alcohols at the aqueous surface: chain length and isomer

Received00thJanuary20xx,Accepted00thJanuary20xx

DOI:10.1039/x0xx00000x

AlcoholsattheAqueousSurface:ChainLengthandIsomerEffectsM.-M.Walz,a,b,*J.Werner,a,cV.Ekholm,aN.L.Prisle,dG.ÖhrwalleandO.Björneholma

Surface-activeorganicmoleculesattheliquid-vaporinterfaceareofgreatimportanceinatmosphericscience.Therefore,westudiedthesurfacebehaviorofalcoholisomerswithdifferentchainlengths(C4-C6)inaqueoussolutionwithsurface-andchemicallysensitiveX-rayphotoelectronspectroscopy(XPS),whichrevealsinformationaboutthesurfacestructureonamolecularlevel.GibbsfreeenergiesofadsorptionandsurfaceconcentrationsaredeterminedfromtheXPSresultsusingastandardLangmuiradsorptionisothermmodel.Thefreeenergiesofadsorption,rangingfromaround-15to-19kJ/mol(C4-C6),scalelinearlywiththenumberofcarbonatomswithinthealcoholswithΔGAdsper–CH2–≈-2kJ/mol.Whileforthelinearalcohols,surfaceconcentrationsliearound2.4x1014molecules/cm2atthebulkconcentrationswheremonolayersareformed,thestudiedbranchedalcoholsshowlowersurfaceconcentrationsofaround1.6x1014molecules/cm2,bothofwhichareinlinewiththemolecularstructureandtheirorientationattheinterface.Interestingly,wefindthatthereisamaximuminthesurfaceenrichmentfactorforlinearalcoholsatlowconcentrations,whichisnotobservedfortheshorterbranchedalcohols.Thisisinterpretedintermsofacooperativeeffect,whichwesuggesttobetheresultofmoreeffectivevanderWaals interactionsbetween the linearalcoholalkyl chainsat theaqueoussurface,making itenergeticallyevenmorefavorabletoresideattheliquid-vaporinterface.

1IntroductionAtmosphericorganicaerosolsformfromlessvolatileoxidationproducts of volatile precursors, either by new particleformation or by condensation into a pre-existing aerosolphase.1,2 A large number of aerosol precursors have beenidentified globally and their oxidation pathways are highlysensitive to the atmospheric environment and ambientconditions, leading to a suite of different oxidation productswith similar functionalities and varying molecular structures,suchaspositionalisomers.3,4Suchstructuralvariationsmayinturnleadtoverydifferentmolecularproperties,suchasvaporpressures and aqueous phase mixing interactions, which arecrucialtotheiraerosol-formingpotential.5Therefore,studyingthevariationofpropertiesoforganiccompoundswithsimilarfunctionalities and different molecular structures, and

ultimately theirmixtures, isofhigh importance for improvingourunderstandingofatmosphericorganicaerosols,whicharerelevant for theglobal radiationbudgetandcloud formation.Still, the major uncertainty in the total radiative forcingestimates, which constitute a crucial element of climateprediction,areaerosoleffects.6,7

Especially, the surface behavior of amphiphilic organicmolecules is of key interest for a better understanding ofatmosphericaerosols,duetothehighsurface-to-bulkratiosofthesenanometer-tomicrometer-sizedairborneparticles.Thesurface and its chemical composition may potentially altertheir physico-chemical interfacial properties. In particularevaporation and condensation processes may be affected,which are important for cloud formation and growth, asdiscussedearlierinmoredetail.8

In this study, we focus on short-chained alcohols andcompare alcohol isomers of different chain lengths (C4-C6).Such surface-active short-chained oxygenated compoundscompose a large fraction of the organic compounds found intropospheric aerosols.9 As with most other atmosphericorganic compounds, the surface structure of short-chainedalcohols in aqueous solution in general is not yet wellunderstood.10,11 The importance of alcohol functional groupsat the liquid-vapor interface is specifically motivated by thenewly found extremely low-volatility organic compounds,ELVOCs,whicharehypothesizedtomakeacrucialcontributionto the "missing secondaryorganicaerosol"mystery,12 aswell

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as humic-like substances (HULIS)13. Both, ELVOCs and HULIS,arebelievedtocontainsignificantalcoholfunctionality.Thesehighly complex molecules are however too difficult to studyimmediately, and the simpler alcohols provide a valuableintermediatestep,astheirspectraareeasierto interpretandtheir physico-chemical properties are much betterconstrained.

