Molecular Fingerprints of Hydrophobicity at Aqueous Interfaces from Theory and Vibrational Spectroscopies - Pezzotti et al. - 2021 - Unk

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pubs.acs.org/JPCLLetterMolecularFingerprintsofHydrophobicityatAqueousInterfacesfromTheoryandVibrationalSpectroscopies,∇∇∇SimonePezzotti,*AlessandraServa,FedericoSebastiani,FlavioSiroBrigiano,DariaRuthGalimberti,LouisPotier,SerenaAlfarano,GerhardSchwaab,MartinaHavenith,*andMarie-PierreGaigeot*CiteThis:J.Phys.Chem.Lett.2021,12,3827−3836ReadOnlineACCESSMetrics&MoreArticleRecommendations*sıSupportingInformationABSTRACT:Hydrophobicity/hydrophilicityofaqueousinterfacesatthemolecularlevelresultsfromasubtlebalanceinthewater−waterandwater−surfaceinteractions.Thisischaracterizedhereviadensityfunctionaltheory−moleculardynamics(DFT-MD)coupledwithvibrationalsumfrequencygeneration(SFG)andTHz-IRabsorptionspectroscopies.Weshowthatwaterattheinterfacewithaseriesofweaklyinteractingmaterialsisorganizedintoatwo-dimensionalhydrogen-bondednetwork(2D-HB-network),whichisalsofoundabovesomemacroscopicallyhydrophilicsilicaandaluminasurfaces.Theseresultsarerationalizedthroughadescriptorthatmeasuresthenumberof“vertical”and“horizontal”hydrogenbondsformedbyinterfacialwater,quantifyingthecompetitionbetweenwater−surfaceandwater−waterinteractions.The2D-HB-networkisdirectlyrevealedbyTHz-IRabsorptionspectroscopy,whilethecompetitionofwater−waterandwater−surfaceinteractionsisquantifiedfromSFGmarkers.ThecombinationofSFGandTHz-IRspectroscopiesisthusfoundtobeacompellingtooltocharacterizethefinestdetailsofmolecularhydrophobicityataqueousinterfaces.hespecificmolecular-levelorganizationofwateratbecomeswhenamolecular-levelperspectiveisadopted.ThisTaqueousinterfacesisattheoriginofmanynaturalcallsforgoingbeyondtheknowledgeofdanglingOHgroups1−8andachievingadeeperrationalizationofhowthewaterHB-phenomena,rangingfrombiologyandcatalysis,topollutant9,10networkrearrangesonceitisexposedtohydrophobicsurfaces.transportingroundwaterandmineraldissolution,toatmosphericchemistry.11−14ItisalsoofcrucialimportanceProgressinthisdirectionhasbeenpossiblethankstoinelectrochemistry,phase-separationprocesses,15andmanymoleculardynamics(MD)simulations,whichshowedhowtheDownloadedviaBUTLERUNIVonMay16,2021at08:03:26(UTC).othertechnologicalapplications.16,17finalarrangementofwateratcomplexinhomogeneousThebalancebetweenhydrophobicandhydrophilicinter-interfacesdependsonnotonlytheamountofhydrophilicactionsinparticulardictatesthemicroscopicarrangementatandhydrophobicsitesexposedtowaterbutalsotheirspatial18−23distributionoverthesurface.18−23Giovambattistaetal.23foraqueousinterfaces.Atanaqueousinterface,waterSeehttps://pubs.acs.org/sharingguidelinesforoptionsonhowtolegitimatelysharepublishedarticles.rearrangesinresponsetotheabruptterminationofthebulkinstancedemonstratedthatthewatersurfacedensityiswaterH-bond(HB)networkinwaysthatdependonthelocalconsiderablyhigherincontactwithahydrophobic“patch”strengthofthewater−surfaceinteractions.Watermaximizessurroundedbyhydrophilicbordersthanitisatapurelywater−surfaceHBsabovealocallyhydrophilicsurface,whilehydrophobicsurface.Forwaterattheboundarywithmodeldangling“free”OHgroups,pointingoutoftheliquidphase,surfaceswithmixedhydrophilic−hydrophobicsites,Erteet19al.furtherfoundthattheexposureofinterfacialwatertotheareobservedinhydrophobicenvironments.SuchdanglingOHhydrophobicareasismaximizedwhenthehydrophobicandgroupshavebeenexperimentallydetectedviasurface-specific24hydrophilicsitesformseparatepatchesoverthesurface,whilevibrationalsumfrequencygeneration(SFG)spectroscopyitisminimizedforhomogeneousdistributions.Inarecentforwaterincontactwithair,oil,organicmonolayers,silica,25−32work,combiningMDsimulationswithSFGexperiments,somealumina,graphene,andboronnitride.ThespectroscopicsignatureofdanglingOHgroupshasbeenhistoricallyinterpretedasamolecular/localmarkerforhydrophobicity.33Received:January25,2021However,recentexperimentalandtheoreticalstudieshaveAccepted:April9,2021shownthatSFG-activedanglingOHgroupscanbedetectedPublished:April14,2021alsoatmacroscopicallyhydrophilicsurfaces,suchasheat-29,31,34−3632,37treatedsilicaand0001-α-alumina.Thesestudiespointedouthowsubtletheconceptofhydrophobicity©2021AmericanChemicalSocietyhttps://doi.org/10.1021/acs.jpclett.1c002573827J.Phys.Chem.Lett.2021,12,3827−3836

