langmuiralsoemphasizedthenon‐universalapplicabilityofanyoneequationormodel including,ofcourse,hisownmonolayeradsorptiontheory ,3andtheneedtoaccountforawidebreadthofdifferentadsorptionmechanismsatplayoncomplicated,real‐worldsurfaces.Hence,thelastingreputationofLangmuir’sadsorptiontheoryisboththatofapowerful,simplemodelwithfar‐reachingapplicabilityandtextbookelegance,andsimultaneouslythatofamodelwithfewstrictlyadheringdemonstrativesystemsinthelaboratory.
Inthisreview,astatisticalmechanicaltreatmentofallsixofLangmuir’soriginalcasesofadsorptionisgiven,followedbyselectedcontemporaryexamplesofgas‐solidadsorbate‐adsorbentsystemsthatexhibitrepresentativebehavior.ReferencetoLangmuir’sworkhasrisensharplysince1999whenmetal‐organicframeworks MOFs wererevolutionizedtoincludematerialswithlargeinnersurfacesofcrystallinestructure;5hence,emphasisisplacedhereinonexamplesofLangmuiradsorptionontheporoussurfacesofMOFs.ThisextensionofLangmuir’s“externalsurface”theorytogasadsorptiononthe“internalsurfaces”ofporousmaterialsisoneofmanynaturalextensionsofLangmuir’soriginaltheorytointerfacesofallkinds,suchasingas‐liquidandliquid‐liquidsystems,molecular‐proteinbinding,andeveninmacromoleculeornanocrystalbinding.WeseektoputLangmuir’snowcentury‐oldtheoryintomoderncontext,withastrongemphasisonthefirst‐principlessimplicityoftheseconceptsandtheir cautious applicabilityinwide‐rangingadsorptionsystems.
HistoricalContext
EarlyAdsorptionTheory.Thephenomenonofadsorptionhasbeenknownsinceantiquity;6quantitativeinvestigationsbeganinthelate18thcentury theearliestknowninvestigationsbyScheele,Priestley,andFontanain1773‐1777,asreviewedmorecomprehensivelyelsewhere7‐8 ,andtheterm“adsorption”asdistinctfrom“absorption”wasoriginallycoinedbyEmilduBois‐
3
Reymond viaHeinrichKayser9 in1881.Adsorption,strictlydefined,istheincreaseindensityofafluidadsorbateataphaseboundaryorsurface.Itremainsdistinguishedfromabsorptionbyitslimitationtothesurfaceorinterfaceofthesorbent;upondiffusionbeyondtheinterfaceintothebulkofthesorbent,theadsorbatebecomestheabsorbate.Theterm“sorption”wasintroducedasreferringtotheambiguouscaseencompassingbothadsorptionandabsorptionbyJamesW.McBainin1909.10Thisterminology,inprinciple,remainstheconventiontoday.
Thehistoryofadsorptiontheoryisintimatelytiedtoboththatofintermolecularinteractionsandtheatomicstructureofsolid‐statematter,subjectsthatwerestillprematureattheturnofthe20thcentury.ItistellingthattheforcesofattractionbetweentwophasesattheirinterfacewerestillreferredtobyLangmuirin1918as“actionatadistance,”inquotationmarks.3Studiesofadsorptionwereprimarilyanexperimental asopposedtotheoretical endeavorbefore1900.WhileJ.WillardGibbsdevelopedtheearliestthermodynamictheoryof“surfacesofdiscontinuity”inthesecondpartofhislandmarkworkOntheequilibriumofheterogeneoussubstancesin1876,11hespecificallyremarkedastothequantityofmoleculesadsorbedbeing“toosmallingeneraltoadmitofdirectmeasurement.”Nevertheless,theterms“surfaceexcess”and“excessquantityofadsorption”werecoinedbyGibbsandtheconceptremainsofgreatimportancetoadsorptiontheory.Thefirstliquidsolute‐solidadsorptionequilibriawerereportedbyFriedrichStohmannandWilhelmHennebergin185812 ammoniumadsorptiononsoil andthefirstgas‐solidadsorptionequilibriawerereportedindependentlybyChappuisin187913andKayserandJoulinin18819,14 smallmoleculargasesoncharcoal ,wherethedelayforgas‐solidexperimentswasduetothemorecomplexapparatusrequired.Heatsofadsorptioncouldbemeasuredasearlyas1854byFavre15‐16andpracticaladsorptionapplicationssuchasgasseparationswerewell‐establishedbytheturnofthecentury.
PriortoLangmuir’smonolayer‐basedtheoryfirstreportedin1916,adsorptionwasunderstoodsuchthatthedensityofafluidataninterfacedecreasesgraduallytowardthebulk.ThetheoryofadsorptionwasfirstfocusedonthesolidsurfaceitselfbyHerbertFreundlichin190717,whoseempiricalequationforadsorptiondemonstratesrespectableutilityacrossrelativelywidetemperatureandpressurerangesstilltoday.However,itiswell‐knownthatFreundlich’sequationisneitherphysicallyfoundednorthermodynamicallyconsistent,preventingitsfundamentaljustification.18‐19MichaelPolanyialludedtotheconceptofacharacteristicadsorptioncurvein1914,20andfurtherlaidthefoundationofwhathasbecomeknownasPolanyi’s“potentialtheory”in1916.21Inatwistoffate,thisnon‐localizedtheorywasrejectedatthetimeforbeingincongruentwiththeemergingtheoriesofintermolecularinteractionsbasedonelectricalforces;nevertheless,itwaslaterdeveloped togetherwithFritzLondon22 intoawidelyacceptedparalleltheorytothatofLangmuir’s.23
Langmuir’stheory,onthecontrary,wasfoundedonafundamentallydiscretizedmolecularperspective,consideringtheatomicstructureofthesurfacetobecentraltoasuccessfuldescriptionofadsorption.HewasstronglyinfluencedbytheemergingpictureofcrystallinesolidmatterpaintedbyWilliamH.BraggandhissonW.LawrenceBragg,especiallyafterhearingseverallecturesgivenbytheelderBraggduringhisvisittotheeasternUnitedStatesaroundthetimeoftheoutbreakofWorldWarI 1914‐1915 .InLangmuir’swords:“theimportanceoftheworkofW.H.BraggandW.L.Bragginitsbearingonchemistryhasnot,asyet,beengenerallyrecognized.”1Thediscretenature i.e.,sequentialfilling ofwell‐definedadsorbedsurfacelayerswastheinsightfirstdevelopedbyLangmuiranddescribedextensivelyinLangmuir1916,withclearreferencetothedefinite,discretizedatomicstructureofcrystalsdiscoveredbytheBraggs.
4
Langmuirpicturedmonolayerformationtooccurbasedonaspecificbindinginteractionbetweenadsorbatemoleculesanddistinctsolidsurfacesites.Thispicturewasoriginallyfoundedontheresultsofexperimentsperformedwithhydrogenandothersmallmoleculesinthepresenceofheatedmetalfilamentsatverylowpressures.InLangmuir1918,however,itwasalreadybroadlygeneralizedtoincludecaseswheremonolayerormultilayerformationoccurs,aswellascasesrangingfromhighlyregularsurfacestocompletelyamorphoussurfacescontaininganabundanceofdifferentadsorptionsites.Thesixfundamentalcasesofadsorption alldesignedforadsorptionatbulksurfaces identifiedbyLangmuirlaidthefoundationforsurfacescience,whichhassincebeenadornedwithmodificationsandnewtheoriesdesignedtodescribecomplex,real‐wordadsorptionphenomena,includingwithinmolecularlyporousmaterials.6
IrvingLangmuir.IrvingLangmuir,bornonJanuary31st,1881,receivedhisprimaryandsecondaryeducationinBrooklyn,Paris,andPhiladelphia,followinghisfather’swork‐relatedmovements asaninsuranceexecutive .Afterthedeathofhisfatherin1899,LangmuirenteredTheSchoolofMinesatColumbiaUniversitytostudymetallurgicalengineering,because“thecoursewasstronginchemistry,ithadmorephysicsthanthechemicalcourse,andmoremathematicsthanthecourseinphysics–andIwantedallthree.”24Afterreceivinghisdegree,Langmuir,likemanyAmericanscientistsatthattime,setofftoGermanytoobtainaPhD.AttheUniversityofGöttingen,hisdesiretocouplescientificresearchwithpracticalandindustrialapplicationswouldbehonedandaffirmedbyseveralgreatprofessors,mostnotablyFelixKleinandWaltherNernst.Nernst,whoservedasLangmuir’sprimaryadvisor despiteFriedrichDolezalekbeingnamedassuch25 ,directedhisthesisprojectofexaminingthedissociativeactionofvariousgasesonthesurfaceofahotplatinumfilament.26Theseearlyexperimentswouldlaythefoundationforhislifelongstudiesofadsorptionphenomena.
Inearly1906,LangmuiracceptedafacultypositionatStevensInstituteofTechnologyinNewJersey,hopingtopursueanacademiccareerinresearch.Afterthreeyearsofintensive,time‐consumingteachingthataffordedlittletimeforresearch,LangmuirleftthispositionandjoinedthestaffoftheGeneralElectricresearchlaboratory,reunitingwithanoldcolleaguefromColumbia,ColinG.Fink.Langmuirfirstsecuredaresearchpositionforthesummerof1909,andimpressedhiscoworkersenoughtogainapermanentpositionattheresearchlab,apositionhewouldholduntilhisretirementin1950.AtGeneralElectric,Langmuir’sindependencewasconsistentlyprotectedbythelaboratory’sdirector,WillisR.Whitney.Hisfirstprojectswerelargelyacontinuationofhisthesisworkinvestigatingchemicalreactionsinthepresenceofheatedmetalfilamentswithinglassbulbs,especiallytungsteninthepresenceofhydrogenandothergases.Thereactionratescouldbeslowedandeffectivelydeconvolutedwhenperformedunderhigh‐vacuumconditions atechnicalachievementofLangmuir’swork ,culminatingintheproposalofafundamentallynewmechanismofheterogeneouschemicalreactions.27Thetwokeyfeaturesofthiswork,namelythelowpressureandinherentroleofthemetalsurface,directlyadvancedLangmuirtowardthebreakthroughtheoryofadsorptionbasedontheconceptofmolecularlayers.Therefore,despitetestinghisnewlyfoundtheoryoncasesofpurephysisorptioninLangmuir1918,theoriginalcollectionofworkonwhichthetheorywasdevelopedconsistedofoveradecadeofexperimentsobservingthebehaviorofgasesinthepresenceofheatedmetalsurfaces:purechemisorption.ItisperhapsunsurprisingthatthemonomolecularlayerthatLangmuirenvisionedremainsmuchmoreaccurateindescribingthephenomenaofchemicaladsorption wherethebindingstrengthisstrongrelativetointermolecularinteractionsinthebulkgas .
5
Langmuir’sTheoryofAdsorption
AKineticModel.Langmuiroriginallydescribedgasadsorptionatasolidinterfaceasbearingmechanisticsimilaritytogas‐phasecondensationataliquidsurface,whereinelasticcollisionsleadtoa“lag”orresidencetimeatthelocationofincidence.1,28Thatis,asgasmoleculesstrikethesurface,theyareheldnearthesurfacebyattractiveintermolecularforces akintocondensation forsomeshortbutfinitetimeuntilleavingintothebulkgasphaseagain akintoevaporation .Withinthispurelykineticviewofadsorption asshowninFigure1 ,andwithanewlydevisedconceptofthediscretized,surface‐orientednatureofadsorptioninhand,asimplerelationshipbetweenequilibriumsiteoccupancy, ,andthepressureofthegasphase, ,wasderived.Langmuirmadethreefundamentalassumptions:
i therateofincidenceofthemoleculesinabulkgasphaseonaunitareaofadsorbentsurface, ,isproportionaltothepressureatconstanttemperatureviathekinetictheoryofgases adaptedfromhispreviousworkonsaturatedmetalvaporsabovetheirrespectivesolids29 ,
ii therateofadsorption asopposedtometalvaporcondensation , ,dependsnotonlyontherateofincidenceofmoleculesonthesurface, ,butalsoontheprobabilityofadsorption asopposedtoelasticreflection , ,andtheprobabilityofincidenceatavacantadsorptionsite asopposedtoatanoccupiedsite , ,and
iii therateofdesorptionisequaltotherateofdesorptionatmaximumsurfacecoverage,
, ,multipliedbythefractionaloccupancyofthesurfacesitesbyadsorbedmolecules, therebyneglectinganyroleofadsorbate‐adsorbateinteractions .
Thesethreeassumptionscanalsobesummarizedbystatingthatboththegasphaseandadsorbedphasebehaveideally thereisnonotionofintermolecularinteractions andthateverybindingsiteisidentical.Combined,theyyieldthefollowingexpressionfortheequilibriumfractionaloccupancyofadsorptionsitesunderagasphaseatpressure :
∙
, ∙ 1 Equation1
TheLangmuirconstant, ,isindependentofpressureandhenceonlydependsontemperature.Whenthetemperatureisinvariant,theisothermcanbemeasuredand experimentallydetermined,whichfromthiskineticmodelisdefinedas:
1
√2∙
,Equation2
TheLangmuirconstanthasunitsofinversepressure orinverseconcentrationforsoluteadsorption .Itisusefultorecognizethatthesurfaceispreciselyhalf‐occupiedatthepressurecorrespondingto ,asshowninFigure1,andsorepresentsa“characteristicinversepressureofadsorption.”AmorethoroughkineticderivationasoriginallypresentedinLangmuir1918isreproducedintheSupportingInformationusingupdatednomenclature.
6
Figure1. a Langmuir’skineticmodelofadsorptionacknowledgesfourprocessesoccurringbetweenagasinequilibriumwithasurface consistingofdiscretizedadsorptionsites :incidenceinc ,reflection ref ,adsorption ads ,anddesorption des .TreatingthesefundamentalprocesseswiththreesimpleapproximationsleadstothesimpleLangmuirisothermequation. b‐c ThedependenceoftheLangmuirisothermontheLangmuirbindingconstant, ,showinghyperbolicshapeinthesimpleplotoffractionaladsorptionsiteoccupancy, ,asafunctionofpressure b andsigmoidalshapeintheequivalentlog‐normalplot c .Themagnitudeofthebindingconstantisindicatedbycolor,increasingfrom0.001‐1000Pa .Thecharacteristicpressureregimecorrespondingtosignificantadsorptionuptakeoccursaround ,asshownforthecaseof 0.1Pa reddashedline .