Suchsurface-activeorganiccompoundsaccumulateat theliquid-vapor interface, and may as a consequence alter thesurface properties, such as lower the surface tension. As forexample observed for alcohol-water solutions, the surfacetension at a given concentration decreases with respect topurewaterwithincreasingalkylchainlength,14andcomparinglinearandnon-linearisomers,highermonolayersurfaceexcessand smaller surface areas per molecule are observed forlonger linearalcohol isomers incomparisontobranchedones(heptanoltododecanol,C7-C12).15Thisexampleindicatesthatchemistry occurring on the surfaces of aerosol droplets is ingeneral affected by the organic content, and the respectivebehavior of the organic molecules at the droplet-airinterface.16-19Thecomparisonofdifferentalcohols isofgreatinterestas it isnotcertainthatallcompoundswiththesamefunctionalgroupwillactandinteractinthesameway.Itisforexample well known that physical properties may alternatewiththechainlength,e.g.intermsofaneven-oddalteration.20Furthermore,thestudyandcomparisonofdifferentisomersisof general interest as in atmospheric oxidation reactionsposition isomerism is ubiquitous,21,22 and such isomers canhaveverydifferentphysicalandchemicalproperties.23,24

The alcohols at the aqueous surface were studied withsurface- and chemically sensitive X-ray photoelectronspectroscopy (XPS), which is commonly applied in solid-statephysics.XPSisnotonlyelement-sensitivebutalsosensitivetothe formal oxidation state of an atom and its local chemicaland physical environment, in this study specifically to theinteraction of the solute with water and other amphiphiles.This information is reflected in the binding energy and theintensity of the core-level photoelectrons. Due to the shorteffective attenuation length of the photoelectrons at theexperimental conditions used here, the photoelectron signalprimarily originates from within a few nanometers of thesurfaceand is thus stronglydependenton theconcentration,orientation and organization ofmolecules at the liquid-vaporinterface.

2ExperimentalAll XPS experiments were performed at MAX-lab, Lund,Swedenat the I411undulatorbeamline.25 ToperformXPSatthe aqueous surface, a liquid micro-jet set-up was applied.Detailsonthistechniquecanbefounde.g.inreference26.

The liquidmicro-jet(∅≈20μm,flowrate≈0.5ml/min(≈26.5m/s),T≈283K)isinjectedthroughaglassnozzleintoanevacuated analysis chamber. Photoionization by linearlypolarized synchrotron light occurs at approx. 1mmafter theinjection point, before the liquid jet breaks up into dropletsand is frozen out in a cold trap. The photoelectrons are

detectedbyahemisphericalelectronenergyanalyzer(ScientaR4000)mountedperpendiculartothepropagationdirectionoftheliquidjet,at54.7°relativetothepolarizationplaneofthesynchrotron light to minimize angular distribution effects.27The total experimental resolution at the applied photonenergy, EPhoton = 360eV, is lower than0.3 eV, asdeterminedfromthewidthofthewatergasphasevalenceband1b1state.All spectrawereenergy-calibratedagainst thebindingenergyofthe1b1state(HOMO)ofliquidwater(EB(1b1,liquidwater)= 11.16 eV)28 and intensity-normalized (against photon fluxandacquisition time). Inorder to facilitate thecomparisonofdifferentexperimentalrunsandtomonitorthestabilityofthemeasurements, the1b1 valenceband stateof liquidwaterofanaqueous sodiumchloride solution (50mM)wasmeasuredbetweenallalcoholsolutionsandusedasaninternalintensityreference. The intensities of these reference measurementswereconstantwithin±5-10%.

Aqueous solutions of 1-butanol, tert-butanol, 1-pentanol,3-pentanol, 1-hexanol and 3-hexanol (purities > 99 %, SigmaAldrich) were prepared from de-ionized water (MilliporeDirect-Q, resistivity > 18.2 MΩcm). To avoid charging of theliquidjetduetophotoionizationandelectrokineticcharging,29all solutions contained 25mM sodium chloride (purity 99%,SigmaAldrich).

TheamphiphilesattheinterfaceweremonitoredviatheC1ssignalusingEPhoton=360eV.Atthisphotonenergy,theC1sphotoelectronshaveakineticenergyofapproximately70eV,making the XPS measurements highly surface-sensitive,27 astheeffectiveattenuationlengthisestimatedtobeintheorderof 1 nm.30,31 The photoelectron spectra were fitted with aleast-squaresmethod,usingtwosymmetricVoigt lineprofilesfor the liquid phase signal and four asymmetric PCI32 lineprofilesforthegasphasesignalfromtherespectivesolutetomodelvibrationalbroadening.ThelifetimewidthforC1scoreholes corresponding to the Lorentzian width was set to 0.1eV.33 Gaussianwidthswere free parameters, but linked suchthat they were the same for the corresponding peaks in allspectra. Energy positions and intensities were also freeparameters. The contributing gas phase signal of the solutewasfittedbylinkingtheenergysplittingandtheintensityratioto its“pure”gasphasespectrum.Suchagasphasespectrumwas recorded with the liquid jet lowered out of the X-raybeam, thus only containing contributions from the vaporphase.