1TheJournalofPhysicalChemistryLetterspubs.acs.org/JPCLLetterofushaveshownthathydrophobicpatchesareformedonsensitivitytothetopmostinterfacialmonolayer(BIL),whichisheat-treatedsilicasurfaces.Forthesesystems,theSFGmarker-idealforthepurposesofthepresentinvestigation.bandfordanglingOHgroupscanbeobserved,althoughtheseDFT-MDtoCharacterizethe2D-HB-Network.Weusesurfacesareclassifiedasmacroscopicallyhydrophilicfromourpreviouslydevelopeddeconvolutionschemetoidentifythe31BILregionatthefiveaqueousinterfaces,45basedonthreecontactanglemeasurements.Whenthepretreatmentofthesilicasurfacewaschanged,theattenuationofthefree-OHbandwater−structuredescriptors,i.e.,thewaterdensityprofilewithintensityintheSFGspectrawasfurthershowntocorrelaterespecttotheverticaldistancefromtheinstantaneouswaterwiththedecreaseinthemeasuredcontactangles.31Thesurface,theaveragenumberofHBs/watermolecule,andthecorrelationbetweenstructural,dynamical,andthermodynamicaverageorientationwithinthewaterHB-network.WefindthatpropertiesofinterfacialwaterwasalsohighlightedbyMonroeallfiveinterfacesarecomposedbyaBIL-watermonolayerofetal.,20concludingthatthesurfacepatternscontrolthe∼3.5Åthickness,directlyfollowedbybulkliquidwater.dynamicsofhydrationwaterandthatthehydrationwaterAsshowninFigure1,waterintheBILhassystematicallyaorientationalentropy,diffusivity,andH-bondingpropertiesarehigherdensitythanbulkwater.Therearehoweverdifferencesintrinsicallyconnected.Alltheseworksshowedhowthespatialvariationinsurfacechemicalandgeometrictopologycantunelocalandglobalsurfacewaterpropertiesinasurprisinglycomplexway.Thefundamentalreasonforthiscomplexityisthatwatermoleculeshavedirectionalinteractionsandcanformawidevarietyofnetworkswithneighboringsurfacegroupsandwater21molecules.InthepresentstudyweprovideadetailedcharacterizationoftheHB-networkwhichisformedbywateratasetofdistinct“hydrophobic”environments,bycombiningdensityfunctionaltheory-basedMD(DFT-MD)simulationswiththeoreticalSFGandexperimental/theoreticalTHz-IRvibrationalspec-troscopies.WhileSFGisanaturalprobeofthevibrationalpropertiesofburiedinterfaces,wecomplementitwithTHz-IRabsorptionspectroscopy,whichhasbeenprovedtobeanextremelysensitivetechniquetorevealtheintermolecular38,39dynamicsofwater.ThestrengthoftheTHzlow-frequencyFigure1.WaterdensityprofilescalculatedfromDFT-MDspectroscopicfingerprintsisthattheycanbedirectlyrelatedtosimulationsforthefiveaqueousinterfaces.TheverticalblackdashedthecorrespondinghydrogenbondnetworkmotifsclosetolinedefinestheseparationbetweenBIL,wherethe2D-HB-networkis38−43hydrophilicandhydrophobicmoieties.formed,andbulkliquid.Forallsystems,thex-axisreportstherWeperformedDFT-MDsimulations(seeMethodsforalldistanceofthewatermoleculesfromtheinstantaneouswater52details)forliquidwaterincontactwith(1)air,theprototypesurface,whichisonaveragepositionedat3.0−3.5Åfromthehydrophobe,(2)graphene,and(3)hexagonalboronnitridefoursolidsurfaces.Byconstruction,rvaluesarepositiveontheliquidside,whilenegativevaluesidentifywatersthatprotrudeoutofthe(BN),chosenasexamplesofnonH-bondingsurfaces;(4)liquidphasetowardthevacuum/solid.heat-treatedsilica(i.e.,amorphoussilicawithlow∼3.5SiOH/231nmsilanoldensity)and(5)(0001)-α-alumina,chosenasinthedensityprofilesofthefivesystems,withthemoreplanarexamplesofH-bonding,macroscopicallyhydrophilicsurfaces.BN−water,graphene−water,andalumina−waterinterfacesTheheat-treatedsilica,withitslowdegreeofsurface229,31,35havingamoreintenseandsharperfirstdensitypeakthanthehydroxylation(∼3.5SiOH/nm),isinparticular31morecorrugatedair−waterandsilica−waterinterfaces(Figureconsideredbecauseitisknownfromourpreviousworkto1).Asrevealedbythesimulations,thiscorrelateswiththeexposeextendedhydrophobicpatches(withalocalsilanolfluctuationsoftheinstantaneouswatersurface,definedusing2density≤1.5SiOH/nm)atthesurfacedespiteitsmacroscopictheWillardandChandlerformalism.52Theinstantaneoushydrophilicity(measuredbycontactangles).Conversely,thesurfaceisfoundwithmoreoscillations(intimeandspace)at(0001)-α-alumina−waterinterfacehasaveryhighdensityofthemorecorrugatedinterfaces,whileitismore“flat”attheAlOHtermination(15.4AlOH/nm2),supposedlyformingamoreplanarinterfaces.Thewaterspatialorderingthusfollowsveryhydrophilicsurface.However,italsoexposes“hydro-themorphologyofthesurface.Itisworthnotingthatthephobic”sitesbecauselessthan1/3ofthealuminolsisfoundoscillationsbeyondthesecondpeakinthedensityprofilesH-bondedtowater(4.7HBs/nm2,similartothevalueof4.3observedinthebulkregionforallinterfaceswereshowninourSiOH−waterHBs/nm2formedbytheheattreatedsilica),53,54recentinvestigationstobeduetothelimitedDFT-MD44becauseoftheirverybasicpKa.Theremaining2/3ofboxdimensions.Theydisappearforlargersimulationboxes,aluminolsaredangling−OHgroupspointingtowardthewithtypicallyaliquidphasecomposedby500watermolecules37,44liquid.Allfiveinterfaceshavebeenconsideredatormore(lessthan256watersareusedtomodeltheliquidisoelectricconditions,wheretheBIL(bindinginterfacialwaterinthepresentsimulations).53Thishoweverhasno45,46layer),indirectcontactwiththesurface/air,isexpectedimpactonthestructuralandvibrationalpropertiesofinterest,tobetheonlyinterfacialwaterlayer,directlyfollowedbybulkasshowninrefs28,31,53,and54.water.Nodiffuselayerispresentatsuchisoelectricconditions,ThehighdensityintheBIL-waterinterfaciallayer,observedaccordingtowhatweandothershaveshowninrefs36andforallfivesystems,issomehowcounterintuitiveforhydro-45−51.TheseconditionsallowustomaximizetheSFGphobicinterfaces:onlyhydrophilicinterfaceswouldbe3828https://doi.org/10.1021/acs.jpclett.1c00257J.Phys.Chem.Lett.2021,12,3827−3836

2TheJournalofPhysicalChemistryLetterspubs.acs.org/JPCLLetterexpectedtohavesuchbehaviorasaresultofwaterstronglygreaterthanthenumber(≤0.8)of“vertical”HBs.Theselatterinteractingwiththesurface,andthusaccumulatingintheBIL.aremostlyformedbetweenBIL-waterandbulk-waterAsdemonstratedinrefs28,48,53,and55,thehighBIL-watermolecules(>85%forallthesimulations),whileonlyafractiondensityobservedattheair−waterinterfaceisduetothe(<15%)isformedwithH-bondingsurfacesites(atsilica−specificHB-structurewhichisformedinordertomaximizethewaterandalumina−wateronly).Thisbehaviorisoppositetowatercoordinationattheboundarywiththevapor.Intheoneobservedforhydrophilicinterfaces.Forexample,whenparticular,waterattheinterfacewiththeair(vacuum)waterisincontactwithhighlyhydroxylated(non-heat-treated)rearrangesbyforminganextendedHB-structureconnectingsilicaorquartz,morethan1.7“vertical”HBs/moleculeare∼90%ofwatermoleculesintheinterfaciallayerthroughformed(≥50%withsurfaceOH-terminations,asdetailedinorientedHBs,preferentiallyformedparalleltotheinstanta-refs31,44,and56).neouswatersurface.28,53Thistwo-dimensional-H-bonded-Theseconddescriptorusedtorevealthe2D-HB-networkisnetwork(2D-HB-network)isfurthermadeofadjacent2D-thetime-evolutionofthemostextendedHB-structuremadeofHB-polygons.54Aslongasthewaterinteractionswiththeintra-BIL(horizontal)HBs.AteachMDstep,allthepossibleothermediumareweakenough,water−waterHBsremainthehorizontalHB-structures(by“structure”wemeananon-dominantdrivingforcefortheinterfacialwaterorganization,interruptedHBondedmotifthatconnectsseveralwaterandtheformationofa2D-HB-networkintheBILcanbemolecules)madebyBILwatermoleculesareidentified,expected.WenowdemonstratethatthehighBIL-waterdensitytogetherwiththenumberofwatermoleculescomposingeachhasthesamemicroscopicoriginatallfiveinterfaces,becauseoneofthesemotifs.WeareinterestedinonlythelargestH-the2D-HB-networkissystematicallyformedintheBIL.bonded-structure,composedofnmaxwatermolecules(seeThreedescriptorsaresufficienttorevealthepresenceoftheFigure3athatreportstheevolutionwithtimeofthe532D-HB-networkataqueousinterfaces.Thefirstdescriptoristhenumberofin-planeintra-BILwater−waterHBs(i.e.,HBsformedwithintheBILonly),whichisgreaterthan1.6HBs/53moleculeatinterfaceswherethe2D-HB-networkisformed,whileitislowerthan1.1HBs/moleculeforhydrophilic31,56interfaceswhereno2D-HB-networkisformed.AsshowninFigure2,morethan1.6intra-BILHBs/molecule(denoted“horizontal”becausetheirorientationisparalleltothewatersurface)areformedinalltheinvestigatedsystems.Thenumberofthese“horizontalHBs”ismuchFigure3.(A)Evolutionwithtime(ps)ofthenumberofwatermolecules(nmax)thatareinterconnectedbyintra-BILHBsintoonesingle2D-HB-network,normalizedbytheaveragenumberofwatermoleculesintheBIL,⟨NBIL⟩.(B)MD-snapshotsillustratingthe2D-HB-network(orangeconnections)formedbyBIL-watermolecules(redoxygensandwhitehydrogens)attheair−waterinterface(sideandtopviews)andatthe(0001)-α-alumina−waterinterface(top-view).Theinstantaneouswatersurfaceisalsoshowninthesideview.normalizedvaluenmax/⟨NBIL⟩,where⟨NBIL⟩istheaveragenumberofwatermoleculeslocatedintheBIL).Ifa2D-HB-networkisformedintheBIL,nmax/⟨NBIL⟩isexpectedto53fluctuatearoundanaveragevalue≥0.85.Forallinterfaces,wefindthatnmax/⟨NBIL⟩oscillatesaroundanaveragevalueof0.9,thusveryclosetothetotalnumberofBIL-watermolecules(horizontalblueline),alongthe50pssimulationtime.A2D-HB-networkcomposedby∼90%ofinterfacialwatermoleculesFigure2.AveragenumberofHBsformedperwatermoleculeinthenotonlyisobservedforalltheinvestigatedinterfacesbutalso2D-HB-network,eitherconsideringintra-BIL(“horizontal”)HBs,i.e.isstablymaintainedintime.IllustrativeMD-snapshotsoftheformingthe2D-HB-network,orconsideringtheremaining“vertical”2D-HB-networkformedattheair−waterandalumina−waterHBs,whichareformedbyBIL-watermoleculesengagedinthe2D-HB-networkeitherwithbulk-watermolecules(>85%)orwithsolidinterfacesarepresentedinFigure3b,wherethe2D-HB-surfaceO−Hterminations(<15%).Aschematicillustrationofpolygonscomposingitarehighlighted.“horizontal”(red)and“vertical”(blue)HBsformedbywaterThethirddescriptoristhedynamicbehaviorofthemoleculesintheBILisprovided.horizontalintra-BILHBs,whichhavebeenshowntobe3829https://doi.org/10.1021/acs.jpclett.1c00257J.Phys.Chem.Lett.2021,12,3827−3836