ExperimentalResults.TheoriginalexperimentsthatinspiredLangmuir’smonolayerhypothesiswereinvestigationsofthebehaviorofhydrogenandoxygenatvacuumpressuresinsideaglasslightbulbcontainingaheatedtungstenfilament,suchasthatreportedin1912.30Langmuirfirstobservedthatthepressureofhydrogendecreaseduponexposuretotheheatedfilamentovertimeespeciallyplatinumorpalladium .Langmuirconcludedthatthehydrogendissociatedonthesurfaceoftheheatedmetal,thenescapedasamonatomicgas,andfinallychemisorbedasatomichydrogenontheinnerglasswallofthebulb.Theadsorptionofatomichydrogenonglasswasfoundtobeirreversible,althoughquantitativeresultsastotheamountadsorbedwerenotreportedatthattime.
Langmuircontinuedhisexperimentswithothergases e.g.,CH4,NH3,PH3,CO,andCO2 adsorbedonheatedmetalsurfacesandfirstoutlinedhismonolayerhypothesisin1916.1Still,equilibriumadsorptionisothermmeasurementswerenotpresentedinLangmuir1916orelsewhereatthattime.InLangmuir1917,thegas‐solidadsorptioninterfacewastemporarilyabandonedinfavoroftheliquid‐solidinterface.Thedevelopmentofmonolayerassembliesataqueousandotherinterfacesisnotwithinthefocusofthepresentreview,andtheinterestedreaderisreferredelsewhere.31‐32
Finally,inLangmuir1918,methodologicalstudiesoftheadsorptionofnumerousgasesonthreemodelmaterialsurfaceswerereported:glass amorphoussilica ,muscovite mica ,andplatinum.Langmuirperformedtheexperimentsalongthedesorptionbranch,permittingdirectobservationoftheinterplaybetweenphysisorptionandchemisorptionphenomena thechemisorbedquantitywouldnotbedesorbedatlowtemperature .Thesewerethefirstsetofquantitativeadsorptionmeasurementsofsmallmoleculargasesonsolidsurfacesofwell‐knownchemistryandgeometry.Hisexperimentswereperformedatextremelylowpressureforthetime,requiringgreatcareandnewlydevelopedvacuumequipment includingaMcLeodgaugeandLangmuir’sinvention,the“condensationpump”33,nowknownasadiffusionpump34 .
7
Figure2. a EquilibriumdesorptionmeasurementsofN2onmicaat90and155KasreportedinLangmuir1918,showingsingle‐siteLangmuir SSL adsorptioncharacter. b EquilibriumdesorptionmeasurementsofCOonglassat90KasreportedinLangmuir1918,showingdual‐siteLangmuir DSL adsorptioncharacter thestronglyboundspeciesattributedtochemisorptionisshownasasolidpurpleline .
Thefirstsetofexperimentsreportedin1918wereperformedona24.3gsampleofmuscovitemica ,havinganactual hand‐measured surfaceareaof0.5750m2 specificsurfacearea:0.0237m2g‐1 .LangmuirconfirmedthatN2adsorptiononmicabetween90‐155Kwaspurelyphysicalinnature i.e.,physisorption andcouldbewell‐describedbyhisequilibriumkineticmonolayermodelseeFigure2 .Modernfittinganalysisofthesedatavalidatethisconclusion,andtheresultscanbeusedtodeterminethesurfaceareaoccupiedbyasinglemoleculeofN2inacompletemonolayer:1.69nm2.Thisvalueissignificantlylarger byafactorof~10 thanthecurrentlyacceptedconventionalvalueforN2adsorptionat77K 0.162nm2 ;thisresultmaybeduetotheelevatedtemperatureofLangmuir’s1918experiments 90K .Langmuirindeedrecognizedthatthemonolayerdensitiesreportedin1918wereroughlyofthecorrectorderofmagnitude,butwerelowerthanexpectedbasedonamonolayerofliquidbyafactorofupto~9,evenat90K.Nevertheless,thisevidencestronglyfavoredLangmuir’s“monolayer”pictureofadsorptionsincethenumberofadsorbedN2moleculesdidnotexceedthatofaliquidmonolayerbutapproachedawell‐definedmaximum.
Langmuir’ssecondsetofexperimentswereperformedonnearly200individualglassmicroscopeslides,eachdefectedslightlytopreventstackingandpermitfreeaccessofthegastotheamorphoussurface.Thetotalsamplemasswas~36.7g basedonareportedvolumeof13.6mL ,andthetotalsurfaceareawasmeasuredtobe0.1966m2 specificsurfacearea:0.00535m2g‐1 .Severalpartsoftheapparatususedintheprecedingmicaexperimentswerereplacedtopermithigheraccuracyadsorptionmeasurementsnecessitatedbytheextremelylowbindingstrengthoftheglasssurface.Impressively,totalquantitiesofadsorbedgasasminuteas20nmolontheentiresample thelowestamountdetectedisthelastpointofLangmuir’sargondesorptionisotherm,at0.1Paand90K couldbedetected.ItisunclearwhichattributeoftheglasssurfacethatLangmuirsetouttocontrastwiththemuscovite:theamorphousnatureoftheglasssurface CaseIII,describedbelow,istheoreticallydevelopedforamorphoussurfaceswithinfinitelymanydifferentsites ormerelyits
8
differentchemistry asSiO2 .Inanycase,themostremarkableresultswereforCOadsorptiononglassat90K;theCOcouldnotbefullydesorbedundervacuum,motivatingamulti‐sitemonolayermodel CaseII whereonetypeofsiteischaracterizedbyweakadsorption physisorption andanotherbystrongadsorption chemicaladsorption,orchemisorption ,asshowninFigure2.
Langmuir’sthirdsetofexperimentswereperformedtodifferentiatethenatureofchemicalandphysicaladsorptiononplatinummetalfoil,andtounravelsomeofthecomplexitiesoftheplatinum‐catalyzedoxidationofH2andCO.Asmallsquareof0.010mmthickfoilwasused,washedandcarefullyfolded,havingatotalmassof4.03gandatotalsurfaceareaof0.0312m2 specificsurfacearea:0.00774m2g‐1 .Langmuirfoundthatatroomtemperatureandbelow,onlyverysmallquantitiesofH2,O2,andCOcouldbeadsorbedonplatinum,farbelowthatofthecalculatedmonomolecularlayer.Aseriesofexperimentsataboveambientpressurewerethenreported,demonstratingseveralkeyfeaturesoftheplatinumsystem,namely:chemicalreaction,catalysis,andhysteresis dependingonwhatisnowknownas“activation” .Theresultinginsightsintothecatalyticreactionmechanismsatplayontheplatinumsurfacerepresentedimportantearlycontributionstoresearchinthebuddingfieldofheterogeneouscatalysis,whereLangmuir’sworkwouldhavelong‐lastingimpact.35
CorroboratingLangmuir’sTheory.Numerousinvestigationswerecarriedoutinthedecadesafter1918totestLangmuir’stheoryinothersimplegas‐solidadsorptionsystems.Ithasbeenfoundthatintheroutinecharacterizationofpracticalmaterialssuchasheterogeneouscatalystsandporousadsorbents usuallywithsubcriticaladsorptivefluidslikeN2 ,multilayeradsorptionisfarmoreoftenverifiedthansimplemonolayeradsorption.ThisledtoamoredetailedtheoryofmultilayeradsorptionchampionedbyStephenBrunauer,PaulEmmett,andEdwardTeller36 aspecificmodificationofLangmuir’sCaseVIreferredtoasBETtheory,describedbelow thatremainswell‐knowntoday.LangmuirremainedsilentduringhislifetimeontherelativeutilityofhistheoryascomparedtothatofBrunauer,Emmett,andTeller,afactthatwas perhapsrathersensitively interpretedbyBrunauertoimplyresentment.37However,itisclearthat,atleastinthecaseofgasadsorptiononexternalsolidsurfaces,nosuchdefenseoftherelativeapplicabilityofeachequationneedbemade.Langmuir’ssimplestmonolayerformulationisusuallyapplicablewhenthestrengthofinteractionbetweentheadsorbateandsolidsurfaceisfarstrongerthanthatbetweentwoadsorbatemolecules,whileBETtheory whichisitselfanextensionofLangmuir’sownmultilayertheory isapplicablewhentheinteractionsaremoresimilar.Itismerelyaquestionofwhichregimerepresentsthesystemofinterest.WenotethatLangmuir’soriginaldata e.g.,N2adsorptiononmicaandCOonglassat90KasshowninFigure2 werenotcollectedathighenoughpressuretodiscerntheaccumulationofmultipleadsorptionlayers.
Numerousadsorptionisothermequationsarenowgenerallyusedtointerpolateand/ormodelexperimentaladsorptionequilibria,typicallybasedononeofthreefundamentalunderlyingapproaches:Freundlich’sempiricalequation,Langmuir’ssurface‐basedequations,orPolanyi’spotentialtheory.Surfaceswithinporoussolids,inparticularwithinmicroporoussolids,presentashighlycomplextoaccuratelymodel,andtypicallycannotbeadequatelyaddressedwithasimpleLangmuir‐typeanalysis.Forfurtherinformation,thereaderisreferredtoseveralmoredetailedreviews6‐7,38ofthesubject.
Hill’sEquilibriumTheoryofProteinBinding.AnanalogoustheorytoLangmuir’stheoryofmonolayergasadsorptiononsolidsurfaceshadinfactalreadybeenindependentlydevelopedsignificantlyearlierthan1916,specificallytoexplaintheoxygen“dissociationcurves”associated
9
withhemoglobininhumanandanimalblood.Likegasadsorptiononafinitenumberofsolidsurfacesitesasafunctionofpressure,thebindingofaligand e.g.,oxygen tofunctionalsitesonafinitenumberofproteinsinsolutionwillreachavalueatequilibriumthatdependsontheconcentrationoftheligand theadsorbate .ArchibaldV.Hill,anundergraduatestudentatTrinityCollegeinCambridge,basedthistheoryondatareviewedbyBarcroftandCamisintheJournalofPhysiologyin1909,39relatingthefractionofoxygen‐saturatedproteintothepartialpressuretension ofoxygeninvariousaqueoussolutions.IthadearlierbeenobservedbyChristianBohretal.thattheshapeofsuchdissociationcurveswassigmoidal,indicatingcooperativebinding.40Hill,whowouldlatergoontoreceivetheNobelPrize inMedicinein1922 ,suggestedthatthedatacouldbeaccuratelyrepresentedbyanequationderivedfromamass‐actionframeworkforthereversiblecombinationofaligand orgeneralbindingmoiety withadiscretesetofreceptorstructures bindingsites atequilibrium.41
ThesamegenerallawsofmassactionatequilibriumunderpinboththeHillandLangmuirequations,withakeydifferenceinemphasisforHill’sdevelopment:theallowanceformultiplebindingsites andhencecooperativeadsorption onasingleprotein.ThesimplerHillequationforsystemswithonlyasinglereceptorsiteperproteinisidenticaltotheLangmuirequationforsimple,single‐sitegasadsorptiononasolidsurface.Itcanthusbederivedusingthesameequilibriumkineticmodel i.e.,thelawofmassactionforasimplecombination“reaction” asgivenabove.Asummaryofthemassactionapproach foreithersingle‐sitegasadsorptionorsingle‐receptorproteinbinding canbemadeasfollows:
Single‐SiteGasAdsorption Single‐MoleculeReceptorBinding
A ↔ A L R ↔ LR Equation3
1 L
1 LEquation4
Foroxygenbindingtomyoglobin,forexample,thissimpletreatmentisconsistentwithexperimentalresults,indicatingthatasinglereceptorbindsoxygenoneachproteinsincenoadsorbate‐adsorbatecooperationisdetected seeFigure3 .
ThemoregeneralHillequationforbindingstructureswithmultiplereceptorsitesperprotein firstreportedinitspresentformin191042 canbederivedusingasimilarmass‐actionatequilibriummodel,butforaligandbinding“reaction”involvingmultiplereceptorsonthesameprotein:
Multi‐MoleculeReceptorBinding
L R ↔ L R Equation5
L1 L
Equation6
Hilloriginallypicturedsingle‐receptorproteinsthatwere“coagulated”together,causinginteractionbetweenthebindingsitesoncloselyheldproteins;thishassincegivenwaytothemodernunderstandingofmultipleligandreceptorsoneachprotein,asdeterminedbyX‐raycrystallography.43Foroxygenbindingtohemoglobin,simpleligand‐bindingtheory correspondingtoEquation4 isnotconsistentwithexperimentalresults,norwereotherempiricalequationssuch
10
asthatproposedbyWolfgangOstwald intheveinofFreundlich’sempiricalequationforgasadsorption .39Instead,HillshowedthatexperimentalresultswereconsistentwitheitheracombinationofEquation4andEquation6 where 2wasimposed orsimplybyEquation6alone where wasallowedtovaryasanindependentparameter .Inthelattercase,thevalueof wasinitiallydeterminedtobe~1.4,indicatingsignificantcooperationbetweentheoxygenbindingsitesonasingle,tetramerichemoglobinprotein seeFigure3 .Modernexperimentaldataconfirmahighervalueof ~2.6.44Thiscooperativebindingofoxygenbetweenthefourferroushemecomplexesinasinglehemoglobinproteinhascrucialimplicationsforoxygentransportanddeliveryinbiologicalsystems.ItisnotablethatHill’stheorydidnotallowforpartialoccupationofeachprotein e.g.,asinimposingindividualcontributionstothetotaladsorbedamountfromstatescorrespondingto 1, 2, 3, 4… infavorofasimplerequationwithfewerindependentparameters.
Figure3. a EquilibriumO2bindingonmyoglobin,asreportedbyRossi‐FanelliandAntonini45,showingsingle‐siteLangmuir SSL character. b EquilibriumO2bindingonhemoglobin,asoriginallymeasuredbyFerryandGreen46andthennormalizedbyPauling47,showingcooperativeadsorption CA character.Myoglobin proteinstructureinsetleft has1bindingsitethatbindsO2,whilehemoglobin proteinstructureinsetright has4bindingsitesthatsemi‐cooperativelybindO2,lendingaHillcoefficientof~2.6.