3ResultsanddiscussionSurfacecoverageandorientationofthemolecules

InFig.1,aC1sXPSspectrumof3-hexanol(25mM)inaqueoussolution is shownwith the fit of the liquid phase signal as arepresentative spectrum for the studied alcohols in aqueoussolution. All studied alcohols in aqueous solutions arecharacterized by two C 1s peaks. The peak at lower bindingenergy (< 290 eV) originates from the carbons in the alkylchain(CC),whilethepeakathigherbindingenergy(>291eV)canbeassignedtothecarbonatomtowhichthehydroxyl

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Fig. 1 C 1s XPS spectrum of 3-hexanol in aqueous solution (25mM)acquired with a photon energy of EPhoton = 360 eV, shown with therespective fit of the liquid phase signal (CC and COH, see skeletalformulainset).Thefitofthegasphaseisnotshown.group is directly attached (COH). The higher electron bindingenergy of COH 1s compared to CC 1s is due to a reducedelectrondensity at theCOH as theattachedhydroxyl group iselectron-withdrawing.TheresultingdecreasedshieldingofthenucleuscausestheCOH1selectronstobemoretightlybound.TheC1ssignalfromtheliquidphaseofthesoluteisshiftedtolower binding energies (by roughly 0.6 eV) compared to thecorresponding signal from thegasphase (fit not shown). Thegas phase signal can be observed as a very small shouldertowardshigherbindingenergies.Thelowerbindingenergiesofthe alcohols in solution are primarily due to polarizationscreening of the charged C 1s core-hole final state by thewatermolecules.34

A concentration-dependent study was performed onseveraldifferentalcohols(1-butanol,tert-butanol,1-pentanol,3-pentanol, 1-hexanol and 3-hexanol), in order to studyvariations in their adsorption behavior at the liquid-vaporinterface with changes in molecular structure and individualconcentration. All acquired spectrawere evaluated regardingthetotalareaAtotof the liquidphaseC1sphotoelectron(PE)signal (Atot = A(CC) + A(COH)) and the PE intensity ratio Rbetween the two liquid phase C 1s peak areas (R = A(CC) /A(COH)).Asdiscussedinmoredetailbefore,35thePEsignalAtotcan be used as a measure for the amount of solutes at thesolution interface,while thePE intensity ratioRbetween thepeak areas of CC and COH reveals information about theorientation of these molecules in the surface region. Thebindingenergyissensitivetotheirlocalchemicalandphysicalenvironment, i.e. to their interaction with water and eachother. This aspect was covered in detail in our previouspaper.35ThechangeinbindingenergysplittingforallalcoholsinvestigatedinthispapercanbefoundintheSI.

The results of the concentration-dependent study aredepictedinFig.2,whereinpanela,they-axisisthetotalC1sPEsignal,Atot,andinpanelb,itisthePEintensityratioR.ThetrendofthetotalPEsignalwithincreasingconcentration(see

Fig.2ResultsofC1sXPSspectraforthedifferentalcoholsinaqueoussolutionatdifferentconcentrations:(a)TotalareaoftheliquidphaseC1sphotoelectronsignal(Atot).Errorbarsareestimatedfromvariationsin intensity stability. (b) Ratio between the liquid phase CC and COHsignal(R).Errorbarsindicatetherangeofvaluesobtainedbydifferentfittingapproaches.Theinsetshows,conceptually,thesurfacebehaviorofthealcoholsatlowandhighconcentration.Fig. 2a) is comparable for all alcohols and resembles aLangmuiradsorptionisotherm.36

The trend of the curve can be divided into two regions,based on the change in the slope. In the first region, the PEsignalincreasesnearlylinearlywithconcentration,whereasinthe second region, no further significant increase in the PEintensity can be observed, i.e. Atot saturates. As discussedearlier,35thetrendofthecurvecanbeinterpretedasaresultof the adsorption of alcohols at the aqueous surface withincreasing bulk concentration and, finally, the formation of amonolayer-likestructure(ML) inthesurfaceregion.Here,thetermmonolayerreferstoacloselypackedlayerofamphiphilicmoleculesattheliquid-vaporinterfacethathasapproximatelyathicknessofasinglemolecularlength,i.e.withthemoleculeshavinganorientationroughlyverticaltotheinterface.

