Water in Narrow Carbon Nanotubes: Roughness Promoted Diffusion Transition

Diffusion of single-file water confined in carbon nanotubes (CNT) has been studied extensively via molecular dynamics simulations in the literature. However, very often, the simulation results do not provide a satisfactory insight to the fundamental mechanism, failing to explain the experimental trend. Sometimes, simulation results are even contradictory to each other. In this work, we first summarize factors that affect the diffusion calculation of single-file water confined in narrow CNTs. In particular, we investigate how the roughness of the CNT model affects diffusion properties of the confined water. A transition of ballistic to Fickian diffusion shall be observable when water is confined in (6, 6) CNTs with roughness. Our simulation results also reveal the single-file diffusion mechanism of confined water when the CNT has significant roughness. This is because the single-file water molecules confined in smooth CNTs move in a collective way, while the motion becomes random in CNTs with roughness. In larger CNTs, even with the roughness change, only the Fickian type diffusion mechanism is observed. Our study provides a new insight into the argument on how confined water diffuses in narrow CNTs.