TheHillcoefficient, ,isrecognizedasanindicatorofcooperativeinteractions,sinceitdescribesthenumberofmoleculesboundperreceptor or,inthegas‐solidadsorptionsystem,thenumberofadsorbedmoleculespersurfacesite .Thiswasoriginallypositedtobeequalto,inthecaseofproteinbinding,theintegernumberofreceptorsperprotein;inreality,however,proteinbindingrequiresmorecomplexitytoaccuratelydescribe,andsotheHillcoefficientisonlylooselyindicativeofthecooperativitybetweenreceptorsites.Indeed,thevalueof isoftenanon‐integer.Langmuiralsoconsideredthepossibilityofcooperativeadsorptionphenomena CaseIV,detailedbelow butdidnotexplicitlyworkouttheequationforadsorptionbetween neighboringadsorbatesin1918.TheHillequationcontinuestoplayanimportantroleinbiologicalanalysisacrossawidevarietyofapplications,includingdrugdiscovery.48Theunderlyingphysicalrationaleforcooperativeeffectsinligandbinding,includingofoxygenonhemoglobin,49remainsanimportanttopicofresearchtoday.
Langmuir’sClassificationsofAdsorption
11
Langmuiracknowledgedthatthediversityofsurfacechemistryandstructuralgeometryexhibitedbysolidmaterials,inconjunctionwiththediversityinsizeandcharacterofadsorbatespecies,manifestsinwidelydifferentphysicalmechanismsofadsorption.1Langmuiridentifiedandclassifiedsixdistinct,simplemechanismsofadsorptioninLangmuir1918,andthenderivedtheequationgoverningtheamountadsorbed, ,asafunctionofpressure, ,atconstanttemperature,,foreachcase.ThesesixclassificationsaredepictedschematicallyinFigure4.Derivationsofeachcase includingsomespecialsub‐cases,indicatedbyanasterisk follow,alongwithexamplesofrepresentativeadsorptionbehaviorreportedintherecentliterature.
AlthoughLangmuiroriginallyemployedequilibriummass‐actiontheorytoderivetheadsorptionisothermforeachcase,amorefundamentalstatisticalmechanicalderivationispresentedherein.Eachbeginsbyprescribingthepotentialenergyfieldcreatedbythesurfaceofthematerial, ,where ∈ isapointonthesolidsurface.ThepotentialenergyfieldarisesfromrelevantelectrostaticandLondondispersionforcesbetweenthesurfaceofthematerialandtheadsorbatemolecules.Usingstandardstatisticalmechanicalmethods,theadsorptionisothermthenfollowsdirectlyfromthepotentialenergyfield, ,andastipulationforhowtheadsorbedmoleculesinteractwitheachother.Thisisaguest‐hostperspectivewhereitisassumedthattheadsorbatedoesnotaffectthestructureoftheadsorbent.
Figure4.SixadsorptionclassificationswereproposedbyLangmuirin1918:single‐siteor“simple”SSL adsorption,multi‐site MSL adsorption ofwhichasub‐caseisdual‐site DSL adsorption ,generalizedmulti‐site GL adsorption,cooperativeadsorption CA ofwhichasub‐caseisquadraticadsorption QA ,dissociativeadsorption DA ,andmultilayeradsorption MLA .
CaseI:Single‐Site Simple LangmuirAdsorption SSL
Thesimplestcaseofgas‐solidadsorptionisthatonasurface coveringaunitgramofadsorbentmaterial composedof identicalelementaryadsorptionsites,eachcapableofhostingonlyasingleadsorbedmolecule,A .Inthesimplestcase,adsorbate‐adsorbateinteractionsonthesurfaceareneglected,andthebulkgasphaseistakentobeideal.Ifthepotentialenergyofanadsorbateatanybindingsiteonthesurfaceisuniformandtakentobeequalto withdimensionsofenergyper
12
molecule ,theadsorptionisothermequation theadsorbedquantity, ,asafunctionofbulkgaspressure, isfoundtobe:
1 Equation7
where havingdimensionsofinversepressure istheLangmuirbindingconstantequivalenttothatdefinedinEquation2.
StatisticalMechanicsDerivation.Weconsideracrystallinesurfacewithaunitcellcontainingasingle,molecularlyaccessibleadsorptionsite.Thepotentialenergyofanadsorbatemoleculeatanypoint withinasingleunitcellisdescribedas:
if ∈
∞ if ∉Equation8
where isthesetofpointscomprisingtheadsorptionsite i.e., isfor“bindingsite” ofvolume| | thenotation| |where isasetofpointsisusedtoindicatethe“volumeof ” .The
potentialenergyisinfinityoutsidetheadsorptionsite.Whenthebindingsiteisunoccupied,thepotentialenergyoftheunitcellis0.Thetotalvolumeoftheunitcell,where isthesetofpointscomprisingtheunitcell,isthen | |.
ThefundamentalassumptionsinherenttotheSSLmodelare:
i everybindingsiteisidentical,ii therearenointeractionsbetweenadsorbedmolecules,andiii therearenointeractionsinthegasphase i.e.,itisanidealgas .
Theadsorbedphaseistakentohaveaconstantvolumeandallowedtoexchangeinternalenergyandparticleswithathermal atconstant and“chemical” atconstant reservoir.Therelevantstatisticalmechanicalrepresentationisthegrandcanonicalensemble;atanygivensite,thegrandcanonicalpartitionfunction, ,isasumoverall two possiblestatesofthesite:
, , 1 1 Equation9
where istheinversetemperature , istheBoltzmannconstant, isthethermaldeBrogliewavelengthoftheadsorbatemolecule,and isthechemicalpotentialofthebulkgasphaseinequilibriumwiththeadsorbedphase.ThefirstandsecondtermsinEquation9correspondtothesingleadsorptionsitebeingemptyandoccupied,respectively.Inthisconfigurationalpartitionfunction,the“state”oftheadsorptivemoleculeisacontinuum andintegrationisperformed ,withthethermaldeBrogliewavelengthaccountingforitsunderlyingdiscretenature.
TheadsorptionsitesintheSSLmodelareindependentandhencethetotalgrandcanonicalpartitionfunctionoftheentireadsorbedphaseoverall adsorptionsites, ,isfactorizable:
, , Equation10
Finally,theexpectednumberofadsorbedmolecules, ,followsfromaderivativeofthetotalpartitionfunction:
13
⟨ ⟩log
, 1 Equation11
Atthermodynamicequilibrium,thebulkgasphase actingasa“chemicalreservoir” imposesitschemicalpotentialontheadsorbedphase,50whichforanidealgasisgivenby:
log Equation12
BysubstitutingEquation12intoEquation11toreplacechemicalpotentialwiththebulkgaspressure,thecommonrepresentationoftheLangmuiradsorptionisotherm Equation7 isobtained,withtheLangmuirconstantexplicitlydefinedas:
Equation13
ThisequationrelatestheLangmuirconstanttothetemperature, via ,thebindingsitevolume,,whichdeterminesthetranslationalentropyofanadsorbedmolecule,andtothepotential
energyofadsorption, .
ModernExample:XenonAdsorptiononSBMOF‐1.TheadsorptionofxenononSBMOF‐1atroomtemperaturebetween0.1‐100kPaisarepresentativeexampleofSSLadsorption.51SBMOF‐1,alsoknownasCa sdb sdb 4,4‐sulfonyldibenzoate ,iscomprisedofone‐dimensionalporeswithconstrictionsof~4.2Åindiameter;insitusingle‐crystalX‐raydiffractionstudiesandmolecularmodelsrevealthatxenonadsorbsatwell‐defined,uniformpockets bindingsites withinthesepores.ThestructureofSBMOF‐1andcontoursofthepotentialenergyofXewithinitspores viacalculationsbasedontheUniversalForceField areshowninFigure5a.Sinceeachwell‐defined,uniformpocketiscapableofholdingonlyasingleatom,theexperimentalXeadsorptionisothermexhibitsclearSSLcharacter.Themaximumbindingsiteoccupancy, ,obtainedbyfittingtheexperimentaldatatotheLangmuirmodel,iscloselymatchedtothetheoreticalmaximumnumberofXeatomsperunitgramofmaterialasestimatedbyinspectingthecrystalstructure shownasadashedblacklineinFigure5a .
Figure5. a EquilibriumadsorptionuptakeofXeonSBMOF‐1at298K,51anexampleofsingle‐siteLangmuir SSL adsorption,and b equilibriumadsorptionuptakeofCO2onMg‐MOF‐74at313K,52anexampleofdual‐siteLangmuir DSL adsorption.
14
CaseII:Multi‐SiteLangmuirAdsorption MSL
Wenowconsiderasurfacethatoffersmorethanonetypeofelementaryadsorptionsite,yetwhereeachsitemaystillonlyaccommodateasingleadsorbedmolecule,allbindingsitesareindependent,andadsorbate‐adsorbateinteractionsarenegligible.Thecontributionofeachtypeofsitetothetotalamountadsorbedissimplyadditive,leadingtotheMSLadsorptionisothermequation:
1 Equation14
wherethesumisoverthenumberofdifferenttypesofbindingsite, istheLangmuirconstantforsitesoftype ,andthetotalnumberofsitesissimply:
Equation15
CaseII*:Dual‐SiteLangmuirAdsorption DSL
ThesimplestcaseofMSLadsorptionisthatwherethesurfaceofferstwodistincttypesofadsorptionsite,knownasthedual‐siteLangmuir DSL model.Ifthesurfaceiscomprisedof adsorptionsitesofsitevolume , andcharacteristicbindingenergy ,for ∈ 1,2 ,theadsorptionisothermequationis:
1 1 Equation16
AkeyillustrationoftheDSLmodelisthat,asmoregasadsorbs,gasmoleculestendtofilltheadsorptionsitesonaheterogenoussurfacethatofferthelowerpotentialenergybeforeoccupyingthosewithahigherpotentialenergy.Thedisparityinbindingenergybetweenthetwositesandthetemperaturedeterminetheextenttowhichthemorefavorablesitesarefilledfirst.Atdiluteconditions,theratioofthenumberofadsorptionsitesoccupied sitesoftype1tothoseoftype2 is:
,
,
,
,Equation17
followingfromthederivationpresentedbelow.Ahighernumberofadsorptionsites, ,andlargerbindingsitevolume, , ,tendtoincreasethediscrepancyinoccupancybetweenthetwotypesofsite.Thediscrepancyinthepotentialenergyofadsorption, ,hasanexponentialinfluenceonthedifferencesinoccupancy,modulatedbythetemperature.Athighertemperatures,thedifferenceinpotentialenergyofadsorptionbetweenthetwotypesofsiteislessconsequentialasentropyincreasestheexplorationofbothtypesofsite;atlowertemperatures,agreaterdifferenceinoccupancyisobservedforagivendifferenceinpotentialenergies.
StatisticalMechanicalDerivation.Weconsiderasurfacecontainingtwodistincttypesofadsorptionsite.Eachsiteprovidesaspatiallyhomogeneouspotentialenergyofadsorption, ,forasinglegasmolecule,where ∈ 1,2 .Thesingle‐sitegrandcanonicalpartitionfunction, ,foreachtypeofsiteisthesameasthatderivedforSSLadsorption:
15
, , 1 , Equation18
Thetotalgrandcanonicalpartitionfunctionisfactorizablesinceeachadsorptionsiteisindependent,andthus:
, , Equation19
Theexpectednumberofadsorbedgasmoleculesisthen:
⟨ ⟩log
,
,
1 ,Equation20
Thisequationcanberewrittenas⟨ ⟩ ⟨ , ⟩ ⟨ , ⟩,where⟨ , ⟩istheexpectednumberofmoleculesadsorbedonthesitesoftype .Thus,thecontributionbyeachtypeofsiteispurelyadditive.UsingEquation12toreplacechemicalpotentialwiththepressureofthebulkgasphase,theDSLadsorptionisotherm Equation16 isachieved.
ModernExample:CO2AdsorptiononMg‐MOF‐74.ArepresentativeexampleofDSLadsorptionisthatofCO2onMg‐MOF‐74at313K,asshowninFigure5b.52TheMgformofMOF‐74,alsoknownasMg2 dobdc dobdc 2,5‐dioxido‐1,4‐benzenedicarboxylate ,iscomprisedofone‐dimensional,hexagonalchannelsboundedbyexposedsquarepyramidalMg2 cationsthatareknowntobesitesofstrongadsorptionformanysmallgasmolecules.Theinflectionintheadsorptionisothermat~105PaisaclearindicationofDSLcharacter,andisduetothelargedisparityinbindingenergybetweentheadsorptionsitesneartheexposedMg2 metalcentersandelsewhereonthesurface~42and~24kJmol ,respectively .FittinganalysisperformedhereinyieldstwodistinctLangmuirconstants: 2.49 10 Pa and 1.28 10 Pa .IncontrastwithCOadsorptiononglassasreportedinLangmuir1918,bothdistinctadsorptionsitesinMg‐MOF‐74areofphysisorptivetype,allowingtheentireadsorptionisothermtobereversibleat313K.
CaseIII:GeneralizedLangmuirAdsorption GL
WhileCaseIIisdesignedtohandlemultipletypesofdistinctadsorptionsites,anamorphousmaterialmaycompriseanintractablenumberofdifferentadsorptionsiteswithdifferingaffinitiestowardtheadsorbate.Langmuirsuggestedtotreatthedistributionofsiteaffinitiesinsuchanamorphousmaterialasacontinuum.3Neglectingadsorbate‐adsorbateattractions,theadsorptionisothermfollowsfromthedistributionofbindingenergiesamongtheadsorptionsites.
Let bethenumberofelementaryspacesontheentiresurfacewithacharacteristicbindingenergybetween and .Weneglecttoemployaunitcell,permittingthetreatmentofeithercrystallineoramorphoussurfaces.Necessarily, ,thetotalnumberofelementaryspacesofferedbythesurface.Whentheroleofadsorbate‐adsorbateattractionscanbeneglected,theadsorptionisothermissimplythecontinuumanalogytotheMSLmodel:
1 Equation21
where istheLangmuirconstantofasiteofferingenergyofadsorption .
