Diameter and helicity effects on static properties of water molecules confined in carbon nanotubes

The behavior of water in carbon nanotubes has recently received increasing attention since some theoretical work has shown that nanotubes have the potential to be used as proton-conducting pores for a variety of biological applications. The properties of a nanotube strongly depend on its diameter and helicity; therefore, their influences on the behavior of water molecules confined in carbon nanotubes have to be fully examined for a better understanding of nanotube's potential biological applications. In this work, molecular dynamics simulations were performed under ambient conditions for armchair and zigzag type nanotube segments of various diameters submerged in water. The results indicate that single-file water chains as the basis of fast proton conduction can be formed only in narrow nanotubes (0.676–0.811 nm diameter). The formation of ice-like water structures inside nanotubes might be sensitive to potential models and corresponding parameters. Obvious variation in average number of H-bonds per molecule can occur only in narrow carbon nanotubes, which is expected to significantly affect the diffusion rate of confined water. The extent of confinement phenomena is dominated by tube diameter. Tube helicity rarely affects the static properties of confined water.