Ingeneral, it isobservedthatAtotofallthelinearalcohols(1-butanol, 1-pentanol and 1-hexanol) is higher at anyconcentrationthanthecorrespondingsignalforthebranchedalcohols (tert-butanol, 3-pentanol and 3-hexanol). Thisindicates that at any concentration there are more linearalcohol molecules than branched alcohols in the immediate

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surface region (comparing the respective isomerpairs,e.g.1-hexanol vs. 3-hexanol etc.). At ML coverage this also meansthat the linear alcohols have a higher molecular packingdensity,which can be explained by steric effects: the bulkiersecondary / tertiary alcohols require more space than theprimaryalcohols. Thisphenomenon is in linewitha studyonlongeralcoholisomersfromheptanoltododecanol.15InFig.2a,alsothephotoelectronsignalforsodiumformatesolutionsisindicated (dotted orange line, multiplied by a factor of 10).This signal corresponds to the anion formate, i.e. just onecarbon atom, which is assumed to be depleted from thesurface.37 Both the lower number of carbon atoms and thelower surface prevalence, explain the lower intensity of theformate signal in comparison to the alcohols. The formatesignal isusedasareferencebulksignalforfurtherevaluation(seebelow).

In Fig. 2b, the PE intensity ratio R between the two C 1speak areas (CC/COH) is plotted versus the alcohol bulkconcentration. As for Atot, two regions are observed; first, RincreasesuptotheconcentrationwhereaMLisformed(cML),thenitdecreasesagain.Thiseffectismorepronouncedforthelinearalcohols,whichisdiscussedinmoredetailbelow.Thesetworegionscoincidewith theonessimilarlyobserved for theLangmuir-like curve, shown in Fig. 2a. Note that the exactvaluesofRdependon theapplied spectral fittingprocedure,whilethegeneraltrendisalwaysobserved.

Assumingthatpossiblemodulationsofthephotoionizationcross-sectionofCCandCOHisnegligible

38andthatthesolutesarerandomlyorparallelorientedattheinterface,oneexpectsa ratio which is close to the stoichiometric ratio, i.e. 3 forbutanol (3:1), 4 for pentanol (4:1) and 5 for hexanol (5:1). Adeviationfromthisratio,ontheotherhand,indicatesthattheamphiphiles change their orientation at the liquid-vaporinterface. The observed ratios, which increase from lowerconcentrationuptocML,thusindicatethatthesignalfromCOHis progressively dampened as themolecules gradually “standup” (see inset in Fig. 2b), with their alkyl chains pointingtowardsthevaporphase,resultinginR>stoichiometricratio.Atverylowconcentrations,wherethealcoholsattheaqueoussurface interactmainlywithwatermolecules,thealkylchainsareoriented“parallel”totheaqueoussurfacewhichis in linewith the observed ratios that are close to the stoichiometricratios,i.e.allcarbonatomscontributeequallytotheacquiredC 1s signal. At higher concentrations, more and moreamphiphiles accumulate at the aqueous surface and interactwith each other. At surface coverage close to one ML, theamphiphilic molecules have a preferential orientation suchthat the hydrophilic hydroxyl groups are immersed into theaqueousphasemaintaininghydrogenbondswithwater,whilethe hydrophobic alkyl chains are pointing out of the solutionphasebeingpartiallydehydratedwhichwasconfirmedbyMDsimulations earlier.35 This preferential orientation of theamphiphilic molecules is proposed to be driven byhydrophobic/hydrophilicinteractionsoftheamphiphileswiththesolventwaterandbyvanderWaals interactionsbetweenthealkylchainstoopposethe lossofentropy. Ingeneral,thelinearalcoholshaveahigherratio(i.e.moredeviatingfromthe

stoichiometric ratio) at ML coverage than the branchedalcohols. These larger ratios for theprimaryalcohols indicateanenhancementoftheCC1sPEsignalandadampeningoftheCOH1sPEsignalbythelongeralkylchainsincomparisontothebranched isomers, which is in line with the observedorientation and themolecular dimensions. At concentrationshigherthancML,Rdecreasesforallisomers.Itissuggestedthatthis decline can be explained by the contributing signal fromthesurface-near-bulkregioninwhichthemoleculesaremorerandomly oriented.35 We can thus conclude that the bulkconcentrationwhereamonolayerstartstoform,cML,isaround40 ± 10mM for hexanol, around 100 ± 20mM for pentanolandaround200±40mMforbutanol.Langmuirisothermmodel

In order to determine the surface coverage and the surfaceconcentrationof thealcohols at theaqueous surface, aswellas to estimate the Gibbs free energy of adsorption, ΔGAds, astandardLangmuiradsorptionmodelisusedtofitthedata.39-41

𝑵𝑺 =𝑵𝑺,𝒎𝒂𝒙𝒙𝒃𝒖𝒍𝒌

𝒙𝒃𝒖𝒍𝒌 + (𝟏 − 𝒙𝒃𝒖𝒍𝒌)𝒆𝒙𝒑∆𝑮𝑨𝒅𝒔𝑹𝑻

Both, NS and NS,max are given in arbitrary units as NS is thesurfacecontributionof the recordedPE signal.NS scaleswiththe surface concentration of the solute, i.e. the number ofmolecules in the probed volume, and NS,max is directlyproportional to themaximumpossible concentration, i.e. theconcentration of the pure compound (for further discussion,seebelow).xbulk isthemolarfractionofthesoluteinthebulkand(1-xbulk)isthusequaltothebulk-watermolarfraction.TheLangmuir model assumes, beyond other assumptions, thatthereismonolayeradsorptionandnointeractionbetweentheadsorbates. In order to calculate a signal, which is directlyproportional to the surface concentration NS, from therecordedphotoelectronsignalAtot(seeFig.2a),thesignalAtotisdividedbythenumberofcarbonatomsinthecorrespondingmolecule and a bulk signal (approximated by means of areferencemeasurement on sodium formate, see above)8,35 issubtracted. Linking the surface signal directly to the surfaceconcentration has certain limitations for longer alkyl chainlengths as the photoemission signal is thenmore attenuatedupon a change in orientation of the molecules. For thepresented data, however, this seems still to be in anacceptable range as the determined surface concentrationsand molecular areas are in good agreement with otherliterature that applies more conventional methods such assurfacetensionmeasurements(formoredetails,seebelow).

In Fig. 3, the corresponding Langmuir fits are shown. Thedata points are fitted well, especially for the branchedalcohols.However,onemightnoticethat inparticular forthelinear alcohols at lower concentrations, there is anoverestimationofthesurfaceconcentrationsbytheLangmuirmodel,whichisadirectresultoffittingthesurface

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Fig. 3 Surface concentration NS (in arbitrary units) versus the solutebulkmolar fraction xbulkwith the respective Langmuir fits. (a) 1- andtert-butanol,(b)1-and3-pentanol,and(c)1-and3-hexanol.Theerrorbarsareestimatedfromexperimentaluncertainties.concentrations at higher concentrationsmore accurately, i.e.in order not to overestimateNS,max. This deviation getsmorepronouncedwithincreasingalkylchainlength(seee.g.Fig.3c,1-hexanol).WewilldiscussthisphenomenoninmoredetailinthesectionSurfaceenrichmentfactor.

The values of ΔGAds for the alcohols, adsorbing at theliquid-vapor interface from thebulk solution,wereestimatedwith the Langmuir model. For the different alcohols, ΔGAdsscales linearly with the number of carbon atoms within themolecule and were determined to be around -15 kJ/mol forbutanol, -17 kJ/mol for pentanol and -19 kJ/mol for hexanol(Table1),resultinginaΔGAdsper–CH2–≈-2kJ/mol.Thisisin

Fig. 4 Gibbs free energies of adsorption ΔGAds in kJ/mol for thedifferent alcohols in aqueous solution plotted versus the number ofcarbonatoms.Theerrorbarsaregiven inTable1andcorrespondtoonestandarddeviationbasedontheLangmuirfit.Table 1 Gibbs free energies of adsorption ΔGAds for the differentalcoholsinkJ/molmolecule ΔGAds(kJ/mol) molecule ΔGAds(kJ/mol)1-butanol -15.3±0.3 tert-butanol -15.0±1.21-pentanol -17.0±0.5 3-pentanol -16.7±0.31-hexanol -19.2±1.5 3-hexanol -19.1±0.6linewithDanovandKralchevskywhodeterminedaΔGAdsvalueof -2.5kJ/molper -CH2- forprimaryalcoholsadsorbingat thewater-air interface.42 By means of this value, one canextrapolate ΔGAds values, for example for shorter chainlengths, such as ethanol (C2) predicting a ΔGAds value ofaround -11 kJ/mol, which was confirmed experimentally byXPSmeasurements.43ThedeterminedΔGAdsvalueforethanolis in line with other literature.44 The linear trend of ΔGAdsversusthenumberofcarbonatomsisvisualizedinFig.4.It isnoticedthatingeneraltheΔGAdsvaluesforthelinearalcoholsareslightlymorenegativecomparedtothebranchedalcohols,which is in line with their higher surface concentrations,althoughthisiswithinthemarginoftheerrorbars.Surfacecoverage,surfaceconcentrationandmoleculararea

BymeansofthemeasuredNSandthedeterminedNS,maxvalues(seeSI), thesurfacecoverageθ (withvalues from0 to1)canbe calculated with θ = NS/NS,max. In Fig. 5, the calculatedsurface coverage θ for the studied alcohols is plotted versusthealcoholbulkconcentration.Forthedifferentisomerpairs,the same trend, i.e. a similar surface coverage at anyconcentrationisobserved,whichsuggeststhatthisquantityisrelatedtothenumberofcarbonatomsinthemoleculeitself.In general, the surface coverage saturates around 0.8 – 0.9,which reveals that at the experimentally observedmaximumsurfacecoverage therearestillwatermolecules incorporatedinto the surface region that is saturated with alcoholmolecules.