Langmuirnotedthatanimpracticallyintimateknowledgeofthesurfacestructurewouldbeneededtowrite explicitly.Nevertheless,Langmuirspeculatedthatconclusionsaboutthecharacterof
16
couldinfactbedrawnfromtheexperimentaladsorptionisotherm,3whichwaslatervalidatedbySips.18AstrongassumptioninherenttoEquation21isthat isthesameforalladsorptionsites,53butSipsnotedthat can,inprinciple,alsobewrittenasafunctionof .19AsimplecaseofEquation21iswherethedistributionisaDiracdeltafunctioncenteredat ,orinotherwordswhere .Inthiscase,alladsorptionsitesareidenticalandtheSSLmodelisrecovered asinEquation7 .
CaseIII*:UniformLangmuirorUnilanAdsorption UL
AnespeciallyusefulcaseofEquation21isknownastheUnilan for“uniformLangmuir” ,orULmodel.54Inthiscase,allofthebindingsitesareuniqueandthebindingenergiesaretakentobeuniformlydistributedbetween .19Thisisexpressedas:
if ∈ ,
0 otherwise
Equation22
Byimposingthisdistribution,Equation21resultsintheUnilanequation:
2ln
1
1 Equation23
where isrelatedtotherangeofadsorptionenergiesas:
2 Equation24
IntheUnilanformalism, istheLangmuirconstantofahypotheticalmaterialwithuniformadsorptionsitesexhibitingtheaveragebindingenergy,givenas:
Equation25
StatisticalMechanicalDerivation.ThestatisticalmechanicalderivationoftheUnilanequationisequivalenttothatforCaseII MSL exceptwithaverylargenumberofdifferentsingle‐sitepartitionfunctionsequaltothenumberoftotalsites .EachpartitionfunctionhasthesameformasEquation18.Thetotalgrandcanonicalpartitionfunctionissimplytheproduct:
, , … Equation26
TheUnilanequation Equation23 followsasusualbydeterminingtheexpectednumberofadsorbedmoleculesatconstanttemperature,volume,andchemicalpotential.18
ThegeneralizationofLangmuir’sSSLadsorptionmodeltoaccountfordifferenttypesofadsorptionsites MSLadsorption permitsitsapplicationtomaterialswithheterogeneoussurfaces.However,theextremecase,acompletelyamorphoussurfacewhereeverybindingsiteisdifferent,wouldrequiremorethan characteristicparameterstobemodelledinthisway,anunreasonablylargenumber.TheULequationisanexampleofapracticalsolutiontothisproblem,allowingforawiderangeofbindingenergiesbutenforcingauniformdistributionofthem,andtherebyminimizingthenumberofcharacteristicparametersofthesystem.TheULmodelisjustifiedforanysystem
17
exhibitingadistributionofbindingsitessolongasadsorbate‐adsorbateinteractionsremaininsignificantandthebulkgasphaseisideal.
ModernExample:H2AdsorptiononZTC.Atypicalcharacteristicofgasadsorptiononamorphoussurfacesisagentleslopingsigmoidaluptakeprofilewhenplottedonlog‐normalaxesasafunctionofpressure.ThisasdemonstratedbyH2adsorptiononzeolite‐templatedcarbon ZTC 55at77K,asshowninFigure6a.56Theexperimentaldatapointsareeachuniquelycoloredtorepresentdifferentcharacteristicbindingsites.TheamorphousnatureofthemicroporoussurfaceofZTCleadstoamultitudeofdifferentbindingenergies;theaverageLangmuirconstantis 2.9 10 Pa .ItisclearthatnooneSSLequationwouldfitthesedataoveranappreciablerangeofpressure.TheSSLisothermforahypotheticalmaterialwithidenticalbindingsitesofthesameaveragebindingenergyasinthedistributiononthesurfaceofZTC correspondingto isshownasapurplelineandissignificantlysteeper.AULmodelfitsthedatawell shownasablackline ,usingonlythreeindependentparameters,owingtothefactthatH2hasweakintermolecularinteractionsandthatZTChasnoshort‐rangeorderedmolecularstructure.57Owingtoitssimplefunctionalformyetcomplexphysicalrepresentation,theULmodelisalsosuccessfulinengineeringapplicationssuchasthedescriptionofH2adsorptiononMOF‐5acrossawiderangeoftemperatureandpressure.58
Figure6. a EquilibriumadsorptionuptakeofH2onzeolite‐templatedcarbon ZTC at77K,56anexampleofgeneralizedLangmuir GL adsorption. b EquilibriumadsorptionuptakeofN2simulatedonIRMOF‐16at77K,59anexampleofmultilayeradsorption MLA withinaporousmaterial.
CaseIV:CooperativeAdsorption CA
Wereturntothecasewhereeachbindingsiteonasurfaceisidentical,butnowwhereeachsiteispermittedtohostmultipleadsorbedmolecules.Theadsorptionofthefirstmoleculeonagivensiteaffectstheenvironment potentialenergyofadsorptionandvolumeavailablefortranslationalentropy ofthesecondadsorbingmolecule,andsoonasmoremoleculesadsorbonthesamesite.Inotherwords,adsorptioniscooperativesincethepresenceofotheradsorbatesonthesamesiteaffectstheenergeticsoffurtheradsorption.Thegeneralcasewhere neighboringmoleculesareallowedtointeractiscomplex,andtheinterestedreaderisreferredelsewhere.53
18
CaseIV*:QuadraticAdsorption QA
ThesimplestCAmodelisthatofasurfacewhereeachbindingsitecanaccommodateuptotwoadsorbedspecies,andwhereanadsorbate‐adsorbateintermolecularinteractiontakesplacewhenbothsitesareoccupied.Inthisparticularcase,thequadraticadsorption QA isothermisobtained:
2 21 2 Equation27
where istheLangmuirbindingconstantcorrespondingtoeachhalf‐site identicaltoEquation13 , isthenumberofadsorptionsites eachbindinguptotwomolecules ,and isafactordescribingthemagnitudeofadsorbate‐adsorbateinteractions whentwomoleculesareboundonasinglesite .Fornon‐interactingmolecules, 1andEquation27reducestothesimpleLangmuiradsorptionisotherm Equation7 withamaximumadsorptionoccupancyof2 .Inthenon‐trivialcase, 1forattractiveadsorbate‐adsorbateinteractionsand 1forrepulsiveadsorbate‐adsorbateinteractions.Inthelattercase,adsorptionofthesecondmoleculeislessfavoredthanthatofthefirst,forexampleduetosterichindrance.
StatisticalMechanicalDerivation.Thepotentialenergyofeachadsorptionsitedependsonitsmicrostate;eachsitecanaccommodateuptotwomoleculesandsohasthreedistinctmicrostatesempty,singlyoccupied,ordoublyoccupied .Welet , ∈ 0,1 determinethestateofeachhalfofanadsorptionsite,where 0correspondstothehalf‐site asemptyand 1tothehalf‐siteasoccupied.Thepotentialenergyofanindividualadsorptionsite, ,isthen:
, Equation28
where isthespatiallyuniformpotentialenergyexperiencedbyasinglemoleculeadsorbedoneitherpartoftheadsorptionsiteduetoitsinteractionwiththesurface,and isthepotentialenergyassociatedwiththeinteractionbetweentwoadsorbedmoleculeswhenthesiteisdoublyoccupied.Thegrandcanonicalpartitionfunctionforthesingleadsorptionsiteisthenfoundbysummingoverallofthemicrostates atotaloffourmicrostates,withtwodegeneratesinglyoccupiedstates :
, , , ,
1 2Equation 29
The termarisesfromtheconfigurationalpartofthepartitionfunctionwheretheBoltzmannfactorisintegratedoverallparticlecoordinates; isthefreevolumeofferedbyeachadsorptionhalf‐siteinthepair.Thefirst,second,andthirdtermsinthesumcorrespondtothethreedistinguishablemicrostatesidentifiedabove.For independentadsorptionsites upto pairsofadsorbedmolecules ,thetotalgrandcanonicalpartitionfunctionisfactorizable:
, , Equation30
Theexpectednumberofadsorbedmoleculesisthen:
⟨ ⟩log
,
2 2
1 2 Equation31
19
ByreplacingthechemicalpotentialwiththepressureoftheidealgasviaEquation12,theresultgivenabove Equation27 isachievedwith asintheSSLmodeland .Thusif 0 forattractiveinteractions ,then 1,andif 0 forrepulsiveinteractions ,then 1,asexpected.
CaseIV*:HillEquation
Wheneachadsorptionsiteisrequiredtobeeitheremptyoroccupiedbythemaximumnumberofoccupants,theHillequation Equation6 isobtained.Ifthemaximumnumberofoccupantspersiteistwo,theformalderivationfollowsthatofthemoregeneralQAmodelabove,exceptwithonlytwotermsinthesingle‐sitepartitionfunction:
, , 1 Equation32
Thetotalgrandcanonicalpartitionfunctionremainsfactorizable:
, , Equation33
TheHillequationfor 2followsfromtheusualderivative:
⟨ ⟩log
,2
1 Equation34
ByreplacingthechemicalpotentialwiththepressureoftheidealgasphaseviaEquation12,thefollowingadsorptionisotherm equivalenttoEquation6 isachieved:
⟨ ⟩ 21 Equation35
Themagnitudeoftheinteraction, ,isdefinedasaboveandtheLangmuirconstantisdefinedasinEquation13.TheHillequationisanover‐simplifiedapproximationformostadsorptionsystemswhereanynumberofmoleculesbetweenzeroandthemaximumnumberofoccupantscanoccupyagivensite ,butremainsacommonlyemployedmodel,nevertheless.
ModernExamples.Cooperativeadsorptionisrareinthemodernadsorptionliterature,andcasesofpureQAbehaviorinagas‐solidsystemareunknowntotheauthors.SomerepresentativeexamplesofCA‐likebehavioraregivenbyCO2adsorptionontheUSO‐2‐MseriesofMOFsat298K,especiallyinthenickelform M Ni .60‐61AsecondexampleisthatofethaneonvariousMOF‐74‐typematerials,especiallyMn‐andCo‐MOF‐74,wherethemaximumoccupancycorrespondsroughlytotwoethanemoleculesperformulaunit.62‐63TheadsorptionofCO2onthealuminumformofMIL‐91alsoexhibitsaCA‐likeinflection.64Lastly,thepresenceofdynamicmoietieswithinaporouscrystalmayleadtophenomenaresemblingCAbehavior.65
CaseV:DissociativeAdsorption DA
Inthepreviouscases,itwasassumedthattheadsorbatedoesnotundergoanysortofchemicalchangeuponadsorptionordesorption.InCaseV,Langmuirconsideredthepossibilityofadsorptionbeingatwo‐foldprocess:residenceatthesurfaceadsorptionsiteincombinationwithmoleculardissociation aresultofchemicalbonding .Thistypeofbehaviorcommonlyoccursatthesurfaceofacatalyst,forexample,whereadiatomicmolecule A adsorbsatabindingsiteandsubsequentlydissociatesintotwoatoms A ,whicharethenpermittedtodiffuseonthesurface freelyexploring
20
theavailableadsorptionsitesasanatomicspecies .Thereverseprocessrequirestwoneighboringatomsonthesurfacetoreassociateintothediatomicmolecule A A → A andleavethesurfacei.e.,undergodesorption .Inthisspecificcaseofhomonucleardiatomic/monatomicdissociativeadsorption DA ,theadsorptionisothermoftheatomicspecies,A,asafunctionofthepressureofthebulkgaseousmolecularspecies,A ,is:
√
1 √Equation36
StatisticalMechanicalDerivation.Atthermodynamicequilibriumofthesimplestdissociationreactiongivenabove A ↔ A A ,thechemicalpotentialsmustsatisfythefollowingrelationship:
2 Equation37
Thatis,thebulkgasphaseofthemolecularspecies,A ,imposesachemicalpotentialontheadsorbedatomicspecies,A,atthermodynamicequilibrium.ThismodeldoesnotpermitanydiatomicA toremainadsorbedonthesurface;dissociationandadsorptionorreassociationanddesorptionareconcomitantinstableequilibrium.Let bethespatiallyhomogeneouspotentialenergyexperiencedbyanatomicspeciesAadsorbedonthesurfaceand bethefreevolumeofthebindingsite thataccommodatesasingleatom .Then,thesingle‐sitegrandcanonicalpartitionfunctionis:
, , 1 Equation38
ThechemicalpotentialanddeBrogliewavelengthbothcorrespondtotheatomicspecies,A,sinceitistherelevantspeciesintheadsorbedphase.Thetotalgrandcanonicalpartitionfunctionforasurfacecontaining independentadsorptionsitesisthen:
, , Equation39
Theexpectednumberofmonatomicadsorbedspecies,A,isthen:
⟨ ⟩log
, 1 Equation40
UsingEquation37andEquation12torelatethebulkgaspressuretothechemicalpotentialoftheatomicadsorbedspecies,A,theDAisothermisobtained Equation36 .TheLangmuirbindingconstantisexplicitlydefinedas:
Equation41
withdimensionsofpressuretothepowerof½,wherethepressurereferstothediatomicgas.
