Ab initio molecular dynamics simulations of water and an excess proton in water confined in carbon nanotubes

Abstract

Ab initio molecular dynamics simulations were performed to investigate the effects of nanoscale confinement on the structural and dynamical properties of water and slightly acidic water. Single-walled carbon nanotubes (CNTs) of two different diameters (11.0 and 13.3 Å) were used as confinement vessels, and the inner walls of the CNT were either left bare or fluorinated to explore the influence of the confined environment on the determined properties. The water molecules in the fluorinated nanotubes were found to preferentially localize near the CNT surface and exhibit highly ordered structures while those in the bare CNTs were more randomly distributed. Additionally, weak interactions that resembled hydrogen bonds between the water molecules and the fluorine atoms were observed which occurred at a greater frequency in the smaller diameter CNT indicating an influence of the confinement dimensions on these interactions. This was further pronounced when an excess proton was added where, on average, approximately half of the water molecules in the smaller tube were involved in these interactions. This also led to a structured hydrogen bond network with regular defect sites that hindered proton transfer along the channel axis. Addition of the proton in the larger fluorinated CNT, however, disrupted the structural ordering and proton transfer down the nanotube axis near the surface of the CNT wall readily occurred. Proton transfer through the channel was also observed in the smaller nonfluorinated system, however, the proton was located closer to the center of the CNT, while in the larger nonfluorinated CNT proton transfer exhibited less directional preference indicating an impact of the scale of confinement and nature of the surface on proton transfer.