In the following part, the molar surface concentration,csurface,inmol/lisestimated.FromtheLangmuirfitting

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Fig. 5 Surface coverage θ for the different alcohols versus their bulkconcentration. The error bars result from the uncertainty in NS,maxgivenbyone standarddeviationbasedon theLangmuir fit (compareTableS1inthesupportinginformation).equation, it is clear that NS will be equal to NS,max when themolar bulk fraction, xbulk, is equal to 1, i.e. for the purecompound. Thus one can determine the molar surfaceconcentrationbymultiplying the surface coverageθwith themaximumpossibleconcentration,i.e.theconcentrationofthepurecompound(listedinTable2),usingcsurface=θcmax. InFig.6, the calculatedmolar surface concentration for the variousalcohol isomers is plotted versus the respective bulkconcentration. It can be seen, that the different pairs ofisomers seem to have similar trends, as observed for thesurfacecoverages.FromC4toC6,themaximummolarsurfaceconcentrationdecreases,fromaround9Mtoaround6M.Thisreflectsthefactthatthelargermoleculesrequiremorespacethan thesmallerones,asaconstantvolumeof thesurface isconsidered. Again one can see that all maximumexperimentallyobservedmolarconcentrationsare lowerthanthe concentrations for the pure compounds, confirming thatwaterisincorporatedintothemonolayer.

From the molar surface concentration, one can estimatethe“surfacemolecularvolume”,i.e.thevolumeatthesurface,which is required by one alcohol molecule plus thesurrounding water molecules at ML coverage. For this weassume that all organicmolecules in the probed volume arelocated directly at the interface, i.e. the molecular lengthdetermines the thicknessof themonolayer. Thus,bydividingthe “surfacemolecular volume” by themolecular length, themolecularareaofthedifferentalcoholscanbeestimated(formore details, see SI). The resulting molecular areas at MLcoverage are summarized in Table 3. For comparison, themolecularareasatNS,maxarealsocalculated.Here,nowaterisincorporatedintothesurfacelayerasthesevaluescorrespondtothepurecompounds.

In line with the larger NS,max values for linear alcoholscompared to the branched alcohols, we find for the linearalcohols at theML concentration thatmolecular areas rangefrom39–44±2Å2/moleculeforC4–C6,whilethemolecularareas for the branched alcohols are around 60 – 68 ± 2Å2/molecule.Thegivenerrorforthemolecularareas

Fig. 6 Molar surface concentration, csurface, (mol/l) for the differentalcohols versus the bulk concentration, cbulk (mmol/l). The error barsresult from theuncertainty inNS,max givenbyone standarddeviationbasedontheLangmuirfit.Table2Molarconcentrationsforthepurecompoundsinmol/lmolecule cmax(mol/l) molecule cmax(mol/l)1-butanol 10.9 tert-butanol 10.51-pentanol 9.2 3-pentanol 9.31-hexanol 8.0 3-hexanol 8.0Table3MolecularareasatMLcoverage(AML)fordifferentalcoholsatthe aqueous surface and minimum molecular areas (AN(S,max))corresponding to NS,max calculated from the pure compounds (seeTable2).molecule length(Å) AML(Å

2) AN(S,max)(Å2)

1-butanol ≈5.5 39±2 28±21-pentanol ≈6.0 41±2 30±21-hexanol ≈6.5 44±2 32±2tert-butanol ≈3.5 68±2 45±23-pentanol ≈4.0 63±2 45±23-hexanol ≈4.5 60±2 46±2(± 2 Å2/molecule) is estimated from the uncertainty in themolecular length,which isassumedtobe±0.5Å (dependingontheexactorientationandconformationofthemoleculesatMLcoverage).IncontrasttothesemolecularareasatcML,thecalculatedvaluesatNS,max arearound30±2Å

2/molecule forthe linear alcohols and around 45 ± 2 Å2/molecule for thebranched alcohols. The difference in the molecular areasbetween linear and branched alcohols is in good agreementwithvaluesreportedelsewhere.15

From the molecular areas, the surface concentration inmolecules/cm2canbecalculated(seeSI).InFig.7,thesurfaceconcentration for the different alcohols is plotted versus thebulk concentration. From this plot it is clear that the linearalcohols have a higher packing density than the branchedalcohols, with a value that is on average 50 % higher. Thelinear alcohols have values around 2.4 × 1014 molecules percm2, which are in good agreement with previous results,45while the values for the branched alcohols in this study arearound 1.6 × 1014 molecules per cm2. This difference in thepackingdensityisaresultofthemolecularorientationatML