ModernExamples.Typicalexamplesofdissociativeadsorption DA areforsimplediatomicmoleculeslikeH2onspecificsurfacessuchaspalladium,platinum,tungsten,andsilicon e.g.,Honsilicon 111 7 7at950K66 .Athoroughreviewofhydrogenonsiliconisgivenelsewhere.67TheauthorsarenotawareofanystrictexamplesofdissociativeadsorptiondirectlyonMOFsurfaceswithoutthepresenceofmetalnanoparticles,asinhydrogenspillover68 ,whichhasbeenattributedtothehighactivationbarriertodissociation.69
21
CaseVI:MultilayerAdsorption MLA
Lastly,Langmuiraddressedthecaseofmultilayeradsorptionuponafinitesetofunderlyingadsorptionsitesonasurface.Inthesimplestsuchcase,eachadsorptionsiteistakentobeidenticalandindependent;afterthefirstmoleculeisadsorbedonagivensite,anadditionalmoleculeispermittedtoadsorbaboveit.Inthisway,despitethatthenumberofadsorptionsitesislimited,thenumberofadsorbedmoleculesisunlimited assumingalargechemicalreservoirisheldincontactwiththesurfaceatconstantchemicalpotential anddivergestoinfinityatthesaturationpressureinthecaseofsubcriticaladsorption .Langmuirdidnotexplicitlywriteaclosed‐formadsorptionisothermforCaseVI,butdidremarkthatsuchanisothermwouldbegreatlysimplifiedifthesecondlayerandallabovewereconsideredunderidenticalconditions oreven,slightlymorecomplexly,thethirdlayerandallabove .3
CaseVI*:Brunauer‐Emmett‐Teller BET MultilayerAdsorption
Themostwell‐knownmultilayeradsorptionisothermistheBrunauer‐Emmett‐Teller BET model;itstipulatesthatadsorptioninthefirstlayeroccurswithaconstantbindingenergyandthatadsorptioninthesecond,third,andallhigherlayersoccurswithabindingenergyequivalenttotheenergyofcondensation liquefaction ofthebulkgasphase.36Langmuir’smoregeneralCaseVIpredatesBETtheorybytwodecadesbutisinfactinclusiveoftheBETstipulation.Hence,inthisreviewwehighlighttheBETequationasaspecificexampleofCaseVIsinceitisthemostwidelyusedadsorptionisothermequation,oftenemployedtocalculatethespecificsurfaceareaofporousandnonporousmaterials.IntheBETmodel,eachbindingsitecanaccommodatean“adsorptioncolumn”consistingofasingleadsorbedmoleculeatthebaseanduptoaninfinitenumberofadsorbedmoleculesaboveit.Adsorbate‐adsorbateinteractionswithineachcolumnareignored,aswellasinteractionsbetweenadsorptionsites.Forasurfacecontaining adsorptionsites,theBETmultilayeradsorptionisothermis:
1 1 Equation42
Asusual, istheLangmuirbindingconstantdescribingtheinteractionbetweentheadsorbateinthefirstmonolayerandthesolidsurface; isananalogousbindingconstantdescribingtheaffinityofanadsorbateinoneofthehigherlayerstowardtheadsorbatemoleculebelowit takentobeequivalenttothatofbulkliquefaction .
StatisticalMechanicalDerivation.AsfortheQAmodel,thepotentialenergyofadsorptionmustfirstbeexpressedasafunctionofeachmicrostateofagivenadsorptionsite.Asbefore,let bethepotentialenergyofamoleculeadsorbedinthefirstlayer,directlyinteractingwiththeadsorbentsurface.Inallsubsequentlayers,thepotentialenergyassociatedwithadsorptionis ,aconsequenceofinteractingwiththegasmoleculeinthelayerbelow.Thereisaninfinitesetofmicrostatesforeachadsorptionsite:empty,singlyoccupied,doublyoccupied,andsoon,uptoinfinitelyoccupied.With asthenumberofmoleculesadsorbedonanadsorptionsite,thepotentialenergyofasinglecolumnofmoleculesasafunctionofitsmicrostateis:
0 if 0
1 if ∈ 1, 2, …Equation43
22
Thegrandcanonicalpartitionfunctionofasingleadsorptionsite column isthenasumoverallpossiblemicrostatesincludingtheemptystate ∈ 0,1, 2, … :
, , 1 ⋯ Equation44
Thecanonicalpartitionfunctionsofasinglemoleculeadsorbedonthefirstlayerandinsubsequentlayersare,respectively:
,
Equation45
Equation46
ByfactoringoutthecontributionineachterminEquation43fromthemoleculeinthefirstlayer,werecognizethegeometricseries:
, , 1 1 ⋯ 11 Equation47
Thetotalgrandcanonicalpartitionfunctionforasurfacecontaining independentadsorptionsitesisfactorizable:
, , Equation48
Thetotalexpectednumberofadsorbedmoleculesisthen:
⟨ ⟩log
, 1 1 Equation49
ReplacingchemicalpotentialinEquation49withthepressureofthebulkgasphaseviaEquation12,wearriveattheBETadsorptionisotherm Equation42 with definedasinEquation13and:
, Equation50
ModernExample:N2AdsorptiononIRMOF‐16.ArepresentativeexampleofBETadsorptionwithinaporousmaterialisN2adsorptiononthesurfaceoflarge‐poreMOFssuchasIRMOF‐12,‐14,and‐16,wherethelatterisshowninFigure6b.59,70The~2nmwideporesofIRMOF‐16canaccommodatemultilayeradsorptionoveralimitedpressurerangeupto~20kPawhencross‐poreinteractionsbecomesignificant.Thiseffectisobviousathigherpressureswherethemeasuredadsorptionuptakedepartsdramaticallyfromtheexpecteduptake,firsttowardhigheruptakeasporefillingcompletes exhibitingstrongintermolecularinteractionsbetweenmultilayersonbothsidesofthepore andthenshowingabruptfillingandnofurtheradsorption.SimilarMLAisothermshavebeenmeasuredforN2adsorptiononotherlarge‐poreMOFssuchasNU‐100,71NU‐1300,72andNU‐109andNU‐11073.
Belowthe“rougheningtemperature,”layer‐by‐layeradsorptiononcrystallinesurfacesisdistinctlyobservedtooccur,resultingfromsignificantlydifferentbindinginteractionscharacteristicofeachlayer e.g.,Xeadsorptiononpalladium 100 between60‐70K74 .SuchanadsorptionsystemisaccuratelydescribedbyamorecomplexMLAequationthanthatoftheBETmodel,withindividualLangmuirconstantsassignedtoeachlayer.
23
ApplicationsandExtensionsofLangmuir’sTheory
TheLangmuirtheoryofadsorptionisusefulformodelinganddesigningengineeringprocesses,bothqualitativelyandquantitatively.SinceeachLangmuirmodelisfoundedonreasonablephysicalassumptions,thermodynamicconsistencyisalsoinherenttoeachadsorptionisotherm,andthecalculationofthermodynamicquantitiessuchastheenthalpyofadsorptionisdirectandanalytical.ThemostcommonmaterialpropertiescalculatedviaLangmuir’stheoryarethespecificsurfaceareaandtheisostericenthalpyofadsorption.Twocommonengineeringprocessesofinteresttogas‐solidadsorptionsystemsarestorageandseparation.
SurfaceAreaMeasurements.Specificsurfaceareaisanimportantcharacteristicofporousmaterials,especiallyfordeterminingtheapplicabilityofamaterialforengineeringapplicationssuchasstorageorseparation.ThetwomostwidelyusedequationsaretheBETandSSLmodels,thelatterofwhichwasfirstusedbyLangmuirhimselfin19183.Whileneithermodelisstrictlyappropriateforthedescriptionofgasadsorptionwithinnarrowmicropores,ithasbeenshownthattheBETmodelisoftensuitablefortheestimationofthetruesurfaceareaofmicroporousandmesoporousmaterialsincludingMOFsandzeolites despitethatthenumberofmultilayersisconstrained whenproperconsistencycriteria75areemployedindeterminingtherangeofpartialpressureoverwhichtofitthedata.59,76‐77
TheBETformalismprevalentintheliteratureiswrittenasfollows:
/1 / 1 1 / Equation51
where isthesaturationpressureoftheadsorbate,definedas:
Equation52
and istheBETconstant,definedas:
Equation53
Acommonlinearizedrearrangement,whereBissimplythe“BETvariable”,is:
/1 /
1 1/ Equation54
Brunauer,Emmett,andTellerdescribedthreeregionsalongtheBETisotherminEquation54:aconcaveregionatlowpressure,aconvexregionathighpressure,andalinearregionatintermediatepressure.Theyoriginallyspecifiedtherelativepressure / rangeof0.05‐0.3asthelinearregionfromwhichthenumberofsurfacesites, ,canbedependablyextracted.Inthecaseofhighsurfacearea,microporousmaterialssuchaszeolitesandMOFs,however,thisrangehasproventobeinadequateasauniversalstandardandanowwidelyacceptedsetofself‐consistencycriteriahavebeenproposed.75Oncethelinearrangeisdeterminedand isextracted,theBETsurfacearea, ,iscalculatedas:
∙ Equation55
24
where isthesurfaceareaofasinglebindingsite.Theconventionalcross‐sectionalareaofaN2moleculeforBETsurfaceareacalculationis0.162nm2,whichallowsresearchersacrossdifferentlaboratoriestohaveastandardformaterialscomparison.Forexample,inthecaseofIRMOF‐16,theBETsurfacearea monolayercapacity,shownasadashedblacklineinFigure6b determinedhereincorrespondsto61.8mmolg or6030m g .Thisisconsistentwiththegeometricalsurfaceareaofthecrystal ~6000m g asdeterminedbyaMonteCarlointegrationtechnique59 .ItshouldbenotedthatchallengesstillpersistinusingtheBETmodeltoaccuratelyestimatethemonolayercapacityofporousmaterials,especiallythosewithadiversityofporesizes.77
EnthalpyofAdsorption.Anotherimportantcharacteristicfordeterminingtheapplicabilityofaporousmaterialtostorageandseparationprocessesistheadsorptionbindingstrength.Whilethebindingenergy orenergies,inthecaseofamulti‐siteadsorptionmodel couldinprinciplebedirectlyextractedfromanyofLangmuir’ssimplemodelsviatherelationshipinEquation13,amoregeneral lessmodel‐dependent approachistocalculatethedifferenceinenthalpybetweentheadsorbedphaseandthebulkgasphase,∆ ,viatheClapeyronequationoravariantthereof.78‐79Whenthebulkgasphaseisideal,therearrangedClausius‐Clapeyronequationisemployed:
∆ ∆ Equation56
Thisrequiresthemeasurementofmultipleisothermsatdifferenttemperatures,andinvestigationofthetemperature‐pressurerelationshipalongalineofconstantadsorbedamount anisostere .Theresultingthermodynamicquantityisreferredtoastheisostericenthalpy oroften“isostericheat” ofadsorption;forSSLadsorptionunderabulkgasphasethatisideal,itisequaltothenegativeofthebindingenergy i.e.,∆ .TheSSL,DSL,andULmodelsareroutinelyemployedtointerpolatemeasureddata,animportantstepinthecalculationoftheisostericenthalpyofadsorption.58,80Whenasufficientquantityofdataaremeasured,theenthalpyofadsorptionmeasuredbythesorptionisostericmethodcomparesreasonablyaccuratelytocalorimetricmeasurements.81Itshouldbenotedthatthereremainsdebateastothevalidityoftheisostericmethodinthehighpressure non‐idealgas regimewherethemeasuredquantityofadsorption the“Gibbssurfaceexcess” departsfromtheactualquantity;undersuchconditions,aso‐called“isoexcess”methodhasbeenproposedwhichdoesnotrelyontheuseofanyadsorptionmodel.82‐83
DeliverableGasStorage.Thekeymaterialdesignmetricforadsorptivestorageisthedeliverablecapacity:theamountadsorbedatthestoragepressure, ,minusthatremainingintheadsorbedphasewhenthepressureisbelowtheusefulthreshold, e.g.,whenthestoragevesselcannolongersupplyafuelcellorcombustionengine .Forexample,theUSDepartmentofEnergy DOE targetfornaturalgasstorageon‐boardmobilevehiclessets 6.5MPaand 0.5MPafordeliveryatroomtemperature.InanadsorptionsystemcharacterizedbySSLtheory Equation7 ,thedeliverablecapacity, ,foranisothermalpressureswingbetween and is:
1 1Equation57
Foragivenstorageanddeliverypressure,thedeliverablecapacitydependsimportantlyonthematerialproperties, and .Whenthenumberofadsorptionsites, ,isfixed i.e.,maximized ,theoptimalLangmuirconstanttomaximize is:
25
1Equation58
Ifthebindingenergyistooweak,correspondingto ,thelowstoragecapacityatthestoragepressure, ,limitsthedeliverableamountofadsorbedgas.Ontheotherhand,ifthebindingenergyistoostrong,correspondingto ,asignificantquantityofadsorbateremainsboundattheminimumdischargepressure, ,limitingthedeliverablecapacityaswell.TheseeffectsareoptimallybalancedforamaterialthatexhibitsaLangmuirconstantgivenby .Then,byassumingatypicalentropyofadsorptionbasedonthedifferenceinentropybetweenthebulkgasandtheadsorbedphases,thisanalysiscanbeusedtoestimatetheoptimalenthalpyofadsorptiontomaximizethedeliverablestoragecapacity.84SeveralMOFshavebeenidentifiedbyasimilarmethodasrecord‐holdingmaterialsforbothmethane85andhydrogen86storageatambientconditions,andthesearchcontinuesforhighercapacitymaterialsusingadesign‐drivenapproach.
Mixed Competitive Adsorption.Anotherchiefapplicationofporousmaterialsistoseparategaseousmixturesthroughselectiveadsorption.Hence,itisusefultoconsiderthecaseofmixedgasadsorptionwheretwodifferentadsorbatespeciescompeteforthesamesetofadsorptionsites.Considerasurfacecomprising adsorptionsitesexposedtoanidealgasmixture A B .IftheAandBmoleculesarecomparableinsizesothateachadsorptionsiteaccommodatesonlyasingleAorBmolecule,theadsorptionisotherm where isthetotalnumberofAandBmolecules is:
1Equation59
where and aretheLangmuirconstantscorrespondingtothebindingofspeciesAandB,andand arethepartialpressuresofspeciesinthemixedgasphase oftotalpressure .87The
statisticalmechanicalderivationuncoversthedependenceoftheLangmuirconstantonthevolumeofthebindingsite, , ,andbindingenergy, ,wherebothcharacteristicsvarydependingonthespecies occupyingtheadsorptionsite,aswellasthetemperature.Thereby,theratioofthenumberofadsorptionsitesoccupiedbyspeciesAtospeciesBatdiluteconditionsis:
,
,
,
,Equation60
whichdependsontheratioofpartialpressures,ontheratiooffreevolumeavailabletoeachadsorbate anentropiceffect ,andexponentiallyonthedifferenceinbindingenergies.AsintheDSLmodel,lowertemperatureservestoamplifytheoccupancyratiodependenceonthedifferenceinpotentialenergiesofadsorptionofthetwospecies.ArepresentativeexampleofthedirectapplicationofEquation59tomeasurementsofmixed‐adsorptionequilibriaisfoundintheadsorptionofbinarymixturesofCO2,H2,andN2onactivatedcarbon.88
Themeasurementofmixed‐adsorptionequilibriarequiresanadditionalstepoverpureadsorptionmeasurements:varyingthecompositionofthebulk gaseousorsupercritical fluidandthenobtainingthecorrespondingcompositionoftheadsorbedphase,whichistime‐intensiveandtechnicallycomplicated.89Tocircumventtheselimitations,adsorptionselectivityisveryoftenestimatedbycomparingthepuregasadsorptionequilibriaofeachofthecomponentsofthedesiredmixtureandapplyingasimplemodel.Idealadsorbedsolutiontheory IAST 90isasimplemodelfordescribingthemixedadsorptionstatebasedonthepurecomponentadsorptionequilibria,analogoustoRaoult’slawforidealvapor/liquidsolutions.Themainprincipleliesintheconceptof
26
aconstantspreadingpressureoftheadsorbedphaseatequilibrium,whereinalladsorbatespeciesareassumedtobenon‐interacting.Fromthepure‐stateadsorptionisotherms, ○ ○, ,thespreadingpressure, ,isestimatedforeachcomponentasafunctionofthepressureofthepurebulkphaseaboveit, ○;inturn,thisrelationshipisextendedtotheco‐adsorptionstatebyassumingthatthe individual adsorbedphaseofeachcomponentwillbehavethesameasafunctionofthepartialpressureofthatcomponentinthemixedstate.Thebinarymixed‐statetotaladsorptionisotherm, , , ,foragivengasphasecompositiondefinedby , canthereforebeestimatedinadditiontotheindividual mixed‐state adsorptionisothermsofeachcomponent,
, , .