3TheJournalofPhysicalChemistryLetterspubs.acs.org/JPCLLettershorter-livedatinterfaceswherea2D-HB-networkisformedinterfaces.TheoreticalheterodynedetectedHD-SFGspectra,thananyHBinbulkwater.Thehydrogenbondlifetime(τHB)wheretheknowledgeontheinterfacialwaterorientationisiscomputedfollowingthesameformalismasusedinref53(2)63gainedfromthesignoftheimaginarycomponentofχ(ω),andfoundtobeequalto0.44±0.03psforallthefivehavebeencalculatedforallsystemsfollowingourpreviousinterfaces.Thisis0.8timessmallerthaninbulkwater(τHB=derivation.28,56,64Im(χ(2)(ω))spectraareplottedinFigure4.0.56psobtainedwiththesamecomputationalsetup,seeref53).ThefasterHBsdynamicswithinthe2D-HB-networkcanberationalizedbythedensityofHBs(donorsandacceptors):thisdensityisindeedfoundhigherwithinthe2D-HB-networkthicknessthaninanequivalentplanerandomlycutinthebulk.ThisinturnpromotesfastHBswitching.Thisrationalizationwasdiscussedinref53.Asimilarresulthasbeenpreviouslyreportedforgraphene−waterandBN−waterinterfacesby57,58meansofabinitioandclassicalMDsimulations.ThepreferentialorientationandshorterlifetimeofinterfacialHBsimposedbytheformationofthe2D-HB-networkhasbeenshownbyrecentworkstostronglyaffecttheprotonhopping48,59mechanismattheair−waterinterface,preferentiallyoccurringviawaterwires(i.e.,HB-chains)runningparalleltothesurface,aswellastheprotonconductivity,whichisgreater60attheair−waterinterfacethaninbulkliquidwater.Thesamerationalizationcanbeappliedalsoforthehighconductivityandlateralhydroxidediffusivityatgraphene−waterandBN−waterinterfaces,asdemonstratedbyGrosjeanFigure4.(A)TheoreticalBIL-SFGIm(χ(2)(ω))spectracalculatedfor61etal.Furthermore,foralltheseinterfacessharingthesameBIL-waterattheinterfacewithboron-nitride(BN),graphene,heat-water−waterHB-structure,similarintermolecularpathwaysfortreatedamorphoussilica,(0001)-α-alumina,andair.(B)TheoreticalBIL-SFGIm(χ(2)(ω))contributionofBIL-watermoleculeshavingultrafastenergytransfersandvibrationalrelaxationprocessesshouldbeexpected.Asaconsequence,wespeculatethattheoneOHgroupH-bondedtobulkwater.(C)Schemeillustratingthepresenceofasimilar2D-HB-networkatair−waterand(0001)-correlationbetweenthepositionofthepositivepeakintheBIL-SFGIm(χ(2)(ω))spectraandthestrengthofwater−surfaceinteractions.α-alumina−waterinterfacescouldbeonemajorreasonforthesimilar(∼250−300fs)vibrationalrelaxationdynamicsmeasuredviatime-resolvedTR-SFGand2D-SFGspectros-Thespectrahavebeencalculatedconsideringthecontribu-copies(atisoelectricconditions,wheretheBILisprobedbytionofwatermoleculesintheBIL,becauseourtargetisto45,46SFG).OurhypothesisthatthefastBIL-waterrelaxationrevealthesespecificsignaturesarisingfromthe2D-HB-measuredbyTR-SFGischaracteristicoftheformationofanetwork.Forthealuminaandsilicasurfaces,thecontributions622D-HB-networkisfurthersupportedbyarecentstudy,oftheO−Hterminationsshouldbeincludedaswellforawherewefindthatadditionofionsat(non-heat-treated)completemodelingoftheSFGsignal.56silica−waterinterfacesinducestheformationofa2D-HB-Figure4AshowsverydifferentIm(χ(2)(ω))spectraforthenetworkaswellasaccelerationofvibrationalrelaxationfiveinterfaces.Inordertorationalizethesedifferenceswedynamicsfrom∼650−700to∼250−300fs.shouldconsiderthecontributionsoftheBIL-waterOHgroupsInsummary,oursimulationsrevealthatthesame2D-HB-orientedtowardthesurface,towardthebulk,andparalleltonetworkisformedatallfiveinterfacesconsideredhere,thusthesurfaceseparately.TheOHgroupsorientedparalleltotheallowingthemaximizationof“horizontal”water−waterHBssurfaceplaneareunfortunatelynotSFG-active(intheandincreasingtheconnectivitybetweenBIL-watermolecules.commonssp/ppppolarizations)becauseoftheirorientation.Waterengagedwithinthe2D-HB-networkformsonaverageAlltheOHgroupsinvolvedin“horizontal”intra-BILHBsandalmosttwointra-BILHBswithothermoleculesinthesameformingthe2D-HB-networkthusprovideanegligibleBIL-layerandlessthanoneHBwithwatermoleculeslocatedIm(χ(2)(ω))intensity,althoughtheyarethemaincomponentsinthesubsequentbulk,resultinginatotal3-foldcoordinationoftheinterfacialstructure.Bycontrast,SFGissensitivetothe(sumofblueandredhistogramsinFigure2).AsshownintheinterfacialOHgroupsnotengagedinthe2D-HB-networkandfigure,such“horizontalordering”ismoremarkedforwateratorientedperpendiculartothesurface.FromtheprevioustheinterfacewithnonH-bondingsurfaces,likeBNanddiscussions,thereisonaverageoneOHgroupperBIL-watergraphene(1.9horizontalHBs/molecule),thanattheinterfacemolecule,orientedeithertowardtheair/surface(1/3ofthewithH-bondingsilica(1.7horizontalHBs/molecule)andOHs)ortowardtheliquidbulk(2/3).Inthelattercase,thesealumina(1.6horizontalHBs/molecule)surfaces.The2D-HB-OHgroupsaresystematicallyHB-donorstobulkwaternetworkisthusweakened(i.e.,lessinterconnectedbecauseofmolecules;thus,theirsignatureisexpectedtobethesameinfewerhorizontalHBs)bytheincreaseinthenumberofwater−alltheSFGspectra.ThisisconfirmedinFigure4B,wherethesurfaceHBsformed,withBIL-watermaking0.0HBs/nm2withSFGcontributionoftheseOHgroupsisdeconvolvedandgrapheneandBNsurfaces,4.3HBs/nm2withsilica,and4.7showntoprovideasimilarnegativebandforallsystems.Thus,HBs/nm2withthealuminasurface.thedifferencesobservedinthetotalspectrainFigure4AariseHD-SFGSpectroscopytoQuantifyWater−SurfacesolelyfromtheOHgroupsofinterfacialwatermoleculesInteractions.Wenowseekthespectroscopicsignaturesofthepointingtowardtheair/surface.TheseOHgroupsmodulate2D-HB-network.WestartbyfocusingonSFGspectroscopy,theshapeofthenegativebandinthefinalSFGspectrabecausewhichisthenaturalspectroscopictooltoinvestigateburiedofcompensationofpositive/negativecontributionsfromthe3830https://doi.org/10.1021/acs.jpclett.1c00257J.Phys.Chem.Lett.2021,12,3827−3836

4TheJournalofPhysicalChemistryLetterspubs.acs.org/JPCLLettertwoOHpopulations.Moreimportantly,theysystematicallyprovideapositiveIm(χ(2)(ω))peak(theorientationtowardthenormaltothesurfacebyconventionisdefinedfromliquidtosolid/air).Suchapositivepeakispositionedat∼3700cm−1fortheair/waterinterface,wherewaterisincontactwithvacuum,i.e.,theultimatehydrophobicmedium.Thisbandfrequencyisthentakenasthereferenceforthehydrophobiccharacterofanysystem.Thestrengthofthewater−surfaceinteractionsformedatallotherinterfacescanbehencerankedbyquantifyingtheextentofred-shiftinthepositiveIm(χ(2)(ω))peakswithrespecttothe∼3700cm−1reference.WecanhencededucethatBIL-waterformsslightlystrongerinteractionswiththenitrogenatomsoftheBNthanwiththecarbonsofgraphene,resultingina∼30cm−1shiftbetweentherelativepositivepeaks(∼3644cm−1forgraphenevs∼3616cm−1forBN),inagreementwithapreviousstudy.30WecanalsoinferthatBIL-watermakestwokindsofinteractionswithsilicasurfaceterminations,providingtwodistinctIm(χ(2)(ω))signaturesat∼3650and∼3450cm−1.Asdemonstratedinref31,the∼3450cm−1bandisduetoBIL-waterOHgroupsdonatingHBstoSiOHsilanolterminations,whilethe∼3650cm−1bandisduetoBIL-waterOHgroupsmoreweaklyinteractingwithSi−O−Sisiloxanebridges.Finally,thein-plane64O−HterminationsatthealuminasurfaceareabletoreceivemuchstrongerHBsfromBIL-waterthansilicaandalltheotherFigure5.THzdifferenceabsorptionspectrashowΔαasobtainedbysurfacesconsidered,thusleadingtothepositivebandlocatedsubtractingthedryBN-nanoplateletsspectrumfromthespectrumofbelow3200cm−1.OneshouldnoteherethatpartoftheeachofthreeaqueousdispersionsofBNnanoplatelets,inthehigh-water−surfaceinteractions,e.g.,theHBspossiblyreceivedby(top),medium-(middle),andlow-(bottom)water-contentregimes.Eachexperimentalspectrum(blackline)wasmodeledasasumofBIL-watermoleculesfromsurfaceOHterminations,cannotbedampedharmonicoscillator(redline),asdescribedinthetextandinferredfromthepresentspectra,becausetheirsignaturestheSupportingInformation.ThedampedharmonicoscillatorswerewouldbeobtainedfromtheSFGactivityofsurfaceOHgroupsassignedtoBIL-waterformingthe2D-HB-network(orange)andtothatarenottakenintoaccountinthepresentcalculations(seebulk-likewater(blue),respectively.ThetheoreticalDFT-MDTHze.g.refs31and56forsuchsignatures).Thesehavealreadysignatureofthe2D-HB-networkattheBN−waterinterface,beenquantifiedatsilica−waterinterfacesinref56,whiletheycalculatedconsideringthecontributionofwatermoleculesformingarenegligibleatthe(0001)-α-alumina−waterinterfacebecausethe2D-HB-network,isalsoshown(green),rescaledtotheofthepKvaluesandorientationoftheAlOHterminations.44experimentalintensityforthesakeofcomparison.SeetheSupportingaThefinalrankingonthestrengthofwater−surfaceInformationformoredetailsonthenormalizationoftheintensityininteractionsformedatthefiveinterfacesobtainedfromthetheexperimentalspectra.positionofthepositivebandinIm(χ(2)(ω))spectraresultstoalumina>silica>BN>graphene>air,asillustratedbythe()ααsample−dryBNspectra(Δα),whichwereobtainedbyΔα=,schemeinFigure4C.Thisisoppositetotherankingobtainedxwforthenumberofintra-BILHBs/moleculeinFigure2,i.e.,forwhereαsampleandαdryBNaretheabsorptioncoefficientofaBNthestrengthofthe2D-HB-network.ThewaterarrangementataqueousdispersionandthatofthecorrespondingdryBNintheinterfaceishencetriggeredbythecompetitionbetweentheconsidereddispersion,respectively,andxwisthemolarwater−waterandwater−surfaceinteractions;thatis,thefractionofwaterineachdispersion.Theabsorptioncoefficientweakerthewater−surfaceinteractions,thehigherthenumberαwasdeducedfromthetransmittedintensity,usingtheofintra-BILHBsintheinterfaciallayerandthestrongertheLambert−Beerlaw(seetheSupportingInformation).2D-HB-network.Theformationofthe2D-HB-networkintheAsdescribedintheSupportingInformationandroutinelyBIL-waterlayeristhusadirectindicatorofthehydrophobic38,39doneinourpreviousTHzstudiesofwatersystems,theΔαcharacterofthesurfaceatthemolecularlevel.However,wespectraarefittedwithasumofdampedharmonicoscillatorswanttostressthatHD-SFGspectraintheOH-stretching(DHOs).Thisanalysisrevealsthattheexperimentaldataarefrequencydomain(2700−4000cm−1)donotprovideanywelldescribedbyoneortwoDHO(s).Asshowninthetopdirectsignatureofthe2D-HB-network.panelofFigure5,aDHOalone,centeredat∼180cm−1(blueTHz-IRAbsorptionSpectroscopytoDetectthe2D-line),dominatesthespectrum,whentheBNnanoplateletHB-Network.TheTHzdifferenceabsorptionspectraofdispersionsareinthehigh-water-contentregime.TheaqueousdispersionsofthinBNnanoplateletshavebeenspectrumofBNnanoplateletsdispersionsinthemedium-recordedinthe80−240cm−1range(seetheSupportingwater-contentregimeresultsfromtwocomponentsat162±2Informationforfurtherdetails).Inthisrange,theintermo-cm−1(orange)and197±4cm−1(blue),whileasingleDHOlecularstretchingofH-bondedwatermoleculescontributetoat160±2cm−1ispresentinthespectrumofBNnanoplatelet65theTHzspectradiscussedhereafter.Threeconcentrationsdispersioninthelow-water-contentregime(orangeline,havebeenmeasured,correspondingto20,50,and100mg/bottompanel).WewanttopointoutthattheresonanceatmL,identifiedashigh-,medium-,andlow-water-content180cm−1inthespectrumofthesampleinthehigh-water-regimes.InFigure5weplottheTHzabsorptiondifferencecontentregimeisaboutatthesamecenterfrequencyasthatof3831https://doi.org/10.1021/acs.jpclett.1c00257J.Phys.Chem.Lett.2021,12,3827−3836

5TheJournalofPhysicalChemistryLetterspubs.acs.org/JPCLLetter65,6628,5328theintermolecularstretchingbandofbulkwater.WecannetworkorHB-wrapissystematicallyformed,hencethusinferthattheDHOcenteredatabout160cm−1(orange)maximizingthenumberofwater−waterHBsorientedcomponentismostlyduetothewaterinthefirstinterfacial“horizontally”(i.e.,paralleltothesurface)withinthetopmostlayer,whichisthedominantcomponentinthelow-water-BILwaterlayer.TheseHBshavelowSFGactivitybecauseofcontentspectrum,whiletheDHOcenteredatabout197cm−1theirorientation;hence,theirspectroscopicsignaturesarenot(blue)isduetobulk-likewater.ThepartialcontributionofthedirectlyprobedinstaticSFGspectra.However,wehaveshown197cm−1componentisincreasedwhenadditionalwaterlayersthatadirectmarkerbandforthe2D-HB-networkcenteredatareintroducedinthemedium-andhigh-water-content∼160cm−1isprovidedbyTHz-IRabsorptionspectroscopyregimes.Wenotethatthecenterfrequencyofthisbandisexperiments.Astrikingresultisthatwaterformsthe2D-HB-similartothatfoundat∼193cm−1inbulkliquidwateratnetworknotonlyattheinterfacewithnonH-bondingsurfaces,38273.2K.likehexagonalboronnitride,graphene,andair,butalsoattheInterestingly,thelower-frequencymodeat160cm−1hasinterfacewithH-bondingsurfaces,likeheat-treatedamorphousbeenalreadyobservedforwaterhydratinghydrophobic67silicaand(0001)-α-alumina,wherehydrophobicityarisesatsolutes,includingalcohols,clathrate,andsemiclathrate38thenanoscalelevelfromsurfacepatcheswhichhavealocallowhydrates.TheselatterarecagesformedbypolygonalwaterdensityofH-bondingsites.Microscopicallyhydrophobicstructuresaroundanapolarguestandthusserveasamodel38water,arrangedinthe2D-HB-network,isthuspresentalsosystemforhydrophobichydration.Wehencetentatively−1atmacroscopicallyhydrophilicsurfaces.The2D-HB-networkassignthe160cmmodetothe2D-HB-networkattheBN-isfurtherfoundtobeweakenedbytheincreaseinthestrengthsurfacewhichmostlyformsintra-BILHBs.Whenmore“bulk-ofwater−surfaceinteractions,whichisdirectlymeasuredbylike”hydrationlayersareadded(medium-water-contentHD-SFGspectroscopy.regime),watermoleculescannowformbothintra-BILandOurworkthusshowsthatbeyondthewell-knownSFGBIL-bulk(orbulk−bulk)HBs,andboththeintermolecularmodesat∼160and∼197cm−1(inorangeandblueinFigurespectroscopicsignaturesofthewaterfreeO−Hgroups,the5,respectively)contributetothespectrum,analogouslytointerfacialmolecularhydrophobicityisencodedintoaplanaralcohol−watersolutions.67Inthehigh-water-contentregime,2D-H-bond-