Evidence of Water Adsorption in Hydrophobic Nanospaces of Highly Pure Double-Walled Carbon Nanotubes

摘要

The adsorption of water vapor by carbon nanotubes and othernnarrow channels is receiving increasing attention in physical,nchemical, and biological sciences owing to its promise for practicalnapplications such as separation and purification, selective chemicalnsensing, and the efficient design and operation of micro- andnnanofluidic devices. As explained by Sansom and Biggin, one wouldnnot intuitively expect water to enter a narrow hydrophobic carbonnnanotube, but computer simulations surprise us to show that it can.1nThis adsorption phenomenon can also provide clues about thenbehavior of biological pores.1,2 Single-walled carbon nanotubesn(SWCNTs) as well as multiwalled carbon nanotubes (MWCNTs)nwith rigid, chemically simple, and well-defined nanopores providena model system for investigation into the behavior of nanoconfinedninterfacial water, and thus water adsorption by SWCNTs andnMWCNTs has been extensively investigated by means of molecularndynamics (MD) simulations.2,3 Recently, nuclear magnetic resonancenmeasurements have been used to construct water adsorptionnisotherms in SWCNTs.4 Hummer et al. have found that, withinnsubnanometer-diameter channels of SWCNTs, water undergoes anone-dimensional ordering, which leads to a concerted, pulse-likenmovement of water molecules within the nanotube channels duento the tightness of the hydrogen bonding network among the waternmolecules.3 Wang et al. have reported that the hydrophobicity ofnnanoconfined SWCNT surfaces should not be considered annabsolute property without considering the structure of interfacialnwater.4 Their study corroborates the proposal that carbon nanoconfinementnfavors the formation of clusters of water moleculesnand that as these clusters grow the SWCNT pores becomeneffectively hydrophilic.5 These intriguing progresses have givennrise to an intensive interest in water adsorption in nanoscale poresnof carbon nanotubes (CNTs). We need to obtain new knowledgenof water adsorption on different types of CNTs.5,6