TheresultsofIASTanalysisareremarkablyaccuratewhencomparedtoexperimentalmixed‐adsorptionequilibria91‐92andalsowhenIASTisappliedtoGrandCanonicalMonteCarlo GCMC pure‐adsorptionsimulationsandcomparedtomixed‐adsorptionGCMCsimulations93‐94.However,theoverallaccuracyofthemethodisdependentonthesimilarityofthecomponentsoftheadsorbedphaseaswellasthebindingstrengthtowardandhomogeneityoftheadsorbentsurface;correctionscanbeappliedforsystemswheretheco‐adsorbedphaseissignificantlynon‐ideal,referredtoasRAST.95‐96TheanalysisofnumerousadsorptionpairsandmixtureshavebeencarriedoutusingIAST,demonstratingextraordinaryprospectsforusingMOFs,97zeolites,andotherdesignableporousmaterialsassolid‐phaseseparationmedia:e.g.,asincarboncapture,98‐99hydrogenrecycling,100hydrocarbonseparations,62,101andharmfulgasseparationsfromair,102amongmanyotherapplications.
Flexible,Gas‐ResponsiveMaterials.Itisusuallyassumedthattheadsorptionofgasmoleculesdoesnotchangethethermodynamicpropertiesorstructureoftheadsorbentitself.Suchaguest‐hostperspectiveisasimplificationthatisnotapplicableinporousmaterialsthatundergostructuralchangesinducedbytheadsorptionofgasmolecules:so‐called“flexible,”“soft,”or“breathing”materials.103‐105ThestructuralchangesinflexibleMOFsoftenhaveaprofoundinfluenceonadsorptionandcanleadtocounterintuitivepropertiessuchasnegativegasadsorptionsteps.106‐107TheLangmuirmodelservesasaplatformontowhichfurthercomplexity,suchasflexibility,canbeintegrated.
Gate‐openingmaterialsexistinacollapsed,nonporousstateinvacuumthatpersistsevenwhentheyareexposedtolowpressures lowchemicalpotential ofthegaseousadsorbate.108Asthechemicalpotentialofthegaseousreservoirisincreased,atsomeconditionthematerialabruptlyexpandstoaporousstateandanadsorptionstepoccurs.Aflagshipexampleofgate‐openingbehaviorisintheadsorptionofCH4onCo bdp wherebdp2‐ 1,4‐benzenedipyrazolate .109Atroomtemperature,gateopeningoccursconcomitantwithanabruptstepat1.6MPaontheadsorptionbranchoftheCH4uptakeisotherm;thereversetransitionisdetectedat0.7MPaonthedesorptionbranch.Importantly,thisstepoccursbetweenthedesiredpressureswingrangeinvolvedinvehicularadsorbednaturalgasstorage,therebyimpartingCo bdp withanextremelyhighdeliverablecapacityofCH4: 197LSTPL‐1 ~10mmolg‐1 .Moreover,thestructuraltransitioninCo bdp isendothermic,offsettingaportionoftheheatofadsorptionandmitigatingstorageinefficienciesuponrefilling.
Breathingmaterials,likegate‐openingmaterials,exhibittwostructuralformsthatarestableunderdifferentconditions:anarrow‐poreandlarge‐porestate.Thelarge‐porestateofabreathingmaterialisstableinvacuumanduptolowpressures lowchemicalpotential ofthegaseousreservoir .Atanintermediatetransitionpressure,thelarge‐porestatecollapsestothenarrow‐
27
porestate,affordingastrongeradsorptioninteraction.Atahighertransitionpressure,theframeworkexpandsagaintothelarge‐porestate,accommodatingmoreadsorbedmoleculeswithaweakercharacteristicbindingenergy.AflagshipexampleofbreathingbehaviorisinCO2adsorptiononM‐MIL‐53 M Cr,Al .110‐111AtroomtemperatureonAl‐MIL‐53,thefirstabruptstepiscenteredat24 15kPaandthesecondat425 75kPawherethepressurerangecorrespondstothewidthofthehysteresisloop.111
Ageneralstatisticalmechanicaltreatmentofgate‐openingandbreathingingas‐solidadsorptionsystemshasbeendeveloped.112Insteadofemployingthegrandcanonicalensembleasintheabovesixderivations wheretheadsorbedphaseisheldatconstant , ,and ,theosmoticensembleispreferred atconstant , ,and .Inthistreatment,thechemicalpotentialimposedbythebulkgasphasecanbedecoupledfromthemechanicalpressuresothattheeffectsofapplyingmechanicalforceonthesolidadsorbentcanbestudied.SuchexperimentshavebeenperformedwithCo bdp ,illustratingthattheapplicationofmechanicalforcecanservetocollapsetheporousstateofagate‐openingmaterialandexpeltheadsorbedgas.Thetwo“rigidhost”adsorptionsystemsforthetwodistinctstatesofthegate‐openingadsorbentcouldbeaccuratelymodeledbySSLequations.109
Structuraltransitionsofadsorbentmaterialsinthepresenceofgaseousadsorbatemayalsooccurwithoutanyexpansionorcontractionoftheunitcell.ArepresentativeexampleofsuchbehavioristhatofXeadsorptiononNi SiF6 pyz 2 pyz pyrazine ,apillaredsquaregridMOFwithrotatablepyrazinestruts.113TheXeadsorptionisothermatroomtemperatureexhibitsapronouncedinflectionat~50kPa;insituX‐raydiffractionmeasurements,densityfunctionaltheorycalculations,andclassicalMonteCarlomolecularsimulationsindicatethattheinflectionisaresultoftherotationalstatesofthepyrazineligandsorganizingtoachieveamorefavorablegas‐hostinteraction.AsimplestatisticalmechanicalmodelofporousMOFswithrotatingligandswassubsequentlydevelopedwhichcouldaccuratelyreproducetheadsorptioninflectionobservedinexperiment.65ThemodelisanextensionoftheSSLmodelwithadditionalcomplexitytoaccountforthetwostatesoftherotatingligands,whichmodifiestheadsorptionbindinginteraction.Studiesofgate‐opening,breathing,androtatingligandbehaviorareleadingtoimportantadsorbentmaterialdesignprinciplesaimedatexploitingthesigmoidal S‐shaped isothermtomaximizedeliverableadsorptioncapacityandseparationselectivity.
Conclusions
ThelegacyofIrvingLangmuiriswidespread,frombiologytoatmosphericscience,butnofieldismorecloselyassociatedwithhisnamethanadsorption:thescienceofinterfaces.Inthecenturysincethepublicationofhissimpletheoryofgas‐solidadsorption,therehasneverbeenatimewhenhisworkwasmorehungrilyconsumed i.e.,referencedinthescientificliterature thanitisnow.DespitethatLangmuir’ssixadsorptionclassificationswereinitiallydevelopedtotreatgasadsorptiononplanar,externalsurfaces withtheexceptionofCaseIIIwhereLangmuirremarksaboutapplicabilitytoporousmaterials ,recentlydiscoveredclassesofnanostructuredandporousmaterialsnowdominatethelandscapeofgas‐solidadsorptionstudies.Mostnotably,metal‐organicframeworks MOFs exhibitwidelytunablecrystallineenvironmentsforbindingadsorbates e.g.,stronglybindingopen‐metalsitesandweaklybindingorganiclinkerandpocketsites ,inadditiontointernalporositieswithunprecedentedsurfaceareasforadsorption e.g., 6000m2g‐1 .Porousframeworkcarbon‐basedmaterialssuchaszeolite‐templatedcarbon ZTC 55andmicroporoussilicates zeolites 114alsopresentunusualandhighlyinterestingenvironmentsforgas,liquid,andionadsorption.Thistypeofadsorptioninconfined“nanospaces”,whereapotentialenergyfieldfor
28
adsorptioniscreatedinathree‐dimensionalspace,bringsanewlevelofcomplexityandrichnesstostudiesofadsorption.ThelimitationsofLangmuir’stheoryintreatingcomplexadsorbentshave,ofcourse,longbeennoted.115Itisoftenappropriatetoemployapotentialtheoryofadsorptioninsuchsystems,116‐118oramorecomplexmodelsuchasonebasedondensityfunctionaltheory119‐120ormolecularsimulations121;nevertheless,theLangmuirmodelstillservesasarobust,simpleplatformontowhichfurthercomplexitycaneasilybeintegratedtomeettheneedsofbothroutineandunusualproblemsinadsorptionscience.Thefundamentaltheoriesofmonolayerandmultilayeradsorption,bothattributedtoLangmuir’spivotal1916‐1918works,remainwidelyusedtoday.
29
AssociatedContent
Nomenclature
ThenomenclatureusedinLangmuir1918hasnotbeenadoptedinthisreviewinfavorofmoremodernnomenclatureandstatisticalmechanicalconventions,asshownbelow.Thephaseoftheadsorptivemolecule“A”isindicatedbyeither“g”forgasor“a”foradsorbed.Theuseoftheterm“gas”orphase“g”isinclusiveofsupercriticalfluids.
Symbol Quantity TypicalUnits Langmuir’sSymbol3
surfacerate m s ,
probability dimensionless
fractionalsiteoccupancy dimensionless
‐‐ relativeadsorptionlifetime m s
number dimensionless
molenumber mol
temperature K
pressure Pa
volume m
molecularmass kg
gasconstant J K mol
Langmuirbindingconstant Pa
Langmuirscalingconstant mmol g
exponentialprefactor varies ‐‐
weightingconstant dimensionless
Hillinteractionconstant dimensionless ‐‐
BETvariable g mmol ‐‐
BETconstant dimensionless ‐‐
numberadsorbed dimensionless
Langmuirscalingconstant g
adsorptionsitebindingenergy J
chemicalpotential J
, single‐sitepartitionfunction dimensionless
, totalpartitionfunction dimensionless
thermaldeBrogliewavelength m
spreadingpressure N m
30
SupportingInformation
TheSupportingInformationisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.ExperimentalNotesandKineticandStatisticalMechanicalDerivationsofLangmuir’sTheory.
I. AuthorInformation
CorrespondingAuthors
*Email:nicholas.stadie@montana.edu
ORCID
NicholasP.Stadie:0000‐0002‐1139‐7846
Notes
Theauthorsdeclarenocompetingfinancialinterest.
Acknowledgements
WethankCorySimonandArniSturlusonforinvaluablecontributionstothestatisticalmechanicalderivationsandacknowledgeseveralimportanthistorical6‐8andbiographical24,122‐125referencesusedextensivelyinthepreparationofthismanuscript.