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Fig. 7 Surface concentration in molecules/cm2 for the differentalcohols versus the bulk concentration. Error bars correspond to therangeof surface concentrations calculatedbyusingavariance in themonolayerthicknessby±0.5Å.Table4SurfaceconcentrationsatMLcoverage(cML)andthemaximumsurfaceconcentrationscalculatedforNS,max (cN(S,max)) inmolecules/cm2(forsurfaceconcentrationsinmol/m2,seeSI).molecule cML(molecules/cm2) cN(S,max)(molecules/cm2)1-butanol 2.5x1014 3.6x10141-pentanol 2.4x1014 3.3x10141-hexanol 2.3x1014 3.1x1014tert-butanol 1.5x1014 2.2x10143-pentanol 1.6x1014 2.2x10143-hexanol 1.7x1014 2.2x1014

Fig. 8 Surface enrichment factor EF for the different alcohols inaqueous solution versus the bulk concentration. Error bars areestimatedfromexperimentaluncertainties.Table5Maximumsurfaceenrichmentfactorsfortheprimaryalcoholsmolecule EFmax cEF(max)(mM) cML(mM)1-butanol 70 60 2001-pentanol 130 30 1001-hexanol 190 15 40coverage(wherethehydroxylgroupspointtowardsthewaterand the alkyl parts towards the vacuum) and the different

molecularstructures(bulkierbranchedvs.linearalcohols).Thesurface concentrations inmolecules/cm2 atML coverage aresummarizedinTable4.Forcomparisonthemaximumsurfaceconcentrations in molecules/cm2 for NS,max are given as well(forvaluesinmol/m2,seeSI).Surfaceenrichmentfactor

Asmentionedabove,thereisanoverestimationofthesurfaceconcentrationbytheLangmuirmodelfortheprimaryalcoholsatlowerconcentrations(compareFig.3)asadirectresultfromfitting the surface concentration at higher concentrationsmore accurately. One aspect that could possibly explain asurface coverage which is lower than expected by theLangmuir model, is a situation where thermodynamicequilibrium has not been established. However, as discussedearlier inmore detail,46we concluded from experiments andtheoretical considerations that our results are comparable tosystemsinequilibrium.Infact,amodificationoftheshapeofaLangmuir isotherm can originate from other factors, such asfor example attractive lateral interactions between themolecules at the interface,which can result in an S-shape oftheisotherm.Here,atwo-stepadsorptionmechanismapplies,where in a first step the molecules adsorb as individualmoleculesandtheninasecondstep,theadsorptionincreasesdramatically as surface aggregates form through interactionsof the hydrophobic chains of the surfactant molecules witheachother.47

In order to explore this phenomenon further, i.e. thedeviation of the surface concentration for linear alcohols atlower concentrations in comparison to the Langmuir model,thesurfaceenrichmentfactor,EF,isdeterminedbycalculatingthe ratio between the molar surface concentration and themolarbulkconcentration,i.e.EF=csurface/cbulk.

In Fig. 8, the enrichment factor EF is plotted versus therespectivemolaralcoholbulkconcentrationcbulk.Interestingly,at lower concentrations there is amaximum for the primaryalcohols,whichpeaksatapprox.1/3of theMLconcentration(seeTable5). Thismaximum is notobserved for tert-butanoland 3-pentanol, but seems to start appearing for 3-hexanol.Also, it is noticed that the observed maximum gets morepronounced, i.e. higher, with increasing chain length.Therefore,itissuggestedthatthismaximumintheenrichmentfactorisconnectedtotheabove-mentionedcooperativeeffectbetweenthealkylchainsbasedonvanderWaalsinteractions,whichincreaseinmagnitudeforincreasingchainlength.Theseintermolecularforcesmakeitenergeticallymorefavorableforthemolecules to resideat the interfaceas soonas there areenoughmoleculesforinteractionresultinginislandformation.Such van der Waals interactions are more pronounced forlinear alcohols than for branched ones, comparingmoleculeswith the same number of carbon atoms, as the part of alkylchainsavailableforinteractionscomprisemorecarbonatoms.Furthermore, the intermolecular distances between linearalcohols should be smaller than in the case of branchedalcoholsduetostericeffects,allowingthusformoreefficientvanderWaalsinteractionsbetweenthelinearalcohols.

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In conclusion, the trendof the surfaceenrichment factor canbe explained as follows: at very low concentrations, thealcoholmoleculesadsorb individuallyat theaqueous surface,asenthalpystabilizesthesolutesthere;acombinationofbothwater-water and alcohol-water interaction energies wassuggested to generate favorable enthalpies for such surfaceconfigurations,48 explaining the initial surface enrichmentfactor.Withincreasingconcentration,moreandmorealcoholmoleculesadsorbatthe interface,whichstartto interactandform molecular islands. The very efficient van der Waalsinteractions between linear alcohols become then moreimportant in theenergybalanceas thesurfacecoveragegetshigher, and the surface enrichment factor increases for thelinear alcohols compared to the shorter branched alcohols.However,with progressing adsorption at the interface,moreand more surface sites are occupied, thus less additionalalcoholmoleculescan residedirectlyat the interfaceand themaximum surface coverage is approached. As the surfaceconcentrationstaysconstantbutbulkconcentrationincreasesfurther,thesurfaceenrichmentfactormustthendecreaseforhigherbulkconcentrations.

