Molecular Simulation of Water in Carbon Nanotubes

MolecularSimulationofWaterinCarbonNanotubesAlessioAlexiadisandStavrosKassinos*ComputationalScienceLaboratory(UCY-CompSci),DepartmentofMechanicalandManufacturingEngineering,UniversityofCyprus,75KallipoleosSt.,P.O.Box20537,1678Nicosia,CyprusReceivedJune6,2007Contents1.Introduction50142.MolecularDynamics50153.MolecularDynamicsForceFields50164.SomePropertiesofCarbonNanotubes50174.1.MolecularDynamicsofEmptyCarbonNanotubes50185.WaterModels50186.Carbon-WaterInteraction50196.1.ContactAngle50197.WaterConfinedinCarbonNanotubes:GeneralOverview50207.1.WaterStructureinCarbonNanotubes50207.2.Radial,Axial,andTotalWaterDensity50237.3.HydrogenBonds50247.4.FillingCarbonNanotubeswithWater50257.5.DipoleMoment50267.6.ProtonTransportinWater50277.7.TransportProperties50278.DrivenFlow50299.ChargedCarbonNanotubes502910.PolarizableCarbonNanotubes503011.SelectivePartitioning503012.FunctionalizedCarbonNanotubes503113.Conclusions503114.Acknowledgments503115.References50311.IntroductionAccordingtoMonthiouxandKuznetsov,1thefirstmentionofcarbonfilamentswasreportedin18892inapatentproposingtheuseofsuchfilamentsinlightbulbs(interest-inglyenough,thesameideawasproposedagain115yearslater3).Later,theexistenceofthecarbonefilamenteuxwasmentionedintwopaperspresentedattheFrenchAcademyofSciences.4,5Thesepapers,however,didnotprovideevi-denceforthetubularnatureofthefilaments,sincetheresolutionoftheavailablemicroscopeswasnothighenoughtorevealtheinnercavityofthefilament.In1952,RadushkevichandLukyanovich6publishedintheJournalofPhysicalChemistryofRussiathefirsttransmissionelectronmicroscopy(TEM)picturesshowingthehollowstructureofthecarbonfilaments.Westernresearchers,however,hadlimitedaccesstotheRussianpressduringtheColdWarandtheworkofthetwoSovietscientistsremainedbasicallyunknownontheothersideoftheironcurtain.In1991,carbonnanotubes(CNTs)werefinally“rediscovered”bySumioIijima7and,thistime,theyimmediatelyawakenedtheinterestofthescientificcommunity.RecentlyMonthiouxandKuznetsov1broughttolighttheworkofRadushkevichandLukyanovich,butthevastmajorityofacademicandpopularliteraturestillattributesthediscoveryofthenano-tubulesofgraphiticcarbontoIijima.Itmustbesaid,however,that,apartfromtheattributionoftheireffectivediscovery,itwasonlyafter1991thatthestudyofcarbonnanotubesbecameoneofthemostactiveareasinmicroscopicphysics.Furthermore,theexistenceofsingle-wallcarbonnanotubes(SWNT)wasactuallydiscov-eredbyIijima82yearslater(andbyBethune9atthesametime).From1991to2007,accordingtothe“WebofScience”10andScopus11databases,CNTsappearedinmorethan30000paperswrittenbymorethan37000authorsandmorethan20000patents.Figure1givesanideaofhowtheinterestinthisarea,whichsupplantedfullerenesasthehottestresearchtopicofthe20thcentury,1,12isgrowingyearafteryear.Thenumberofpapersandpatentsisincreasingexponentially;morethanafifthofallthepapersandmorethanaquarterofthepatentshavebeenpublishedorregisteredin2007andthistrendseemstocontinuein2008.Toalargeextent,thispopularityisduetotheremarkablestructural,mechanical,andelectromechanicalproperties13,14thatmakeCNTsprom-isingcandidatesforalargevarietyofapplicationssuchasgasstorage,15,16nanoelectronics,17,18moleculardetection,19,20membraneseparation,21,22nanopipets,23nanotweezers,24nanopipesfortheprecisedeliveryofgasesorliquids,25,26anddrugdelivery.27,28CertainexperimentalstudiessuggeststhatSWNTscanbeinternalizedintolivingcellsthroughendocytosiswithoutapparenttoxiceffects29,30(thisassess-ment,however,isunderdebatewithsomerecentresults*Correspondingauthor.E-mail:[e-mail].:[phone].Fax:[phone].Figure1.Numberofarticles/patentspublished/registeredeachyearandhaving“nanotubes”astopic.Chem.Rev.2008,108,5014–5034501410.1021/cr078140fCCC:$71.002008AmericanChemicalSocietyPublishedonWeb11/04/2008indicatingthatcarbonnanotubescanbetoxic31).Asamoleculartransporter,SWNTscanshuttlevariouscargoesacrossthecellularmembrane,thusopeninganewrouteformedicinedeliveryandgivingrisetoanovelmechanismforcancertherapy.32Centraltomanyoftheseapplicationsisthecapacitytostoreorconveyfluids,andinparticularaqueoussolutions,atnanoscaleprecision.Forthisreason,thestudyofCNT/watersystemsisofparamountimportanceforthedevelopmentandevolutionoftherelatedtechnologies.However,evenifCNTsenduphavingnopracticalapplica-tions,asithappened,forinstance,withfullerenes,whosecommercialapplicationsremainlimiteddespiteallinitialexpectations,thestudyofwaterconfinementinCNTswouldstillmaintainahightheoreticalimportancetochemistry,biology,andmaterialsscience.33,34Infact,water-filledandwater-permeableporesarepresentinbiologicalcells,mem-branes,andsurfaceofproteins35andinotherrelevantbiologicalandgeological(e.g.,zeolites36)systemsthatmaypresentastrictanalogywithwaterconfinedincarbonnanotubes.37Forallthesereasons,themathematicalmodelingoftheflowoffluids,andinparticularofwater,throughcarbonnanotubeshasincreasinglyattractedtheinterestofresearchersinthelastyears.Moleculardynamics(MD),whichcanbeasaccurateasexperimentsandmucheasiertoperformatthenanoscale,hasproventobethemostnaturalandflexibletoolforthiskindofanalysis.Thefir