31
References
1. Langmuir,I.,TheConstitutionandFundamentalPropertiesofSolidsandLiquids.PartI.Solids.J.Am.Chem.Soc.1916,38 11 ,2221‐2295.2. Langmuir,I.,TheConstitutionandFundamentalPropertiesofSolidsandLiquids.II.Liquids.J.Am.Chem.Soc.1917,39 9 ,1848‐1906.3. Langmuir,I.,TheAdsorptionofGasesonPlaneSurfacesofGlass,MicaandPlatinum.J.Am.Chem.Soc.1918,40 9 ,1361‐1403.4. NobelLectures:Chemistry1922‐1941.ElsevierPublishingCompany:Amsterdam,1966.5. Furukawa,H.;Cordova,K.E.;O’Keeffe,M.;Yaghi,O.M.,TheChemistryandApplicationsofMetal‐OrganicFrameworks.Science2013,341 6149 ,1230444.6. Rouquerol,F.;Rouquerol,J.;Sing,K.S.W.,AdsorptionbyPowdersandPorousSolids:Principles,Methodology,andApplications.AcademicPress:SanDiego,1999;pxvi,467p.7. Dąbrowski,A.,Adsorption—fromTheorytoPractice.Adv.ColloidInterfaceSci.2001,93 1‐3 ,135‐224.8. Kiefer,S.;Robens,E.,SomeIntriguingItemsintheHistoryofVolumetricandGravimetricAdsorptionMeasurements.J.Therm.Anal.Calorim.2008,94 3 ,613‐618.9. Kayser,H.,UeberDieVerdichtungVonGasenanOberflächeninIhrerAbhängigkeitvonDruckundTemperatur.AnnalenderPhysik1881,250 11 ,450‐468.10. McBain,J.W.,XCIX.TheMechanismoftheAdsorption “Sorption” ofHydrogenbyCarbon.TheLondon,Edinburgh,andDublinPhilosophicalMagazineandJournalofScience1909,18 108 ,916‐935.11. Gibbs,J.W.,OntheEquilibriumofHeterogeneousSubstances PartII .Trans.Conn.Acad.1877,III,343‐524.12. Henneberg,W.;Stohmann,F.,UeberdasVerhaltenderAckerkrumeGegenAmmoniakundAmmoniaksalze.JustusLiebigsAnnalenderChemie1858,107 2 ,152‐174.13. Chappuis,P.,UeberdieVerdichtungderGaseaufGlasoberflächen.AnnalenderPhysik1879,244 9 ,1‐29.14. Joulin,M.L.,RecherchesExpérimentalesSurLaDiffusion.AnnalesdeChimieetdePhysique1881,5 23 ,398‐421.15. Favre,P.A.,SurlaCondensationdesGazparlesCorpsSolidesetsurlaChaleurDégagéedansl'ActedecetteAbsorption‐surlesRelationsdecesEffetsaveclesChaleursdeLiquéfactionoudeSolidificationdesGaz.C.R.Acad.Sci.1854,39 16 ,729‐733.16. Favre,P.A.,RecherchesThermiquessurlaCondensationdesGazparlesCorpsSolidesetlaChaleurDégagéedansl'ActedecetteAbsorption‐RelationsdecesEffetsaveclesChaleursdeLiquéfactionetdeSolidificationdesGaz.Ann.Chim.Phys.1874,1 5 ,209‐261.17. Freundlich,H.,ÜberdieAdsorptioninLösungen.Z.Phys.Chem.1907,57 1 ,385‐470.18. Sips,R.,OntheStructureofaCatalystSurface.J.Chem.Phys.1948,16 5 ,490‐495.19. Sips,R.,OntheStructureofaCatalystSurface.II.J.Chem.Phys.1950,18 8 ,1024‐1026.20. Polanyi,M.,ÜberdieAdsorptionvomStandpunktdesDrittenWärmesatzes.Verh.Dtsch.Phys.Ges.1914,16,1012‐1016.21. Polanyi,M.,AdsorptionvonGasen Dämpfen DurcheinFestesNichtflüchtigesAdsorbens.Verh.Dtsch.Phys.Ges.1916,18,55‐80.22. Polanyi,M.;London,F.,ÜberdieAtomtheoretischeDeutungderAdsorptionskräfte.Naturwissenschaften1930,18 50 ,1099‐1100.23. Polanyi,M.,ThePotentialTheoryofAdsorption.Science1963,141 3585 ,1010‐1013.24. Rosenfeld,A.,MenofPhysics:IrvingLangmuir.PergamonPressLtd.:1966.25. Langmuir,I.UeberPartielleWiedervereinigungDissociierterGaseimVerlaufeinerAbkühlung.Göttingen,1906.
32
26. Langmuir,I.,TheDissociationofWaterVaporandCarbonDioxideatHighTemperatures.J.Am.Chem.Soc.1906,28 10 ,1357‐1379.27. Langmuir,I.,ChemicalReactionsatLowPressures.J.Am.Chem.Soc.1915,37 5 ,1139‐1167.28. Langmuir,I.,TheEvaporation,CondensationandReflectionofMoleculesandtheMechanismofAdsorption.Phys.Rev.1916,8 2 ,149.29. Langmuir,I.,TheVaporPressureofMetallicTungsten.Phys.Rev.1913,2 5 ,329.30. Langmuir,I.,AChemicallyActiveModificationofHydrogen.J.Am.Chem.Soc.1912,34 10 ,1310‐1325.31. Roberts,G.,Langmuir‐BlodgettFilms.PlenumPress:NewYork,NY,1990.32. Ulman,A.,AnIntroductiontoUltrathinOrganicFilms:FromLangmuir‐BlodgetttoSelf‐Assembly.AcademicPressInc.:SanDiego,CA,1991.33. Langmuir,I.,TheCondensationPump:AnImprovedFormofHighVacuumPump.JournaloftheFranklinInstitute1916,182 6 ,719‐743.34. Roth,A.,VacuumTechnology.Elsevier:NewYork,2012.35. Gaines,G.L.;Wise,G.,Insiders,Outsiders,andSurfaces:IrvingLangmuir'sContributiontoCatalysis.HeterogeneousCatalysis,Davis,B.H.;Hettinger,W.P.,Eds.AmericanChemicalSociety,1983,222,13‐22.36. Brunauer,S.;Emmett,P.;Teller,E.,AdsorptionofGasesinMultimolecularLayers.J.Am.Chem.Soc.1938,60,309‐319.37. Brunauer,S.,AboutSomeCriticsoftheBETTheory.Langmuir1987,3,3‐4.38. Adamson,A.W.;Gast,A.P.,PhysicalChemistryofSurfaces.JohnWiley&Sons,Inc.:NewYork,1967.39. Barcroft,J.;Camis,M.,TheDissociationCurveofBlood.J.Physiol.1909,39 2 ,118‐142.40. Bohr,C.;Hasselbalch,K.;Krogh,A.,UebereineninBiologischerBeziehungWichtigenEinfluss,dendieKohlensäurespannungdesBlutesuufdessenSauerstoffbindungübt.SkandinavischesArchivfürPhysiologie1904,16 2 ,402‐412.41. Hill,A.V.,TheModeofActionofNicotineandCurari,DeterminedbytheFormoftheContractionCurveandtheMethodofTemperatureCoefficients.J.Physiol.1909,39 5 ,361‐373.42. Hill,A.V.,ThePossibleEffectsoftheAggregationoftheMoleculesofHaemoglobinonItsDissociationCurves.J.Physiol.1910,40,4‐7.43. Edsall,J.T.InHemoglobinandtheOriginsoftheConceptofAllosterism,Federationproceedings,1980;pp226‐235.44. Garrett,R.H.;Grisham,C.M.,Biochemistry.5thed.;Brooks/Cole,CengageLearning:Boston,2010.45. Rossi‐Fanelli,A.;Antonini,E.,StudiesontheOxygenandCarbonMonoxideEquilibriaofHumanMyoglobin.Arch.Biochem.Biophys.1958,77 2 ,478‐492.46. Ferry,R.M.;Green,A.A.,StudiesintheChemistryofHemoglobin.III.TheEquilibriumbetweenOxygenandHemoglobinandItsRelationtoChangingHydrogenIonActivity.J.Biol.Chem.1929,81 1 ,175‐203.47. Pauling,L.,TheOxygenEquilibriumofHemoglobinandItsStructuralInterpretation.Proc.Natl.Acad.Sci1935,21 4 ,186‐191.48. Gesztelyi,R.;Zsuga,J.;Kemeny‐Beke,A.;Varga,B.;Juhasz,B.;Tosaki,A.,TheHillEquationandtheOriginofQuantitativePharmacology.ArchiveforHistoryofExactSciences2012,66 4 ,427‐438.49. Eaton,W.A.;Henry,E.R.;Hofrichter,J.;Mozzarelli,A.,IsCooperativeOxygenBindingbyHemoglobinReallyUnderstood?Nat.Struct.Mol.Biol.1999,6 4 ,351.50. Hill,T.L.,TheoryofPhysicalAdsorption.InAdvancesinCatalysisandRelatedSubjects,Frankenburg,W.G.,Ed.AcademicPress:NewYork,1952;Vol.IV,pp211‐258.
33
51. Banerjee,D.;Simon,C.M.;Plonka,A.M.;Motkuri,R.K.;Liu,J.;Chen,X.;Smit,B.;Parise,J.B.;Haranczyk,M.;Thallapally,P.K.,Metal–OrganicFrameworkwithOptimallySelectiveXenonAdsorptionandSeparation.Nat.Commun.2016,7,ncomms11831.52. Mason,J.A.;Sumida,K.;Herm,Z.R.;Krishna,R.;Long,J.R.,EvaluatingMetal–OrganicFrameworksforPost‐CombustionCarbonDioxideCaptureviaTemperatureSwingAdsorption.Energ.Environ.Sci.2011,4 8 ,3030‐3040.53. Hill,T.L.,StatisticalMechanicsofAdsorption.VI.LocalizedUnimolecularAdsorptiononaHeterogeneousSurface.J.Chem.Phys.1949,17 9 ,762‐771.54. Honig,J.;Reyerson,L.,AdsorptionofNitrogen,OxygenandArgononRutileatLowTemperatures;ApplicabilityoftheConceptofSurfaceHeterogeneity.J.Phys.Chem.1952,56 1 ,140‐144.55. Nishihara,H.;Kyotani,T.,Zeolite‐TemplatedCarbons‐Three‐DimensionalMicroporousGrapheneFrameworks.Chem.Commun.2018,54,5648‐5673.56. Stadie,N.P.;Vajo,J.J.;Cumberland,R.W.;Wilson,A.A.;Ahn,C.C.;Fultz,B.,Zeolite‐TemplatedCarbonMaterialsforHigh‐PressureHydrogenStorage.Langmuir2012,28,10057‐10063.57. Nishihara,H.;Fujimoto,H.;Itoi,H.;Nomura,K.;Tanaka,H.;Miyahara,M.T.;Bonnaud,P.A.;Miura,R.;Suzuki,A.;Miyamoto,N.,Graphene‐BasedOrderedFrameworkwithaDiverseRangeofCarbonPolygonsFormedinZeoliteNanochannels.Carbon2018,129,854‐862.58. Purewal,J.;Liu,D.;Sudik,A.;Veenstra,M.;Yang,J.;Maurer,S.;Müller,U.;Siegel,D.J.,ImprovedHydrogenStorageandThermalConductivityinHigh‐DensityMOF‐5Composites.J.Phys.Chem.C2012,116 38 ,20199‐20212.59. Walton,K.S.;Snurr,R.Q.,ApplicabilityoftheBETMethodforDeterminingSurfaceAreasofMicroporousMetal‐OrganicFrameworks.J.Am.Chem.Soc.2007,129 27 ,8552‐8556.60. Arstad,B.;Fjellvåg,H.;Kongshaug,K.O.;Swang,O.;Blom,R.,AmineFunctionalisedMetalOrganicFrameworks MOFs asAdsorbentsforCarbonDioxide.Adsorption2008,14 6 ,755‐762.61. Dybtsev,D.N.;Chun,H.;Kim,K.,RigidandFlexible:AHighlyPorousMetal‐OrganicFrameworkwithUnusualGuest‐DependentDynamicBehavior.Angew.Chem.Int.Ed.2004,43 38 ,5033‐5036.62. Geier,S.J.;Mason,J.A.;Bloch,E.D.;Queen,W.L.;Hudson,M.R.;Brown,C.M.;Long,J.R.,SelectiveAdsorptionofEthyleneoverEthaneandPropyleneoverPropaneintheMetal‐OrganicFrameworksM2 dobdc M Mg,Mn,Fe,Co,Ni,Zn .Chem.Sci.2013,4 5 ,2054‐2061.63. Bao,Z.;Alnemrat,S.;Yu,L.;Vasiliev,I.;Ren,Q.;Lu,X.;Deng,S.,AdsorptionofEthane,Ethylene,Propane,andPropyleneonaMagnesium‐BasedMetal‐OrganicFramework.Langmuir2011,27 22 ,13554‐13562.64. Llewellyn,P.L.;Garcia‐Rates,M.;Gaberová,L.;Miller,S.R.;Devic,T.;Lavalley,J.‐C.;Bourrelly,S.;Bloch,E.;Filinchuk,Y.;Wright,P.A.,StructuralOriginofUnusualCO2AdsorptionBehaviorofaSmall‐PoreAluminumBisphosphonateMOF.J.Phys.Chem.C2015,119 8 ,4208‐4216.65. Simon,C.M.;Braun,E.;Carraro,C.;Smit,B.,StatisticalMechanicalModelofGasAdsorptioninPorousCrystalswithDynamicMoieties.Proc.Natl.Acad.Sci.2017,114 3 ,E287‐E296.66. Raschke,M.;Höfer,U.,ChemisorptionEnergyofHydrogenonSiliconSurfaces.Phys.Rev.B2001,63 20 ,201303.67. Rauscher,H.,TheInteractionofSilaneswithSiliconSingleCrystalSurfaces:MicroscopicProcessesandStructures.Surf.Sci.Rep.2001,42 6‐8 ,207‐328.68. Conner,W.C.;Falconer,J.L.,SpilloverinHeterogeneousCatalysis.Chem.Rev.1995,95 3 ,759‐788.69. Cabria,I.;Lopez,M.;Alonso,J.,ShapeoftheHydrogenAdsorptionRegionsofMOF‐5andItsImpactontheHydrogenStorageCapacity.Phys.Rev.B2008,78 20 ,205432.
