To probe the ability of ions to enter and leave a nanotube interior, calculations were made of the energetics of a direct diffusion pathway for H+, Li+, Na+, K+ cations to move through the (3,3), (4,4) and (5,5) open-ended single-walled carbon nanotubes with ab initio techniques. For three nanotubes, there was no energy barrier for the Li+ ions to enter the open tubes. In fact, there was a net decrease in the energy of the system, showing that the nanotubes actually act as an attractor for the Li+ ions. Similar results also held for the H+ and Na+ ions into (4,4) nanotube. These ions could easily diffuse and leave the nanotube from other side. H+ strongly attract with the (3,3) nanotube but could not diffuse and was trapped. The results showed that the entry of K+ and Na+ ions into (3,3) nanotube was forbidden. In contrast, (4,4) and (5,5) nanotubes for H+ and K+ ions and (5,5) nanotubes for Na+ act as ion channels that conducted these ions. In all NT-H systems and (3,3) NT-Li system, the charge transferred between ion and nanotube and nanotube achieve the positive charge.

Ab initio Study of Direct Diffusion Pathway for H+, Li+, Na+, K+ Cations into the (3,3), (4,4), and (5,5) Open-Ended Single-Walled Carbon Nanotubes. Monajjemi, M., Faham, R., Mollaamin, F.: Fullerenes Nanotubes and Carbon Nanostructures, 2012, 20[2], 163-9