networkformedbythewatermoleculesinthetheHBsformedbetweenbulkwatermoleculesnaturallyBIL,whichisdirectlyrevealedbyonespecificTHzbecomethemajorcontributorstotheTHzspectrum,whichisspectroscopicfingerprint.Generallyspeaking,thewaterHB-accordinglydominatedbyonebroadbandat∼180cm−1networksformedatanyaqueousinterfaceresultfromthe(blue),similartobulkwater(seerefs65and66andthesubtlebalancebetweenhorizontalwater−water(intra-BIL)andSupportingInformation).verticalwater−surfaceinteractions,whichreflects(i)thelocalTosupporttheassignment,wealsopresentinFigure5thedensityofsurfacesitesthatcaninteractwithwatermolecules,theoreticalTHzsignalcalculatedforthe2D-HB-networkatthe(ii)thestrengthoftheverticalwater−surfaceH-bondBN−waterinterface(greenline).Thespectrumhasbeeninteractions(i.e.,pKa’s),and(iii)howmuchthestructuralobtainedbycomputingthetheoreticalfar-IRspectrumofthepattern(s)/patch(es)formedbythesurfacesitescanbewatermoleculesformingthe2D-HB-network,sothatthecommensuratetotheH-bondedstructuresthatcanbeformedintermolecularstretchingofintra-BILHBssolelycontributeshorizontallybyinterfacialwaterabovethesurface.The2D-H-inthe80−240cm−1frequencyrangeofinterest.Asshowninbond-networkrevealedbythepresentMDsimulationsandthefigure,anexcellentagreementisobtainedbetweenTHzspectroscopyatvarioussolid−waterinterfaces(andair−theoreticalandexperimentalspectra(greenversusorangewaterinterface)istheultimatehorizontalorderingthatcanbeline),withthe2D-HB-networksignalcalculatedfromtheobtained.Thesubtlebalanceamongpointsi−iiicanthusleadDFT-MDsimulationoftheBN−waterinterfacegivingatotheappearanceofthe2D-H-bond-networkevenatmaximumcenteredat∼160cm−1,asintheexperiments.Themacroscopicallyhydrophilicinterfaces,asshownhere.∼160cm−1bandishencethedirectTHz-markerofthe2D-WefurtherinferthatthehighconnectivityandfastHB-HB-network.dynamicswithinthe2D-HB-networkrevealedherecouldbeFinally,itisworthnotingthatwaterinBNnanoplateletusedtorationalizetheultrafastvibrationalrelaxationprocessesdispersionsshowsanarrowerlinewidthwithrespecttothatof37,62observedatinterfaceswherea2D-HB-networkisexpected.bulkwaterat293K(seetheSupportingInformation).Such+−Inaddition,thelateraldiffusivityofH3O/OHionsshouldbenarrowingisslightlyincreasingupondecreasingthewaterenhancedwithinthe2D-HB-networkinterconnectedplane.contentinthedispersion.AnydecreaseinlinewidthhasbeenWearguethatsuchenhanceddiffusivitycontributestothehighascribabletoareducednumberofdegreesoffreedom,i.e.,anconductivityofinterfacessuchasgraphene−water,BN−water,entropicsignatureofamorerestrictedsetofmolecularandair−water,asdiscussedinrefs60and61aswellastotheconfigurationsthatareavailabletowatermoleculesinconfinedenvironments.66,68Here,wesuggestthatthisresultsfromtheacidity/basicityof,forexample,silica−waterandair−waterinteractionofwaterwiththeBNnanoplateletsurface.interfaces,asdiscussedinrefs48,59,and69.Interestingly,thesamereducedvaluesforthelinewidthsAsafinalnote,thecomplementaritybetweenTHzandSFGwereobservedforwatermoleculesinthehydrationshellsofspectroscopiesisprovedtobeacompellingtooltoshedlightalcohols,wherethe2D-HB-networkwasalsofound.67onthehorizontal/verticalHBbalanceandrevealmolecularWhenDFT-MDsimulationsarecombinedwiththeoreticalhydrophobicity,providingdirectmarkerbandsforboththeSFGandexperimental/theoreticalTHz-IRspectroscopies,theformationofa2D-HB-network(THz)andthestrengthofmolecular-levelunderstandingofthewaterarrangementatwater−surfaceinteractions(SFG).Thistoolopensperspec-aqueousinterfacesanditsspectroscopicmarkerscanbetivesintherationalizationandoptimizationoflocalhydro-obtained.Wehavedemonstratedthat,forwaterattheinterfacephobiceffectsandthereforeinthedesignofaqueouswithweaklyinteractingsurfaces,theair−water-like2D-HB-interfaces.3832https://doi.org/10.1021/acs.jpclett.1c00257J.Phys.Chem.Lett.2021,12,3827−3836

6TheJournalofPhysicalChemistryLetterspubs.acs.org/JPCLLetter■METHODStemperature-controlledliquidtransmissioncellfromHarrickwithz-cutquartzwindowsanda25μmthickKaptonspacer,Forallsystems,Born−OppenheimerDFT-MDsimulations−170,71and64scanswitharesolutionof2cmwereaveragedforhavebeencarriedoutusingtheCP2Kpackage.The72,7374eachspectrum,whichwassmoothedwitha3point-wideBLYPfunctionalplusGrimmeD2correctionforvandermovingaverage.TheapparentcenterfrequenciesoftheWaalsinteractionsareadopted,withacombinationofdampedresonanceshavebeencorrectedforthedampingGaussian(DZVP-MOLOPT-SR)plusplanewavesbasissets75factorsofthemodestoobtainthecorrespondingundamped(400Ry)andGTHpseudopotentials.Thenucleidisplace-peakfrequencies,whicharereportedinthetext.TheerroronmentshavebeenpredictedthroughclassicalNewton’sthefitparametersislessthan5%.equationsofmotionsintegratedthroughthevelocity-VerletBNnanoplateletaqueousdispersionswerepurchasedfromalgorithm,withatime-stepof0.4fs.Three-dimensionalSigma-Aldrich(ataconcentrationof20mg/mL),andalltheperiodicboundaryconditionshavebeenapplied.otherconcentrationsinvestigatedherewereobtainedbyAllthesimulationshavebeenperformedfor50psintheevaporationofthedispersiondirectlyinthemeasurementNVEensemble,afterequilibrationdynamicsintheNVEcell.Thesizedistributionofthethinnanoplateletsindilutedensemble,withpossiblerescalingofvelocities.AllthedispersionswasmeasuredbydynamiclightscatteringsimulationdetailsarereportedinTable1.Notethatforthe(DynaProNanostar,WyattTechnology),resultingina90%silica/waterinterface,themodelforthesilicasurfaceistakensizefractionwitha140nmequivalentradiusanda10%sizefromref76.fractionwitha1500nmequivalentradiusforthenanoplatelets.ThethicknessofthethinBNnanoplateletsisabout2.4nm,asTable1.ComputationalDetailsoftheSimulationsmeasuredinref81.Forfurtherdetails,seetheSupportingPerformedInformation.simulationN(HO)N(solid)size(Å3)mol2atoms28air/water25619.76×19.76×35.0■ASSOCIATEDCONTENTgraphene/water25618021.49×22.22×35.0*sıSupportingInformationboronnitride/water25618021.85×22.71×35.0TheSupportingInformationisavailablefreeofchargeat31silica/water11619812.67×13.27×37.0https://pubs.acs.org/doi/10.1021/acs.jpclett.1c00257.alumina/water16840814.20×16.40×35.0DetailsregardingtheTHzexperiments(PDF)Forallstructuralanalyses,theH-bonddefinitionproposed77byWhiteandco-workershasbeenadopted,withO(−H)···O■≤3.2ÅandtheO−H···Oangleintherangeof[140−220]°.AUTHORINFORMATIONFortheanalysisreportedinFigure2,theHBsformedbyBILCorrespondingAuthorswatermoleculesareclassifiedas“horizontal”iftheyareSimonePezzotti−UniversitéParis-Saclay,UnivEvry,91025orientedparalleltotheinstantaneouswatersurfacewithinEvry-Courcouronnes,France;orcid.org/0000-0003-2023-±30°fluctuationandas“vertical”otherwise.3648;Email:simone.pezzotti@rub.deTheSFGsignal,comingfromtheimaginarycomponentofMartinaHavenith−DepartmentofPhysicalChemistryII,thetotalresonantelectricdipolenonlinearsusceptibilityRuhrUniversityBochum,D-44801Bochum,Germany;χ(2)(ω),hasbeencalculatedfollowingthetime-dependentorcid.org/0000-0001-8475-5037;methodintroducedbyMoritaetal.78,79Themodelfromref64Email:martina.havenith@rub.deisused,largelyvalidatedonavarietyofwater−vaporandMarie-PierreGaigeot−UniversitéParis-Saclay,UnivEvry,water−solidinterfaces.28,31,45,47,48TheTHzsignaliscalculated91025Evry-Courcouronnes,France;orcid.org/0000-usingtheatomicpolartensors(APT)-basedmethod,as0002-3409-5824;Email:mgaigeot@univ-evry.frdeveloped,applied,andvalidatedinrefs67and80.TheTHzAuthorscalculatedspectrumincludesself-correlationtermsandallAlessandraServa−UniversitéParis-Saclay,UnivEvry,91025cross-correlationtermsbetweenthewatermolecules(BIL-BILEvry-Courcouronnes,France;orcid.org/0000-0002-7525-andBIL-Bulk)(seeref67).The2D-HB-networkmarker-band−12494at∼160cmisduetothesumoftheself-correlationandFedericoSebastiani−DepartmentofPhysicalChemistryII,cross-correlationtermsbetweenthewatermoleculesthatRuhrUniversityBochum,D-44801Bochum,Germanybelongtothe2D-HB-network.Inref67wehaveshownthatFlavioSiroBrigiano−UniversitéParis-Saclay,UnivEvry,theremainingcross-correlationtermsbetweenthe2D-HB-91025Evry-Courcouronnes,Francenetworkandtheadjacentliquidbulkprovideadistinctband−1DariaRuthGalimberti−UniversitéParis-Saclay,UnivEvry,centeredat∼190cm,whichhasamorebulk-likecharacter91025Evry-Courcouronnes,France;orcid.org/0000-andthereforecannotbeusedasamarkerfortheformationof0003-2766-3325the2D-HB-network.ThetheoreticalassignmentofsuchaLouisPotier−UniversitéParis-Saclay,UnivEvry,91025bandisthusnotdiscussedinthepresentLetter,whiledetailsEvry-Courcouronnes,Francecanbefoundinref67.SerenaAlfarano−DepartmentofPhysicalChemistryII,RuhrTHzabsorptionspectraofBNnanoplateletaqueousUniversityBochum,D-44801Bochum,Germanydispersionswererecordedinthefrequencyrangefrom50to−1GerhardSchwaab−DepartmentofPhysicalChemistryII,240cmbyFTIRabsorptionspectroscopyatroomRuhrUniversityBochum,D-44801Bochum,Germany;temperature.THz-FTIRmeasurementswereperformedusingorcid.org/0000-0003-2136-907XaBrukerVertex80vspectrometerequippedwithamercuryarclampassourceandaliquidhelium-cooledbolometerfromCompletecontactinformationisavailableat:InfraredLaboratoriesasdetector.Thesampleswereplacedinahttps://pubs.acs.org/10.1021/acs.jpclett.1c002573833https://doi.org/10.1021/acs.jpclett.1c00257J.Phys.Chem.Lett.2021,12,3827−3836

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