34
70. Eddaoudi,M.;Kim,J.;Rosi,N.;Vodak,D.;Wachter,J.;O'Keeffe,M.;Yaghi,O.M.,SystematicDesignofPoreSizeandFunctionalityinIsoreticularMOFsandTheirApplicationinMethaneStorage.Science2002,295 5554 ,469‐472.71. Farha,O.K.;Yazaydın,A.Ö.;Eryazici,I.;Malliakas,C.D.;Hauser,B.G.;Kanatzidis,M.G.;Nguyen,S.T.;Snurr,R.Q.;Hupp,J.T.,DeNovoSynthesisofaMetal‐OrganicFrameworkMaterialFeaturingUltrahighSurfaceAreaandGasStorageCapacities.Nat.Chem.2010,2 11 ,944.72. Li,P.;Vermeulen,N.A.;Gong,X.;Malliakas,C.D.;Stoddart,J.F.;Hupp,J.T.;Farha,O.K.,DesignandSynthesisofaWater‐StableAnionicUranium‐BasedMetal‐OrganicFramework MOF withUltraLargePores.Angew.Chem.Int.Ed.2016,55 35 ,10358‐10362.73. Farha,O.K.;Eryazici,I.;Jeong,N.C.;Hauser,B.G.;Wilmer,C.E.;Sarjeant,A.A.;Snurr,R.Q.;Nguyen,S.T.;Yazaydın,A.O.z.r.;Hupp,J.T.,Metal‐OrganicFrameworkMaterialswithUltrahighSurfaceAreas:IstheSkytheLimit?J.Am.Chem.Soc.2012,134 36 ,15016‐15021.74. Miranda,R.;Albano,E.;Daiser,S.;Wandelt,K.;Ertl,G.,RougheningTransitioninAdsorbedXenonMultilayers.J.Chem.Phys.1984,80 6 ,2931‐2938.75. Rouquerol,J.;Llewellyn,P.;Rouquerol,F.,IstheBETEquationApplicabletoMicroporousAdsorbents?Stud.Surf.Sci.Catal.2007,160,49‐56.76. Bae,Y.‐S.;Yazaydın,A.O.z.r.;Snurr,R.Q.,EvaluationoftheBETMethodforDeterminingSurfaceAreasofMOFsandZeolitesThatContainUltra‐Micropores.Langmuir2010,26 8 ,5475‐5483.77. Gómez‐Gualdrón,D.A.;Moghadam,P.Z.;Hupp,J.T.;Farha,O.K.;Snurr,R.Q.,ApplicationofConsistencyCriteriatoCalculateBETAreasofMicro‐andMesoporousMetal–OrganicFrameworks.J.Am.Chem.Soc.2015,138 1 ,215‐224.78. Sircar,S.;Mohr,R.;Ristic,C.;Rao,M.B.,IsostericHeatofAdsorption:TheoryandExperiment.J.Phys.Chem.B1999,103,6539‐6546.79. Stadie,N.P.SynthesisandThermodynamicStudiesofPhysisorptiveEnergyStorageMaterials.Ph.D.thesis,CaliforniaInstituteofTechnology,2013.80. Stadie,N.P.;Murialdo,M.;Ahn,C.C.;Fultz,B.,AnomalousIsostericEnthalpyofAdsorptionofMethaneonZeolite‐TemplatedCarbon.J.Am.Chem.Soc.2013,135 3 ,990‐993.81. Shen,D.;Bülow,M.;Siperstein,F.;Engelhard,M.;Myers,A.L.,ComparisonofExperimentalTechniquesforMeasuringIsostericHeatofAdsorption.Adsorption2000,6 4 ,275‐286.82. Sircar,S.,GibbsianSurfaceExcessforGasAdsorption–Revisited.Ind.Eng.Chem.Res.1999,38,3670‐3682.83. Sircar,S.,TheGeniusofGibbsianSurfaceExcess GSE FrameworkforFluid GasorLiquid –SolidAdsorption:APowerfulPracticalTool.Sep.Purif.Technol.2019,213,235‐245.84. Bhatia,S.K.;Myers,A.L.,OptimumConditionsforAdsorptiveStorage.Langmuir2006,22,1688‐1700.85. Casco,M.E.;Martínez‐Escandell,M.;Gadea‐Ramos,E.;Kaneko,K.;Silvestre‐Albero,J.;Rodriguez‐Reinoso,F.,High‐PressureMethaneStorageinPorousMaterials:AreCarbonMaterialsinthePolePosition?Chem.Mater.2015,27 3 ,959‐964.86. Kapelewski,M.T.;Runčevski,T.e.;Tarver,J.D.;Jiang,H.Z.;Hurst,K.E.;Parilla,P.A.;Ayala,A.;Gennett,T.;FitzGerald,S.A.;Brown,C.M.,RecordHighHydrogenStorageCapacityintheMetal–OrganicFrameworkNi2 m‐dobdc atnear‐AmbientTemperatures.Chem.Mater.2018,30 22 ,8179‐8189.87. Ruthven,D.;Loughlin,K.;Holborow,K.,MulticomponentSorptionEquilibriuminMolecularSieveZeolites.Chem.Eng.Sci.1973,28 3 ,701‐709.88. Schell,J.;Casas,N.;Pini,R.;Mazzotti,M.,PureandBinaryAdsorptionofCO2,H2,andN2onActivatedCarbon.Adsorption2012,18 1 ,49‐65.89. Talu,O.,Needs,Status,TechniquesandProblemswithBinaryGasAdsorptionExperiments.Adv.ColloidInterfaceSci.1998,76‐77,227‐269.
35
90. Myers,A.L.;Prausnitz,J.M.,ThermodynamicsofMixed‐GasAdsorption.AlChEJ.1965,111 ,121‐127.91. Hefti,M.;Marx,D.;Joss,L.;Mazzotti,M.,AdsorptionEquilibriumofBinaryMixturesofCarbonDioxideandNitrogenonZeolitesZSM‐5and13X.Micropor.Mesopor.Mater.2015,215,215‐228.92. Schell,J.;Casas,N.;Blom,R.;Spjelkavik,A.I.;Andersen,A.;Cavka,J.H.;Mazzotti,M.,MCM‐41,MOFandUIO‐67/MCM‐41AdsorbentsforPre‐CombustionCO2CapturebyPSA:AdsorptionEquilibria.Adsorption2012,18 3‐4 ,213‐227.93. Yang,Q.;Zhong,C.,MolecularSimulationofCarbonDioxide/Methane/HydrogenMixtureAdsorptioninMetal‐OrganicFrameworks.J.Phys.Chem.B2006,110,17776‐17783.94. Cessford,N.F.;Seaton,N.A.;Düren,T.,EvaluationofIdealAdsorbedSolutionTheoryasaToolfortheDesignofMetal–OrganicFrameworkMaterials.Ind.Eng.Chem.Res.2012,51 13 ,4911‐4921.95. Costa,E.;Sotelo,J.;Calleja,G.;Marrón,C.,AdsorptionofBinaryandTernaryHydrocarbonGasMixturesonActivatedCarbon:ExperimentalDeterminationandTheoreticalPredictionoftheTernaryEquilibriumData.AlChEJ.1981,27 1 ,5‐12.96. Talu,O.;Zwiebel,I.,MulticomponentAdsorptionEquilibriaofNonidealMixtures.AlChEJ.1986,32 8 ,1263‐1276.97. Li,J.‐R.;Kuppler,R.J.;Zhou,H.‐C.,SelectiveGasAdsorptionandSeparationinMetal–OrganicFrameworks.Chem.Soc.Rev.2009,38,1477‐1504.98. Hudson,M.R.;Queen,W.L.;Mason,J.A.;Fickel,D.W.;Lobo,R.F.;Brown,C.M.,Unconventional,HighlySelectiveCO2AdsorptioninZeoliteSSZ‐13.J.Am.Chem.Soc.2012,134 4 ,1970‐1973.99. Li,J.‐R.;Ma,Y.;McCarthy,M.C.;Sculley,J.;Yu,J.;Jeong,H.‐K.;Balbuena,P.B.;Zhou,H.‐C.,CarbonDioxideCapture‐RelatedGasAdsorptionandSeparationinMetal‐OrganicFrameworks.Coord.Chem.Rev.2011,255,1791‐1823.100. Guo,H.;Zhu,G.;Hewitt,I.J.;Qiu,S.,“TwinCopperSource”GrowthofMetal‐OrganicFrameworkMembrane:Cu3 BTC 2withHighPermeabilityandSelectivityforRecyclingH2.J.Am.Chem.Soc.2009,131,1646‐1647.101. Bloch,E.D.;Queen,W.L.;Krishna,R.;Zadrozny,J.M.;Brown,C.M.;Long,J.R.,HydrocarbonSeparationsinaMetal‐OrganicFrameworkwithOpenIron II CoordinationSites.Science2012,335 6076 ,1606‐1610.102. Britt,D.;Tranchemontagne,D.;Yaghi,O.M.,Metal‐OrganicFrameworkswithHighCapacityandSelectivityforHarmfulGases.Proc.Nat.Acad.Sci.2014,105 33 ,11623‐11627.103. Salles,F.;Ghoufi,A.;Maurin,G.;Bell,R.G.;Mellot‐Draznieks,C.;Férey,G.,MolecularDynamicsSimulationsofBreathingMOFs:StructuralTransformationsofMIL‐53 Cr UponThermalActivationandCO2Adsorption.Angew.Chem.Int.Ed.2008,47 44 ,8487‐8491.104. Horike,S.;Shimomura,S.;Kitagawa,S.,SoftPorousCrystals.Nat.Chem.2009,1 9 ,695.105. Alhamami,M.;Doan,H.;Cheng,C.‐H.,AReviewonBreathingBehaviorsofMetal‐Organic‐Frameworks MOFs forGasAdsorption.Materials2014,7 4 ,3198‐3250.106. Krause,S.;Bon,V.;Senkovska,I.;Stoeck,U.;Wallacher,D.;Többens,D.M.;Zander,S.;Pillai,R.S.;Maurin,G.;Coudert,F.‐X.,APressure‐AmplifyingFrameworkMaterialwithNegativeGasAdsorptionTransitions.Nature2016,532 7599 ,348.107. Evans,J.D.;Bocquet,L.;Coudert,F.‐X.,OriginsofNegativeGasAdsorption.Chem2016,16 ,873‐886.108. Kitaura,R.;Seki,K.;Akiyama,G.;Kitagawa,S.,PorousCoordination‐PolymerCrystalswithGatedChannelsSpecificforSupercriticalGases.Angew.Chem.Int.Ed.2003,42 4 ,428‐431.109. Mason,J.A.;Oktawiec,J.;Taylor,M.K.;Hudson,M.R.;Rodriguez,J.;Bachman,J.E.;Gonzalez,M.I.;Cervellino,A.;Guagliardi,A.;Brown,C.M.,MethaneStorageinFlexibleMetal–OrganicFrameworkswithIntrinsicThermalManagement.Nature2015,527 7578 ,357‐361.
36
110. Serre,C.;Bourrelly,S.;Vimont,A.;Ramsahye,N.A.;Maurin,G.;Llewellyn,P.L.;Daturi,M.;Filinchuk,Y.;Leynaud,O.;Barnes,P.,AnExplanationfortheVeryLargeBreathingEffectofaMetal–OrganicFrameworkDuringCO2Adsorption.Adv.Mater.2007,19 17 ,2246‐2251.111. Boutin,A.;Coudert,F.‐X.;Springuel‐Huet,M.‐A.;Neimark,A.V.;Férey,G.;Fuchs,A.H.,TheBehaviorofFlexibleMIL‐53 Al UponCH4andCO2Adsorption.J.Phys.Chem.C2010,114 50 ,22237‐22244.112. Coudert,F.‐X.;Jeffroy,M.;Fuchs,A.H.;Boutin,A.;Mellot‐Draznieks,C.,ThermodynamicsofGuest‐InducedStructuralTransitionsinHybridOrganic‐InorganicFrameworks.J.Am.Chem.Soc.2008,130 43 ,14294‐14302.113. Elsaidi,S.K.;Mohamed,M.H.;Simon,C.M.;Braun,E.;Pham,T.;Forrest,K.A.;Xu,W.;Banerjee,D.;Space,B.;Zaworotko,M.J.,EffectofRingRotationUponGasAdsorptioninSIFSIX‐3‐MM Fe,Ni PillaredSquareGridNetworks.Chem.Sci.2017,8 3 ,2373‐2380.114. Smit,B.;Maesen,T.L.,MolecularSimulationsofZeolites:Adsorption,Diffusion,andShapeSelectivity.Chem.Rev.2008,108 10 ,4125‐4184.115. Bering,B.;Dubinin,M.;Serpinsky,V.,TheoryofVolumeFillingforVaporAdsorption.J.ColloidInterfaceSci.1966,21 4 ,378‐393.116. Dubinin,M.M.,ThePotentialTheoryofAdsorptionofGasesandVaporsforAdsorbentswithEnergeticallyNonuniformSurfaces.Chem.Rev.1960,60 2 ,235‐241.117. Monsalvo,M.A.;Shapiro,A.A.,StudyofHigh‐PressureAdsorptionfromSupercriticalFluidsbythePotentialTheory.FluidPhaseEquilib.2009,283 1‐2 ,56‐64.118. Dundar,E.;Zacharia,R.;Chahine,R.;Bénard,P.,ModifiedPotentialTheoryforModelingSupercriticalGasAdsorption.Int.J.HydrogenEnergy2012,37,9137‐9147.119. Henderson,D.,FundamentalsofInhomogeneousFluids.MarcelDekker,Inc.:NewYork1992.120. Lastoskie,C.;Gubbins,K.E.;Quirke,N.,PoreSizeDistributionAnalysisofMicroporousCarbons:ADensityFunctionalTheoryApproach.J.Phys.Chem.1993,97 18 ,4786‐4796.121. Gelb,L.D.;Gubbins,K.;Radhakrishnan,R.;Sliwinska‐Bartkowiak,M.,PhaseSeparationinConfinedSystems.Rep.Prog.Phys.1999,62 12 ,1573.122. Rideal,E.K.,Dr.IrvingLangmuir,For.Mem.R.S.Nature1957,180,581‐582.123. Hull,A.W.,Dr.IrvingLangmuir'sContributionstoPhysics.Nature1958,181,148‐149.124. Taylor,H.,IrvingLangmuir,1881‐1957.BiographicalMemoirsofFellowsoftheRoyalSociety1958,4,167‐184.125. Suits,G.,TheCollectedWorksofIrvingLangmuir.PergamonPressLtd.:Oxford,1961;Vol.1‐12.
37
TableofContentsGraphic
38
Biographies
HansSwensongraduatedfromhighschoolinBozeman,Montana,andiscurrentlyanundergraduatestudentatMontanaStateUniversity,withaspirationsofpursuingaPhDinchemistry.Hisresearchhasbeenfocusedonmeasuringtheadsorptionofheliumonmicroporousmaterialstoelucidatetheeffectsofnarrowporesizeonerrorsinvoidvolumedetermination.
NicholasP.StadiereceivedaB.S.inChemistryfromArizonaStateUniversitywhereheperformedresearchonzeoliticimidazolateframeworks ZIFs underProf.MichaelO’Keeffe.AttheCaliforniaInstituteofTechnology,heundertookstudiesofhighsurfaceareacarbonaceousframeworkmaterialsandtheirgasadsorptionpropertiesunderProf.BrentFultz,receivingaPh.D.inMaterialsSciencein2013.Hisattentionturnedtoreactive,unstableporousphasesofhydrogen‐richmaterialssuchasmagnesiumborohydrideunderProf.AndreasZüttelattheSwissFederalLaboratoriesforMaterialsScience&Technology Empa .AfterasecondpostdoctoralpositionatETHZürichapplyingporouscarbonmaterialsasexoticbatteryelectrodesinthelaboratoryofProf.MaksymKovalenko,hereturnedtoNorthAmericain2017asanassistantprofessoratMontanaStateUniversity.Hisresearcheffortscombinesyntheticsolid‐statechemistrywithexperimentalstudiesofgas‐surfaceinteractions,especiallyunderhigh‐pressureconditions.
Recommended