Exploring the effect of confinement on water clusters in carbon nanotubes.

Abstract

Using armchair-type single-walled carbon nanotubes (SWCNTs) of different sizes as model compounds for lignite, the effect of water molecule confinement on the water-holding capacity of lignite pores was investigated. Results indicated that the water-holding capacity of pores with diameters of <10nm was eight times larger than that of pores with diameters of 100nm. The configuration of the cluster of water molecules in each SWCNT and the binding energy between each SWCNT and the water molecules within it were calculated by means of density functional theory using a hybrid functional: M06-2X/6-311+G**, 6-31G*. The results prove that the configurations of the water molecules in the SWCNTs are very different to their configuration in the unconfined state. In vacuum, the cluster of three water molecules adopted a trimer configuration, while they presented a linear configuration in the 6.78Å SWCNT. Similarly, in vacuum, the cluster of five water molecules formed a five-membered ring, while they favored a linear configuration in the 6.78Å SWCNT, a zigzag configuration in the 8.14Å SWCNT, and a trimer + 1 + 1 configuration (i.e., a trimer plus two isolated water molecules) in the 9.49Å, 10.85Å, and 13.75Å SWCNTs. There was found to be a degree of competition between the coupling energy of the water molecules with the SWCNT and the hydrogen bonding among the water molecules. When the diameter of the SWCNT was >1nm, the hydrogen bonding among the water molecules dominated, while the coupling energy of the water molecules with the SWCNT amounted to only 30-40% of the total interaction energy of the water molecules. Graphical Abstract Computed equilibrium structures of five water molecules confined in SWCNTs with diameters of 6.78Å, 8.14Å, 9.49Å, 10.85Å, and 13.75Å, and in vacuum.