TheLangmuirmodeldoesnotaccountforattractivelateralinteractions between the adsorbed molecules within thesurface phase, which is of higher importance for the linearalcohols.Ifoneconsidersforexample1-hexanol,aLangmuirfitwhich solely describes the lower concentrations moreaccurately(upto30mM),amuchlowerΔGAdsvalueofaround-9 kJ/mol is determined. However, with this fit NS,max issignificantly overestimated as compared to the experimentaldata points that saturate at significantly lower values forhigher concentrations. By fitting the data points at higherconcentrationsmoreaccurately,theNS,maxvalueisreasonableandahigherΔGAdsvalueofaround-19kJ/molisestimated.Inthis case, the Langmuir fit overestimates the surfaceconcentration at lower bulk concentrations, as a result ofextrapolatingthebeneficialvanderWaalsinteractionsthatareprevalent at higher surface coverage to lower surfacecoverage, predicting thus higher surface concentrations.Therefore, another consequence of our observations is thatthe Gibbs free energy of adsorption is dependent on thesurface coverage as the molecules are able to interact witheachotherandcanstabilizethemselvesatthesurface.Thisisin linewith theequation that canbe applied to approximatethe free energy of adsorption of the surfactant for dilutesolutions: ∆GAds = -RT log EF (ref. 47) as EF is changing withconcentration. Using the determined EF value for dilutesolution,aninitialΔGAdsfor1-hexanolcanbeestimatedtobearound-12kJ/mol,whichliesbetweenthedeterminedvaluesmentionedabove.Thus,withincreasingsurfacecoverage,thefreeenergyofadsorptiongetsmorenegativeasa resultofachange in orientation of the molecules allowing for van derWaalsinteractionsbetweenthealkylchains.

ConclusionsInthisstudy,weinvestigatedtheadsorptionofvariousalcoholisomers with different chain lengths (C4 – C6) at the liquid-

vapor interface with surface-sensitive X-ray photoelectronspectroscopy (XPS) and model the data with a standardLangmuir adsorption isotherm to determine Gibbs freeenergies of adsorption and estimate surface concentrations.From butanol to hexanol, we find ΔGAds values, which scalelinearlyfrom-15kJ/molto-19kJ/molwithaΔGAdsper–CH2–valueof-2kJ/mol.TheΔGAdsvaluesforthelinearalcoholsareslightly more negative than the values determined for thebranched alcohols, which is in line with their higher surfaceconcentrations. At monolayer coverage, the surfaceconcentrations of the linear alcohols are around 2.4 x 1014molecules/cm2,whilethebranchedalcoholsinthisstudyhavevalues around 1.6 x 1014 molecules/cm2. Thus, the packingdensityof theprimaryalcoholsatMLcoverage isonaverage50%higher than the one of the branched alcohols,which isconsistentwiththemolecularstructuresandtheirorientationat the interface.Most interestingly, we find that the surfaceenrichment factor for the linear alcohols has a maximum atlower concentrations, which is not observed for the shorterbranchedalcohols. This is interpretedas a cooperativeeffectbetweenthelinearalcohols,whichmakesitenergeticallymorefavorabletoresideattheaqueoussurfaceassoonasthereisasufficient amount of molecules at the interface allowing forvanderWaals interactionsbetweenthealkylchainsandthustheformationofislands.

Different oxidation pathways in the atmosphere can leadtotheformationofdifferentisomersoftheoxidationproductsforagivenorganicprecursor.Weseehere,thatdependingonwhich isomers are formed and eventually end up in thecondensed aerosol phase will affect the surface structuralproperties of aqueous aerosols. The impact of this on e.g.cloudmicrophysicsisofinterestforfurtherinvestigations.

AcknowledgementsFinancial support from the Swedish Research Council (VR),Swedish Foundation for Strategic Research, NICITA and theCarlTryggersStiftelseförVetenskapligForskning(M.-M.W.)isgratefully acknowledged. NPL gratefully acknowledges thepersonal funding received from the Carlsberg Foundation(grants 2009_01_0366 and 2010_01_0391) and the FinnishAcademy of Sciences (257411). MAX IV Laboratory, LundUniversity, Sweden, is acknowledged for the allocation ofbeamtime and laboratory facilities. Dr. Daniel Lundberg isacknowledgedfordiscussionandproof-reading.

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alcohols at the aqueous surface: chain length and isomer

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