Molecular dynamics of zigzag single walled carbon nanotube immersion in water

Abstract

The results of enthalpy of immersion in water for finite single-walled carbon nanotubes arereported. Using molecular dynamics simulation, we discuss the relation between the valueof this enthalpy and tube diameters showing that the obtained plot can be divided into threeregions. The structure of water inside tubes in all three regions is discussed and it is shown thatthe existence of the strong maximum of enthalpy observed for tube diameter ca. 1.17 nmis due to freezing of water under confinement. The calculations of hydrogen bond statistics andwater density profiles inside tubes are additionally reported to confirm the obtained results. Next,we show the results of calculation for the same tubes but containing surface carbonyl oxygengroups at pore entrances. A remarkable rise in the value of enthalpy of immersion in comparisonto the initial tubes is observed. We also discuss the influence of charge distribution betweenoxygen and carbon atom forming surface carbonyls on the structure of confined water.It is concluded for the first time that the presence of surface oxygen atoms at the pore entrancesremarkably influences the structure and stability of ice created inside nanotubes, andsurface carbonyls appear to be chaotropic (i.e. structure breaking) for confined water. This effectis explained by the pore blocking leading to a decrease (compared to initial structure) in thenumber of confined water molecules after introduction of surface oxygen groups at poreentrances.