Molecular dynamics simulation of ion selectivity process in nanopores

The behaviour of cations in nanopores plays an important role in transport processes in the biological and industrial membranes, porous electrodes, ion channels, and nanomaterials in general. Molecular dynamics simulations have been carried out to study the selectivity mechanism of nanopores by studying the ion permeation events through non-charged carbon nanotubes (10,0) which have an effective diameter of 0.4374 nm. We have investigated pure NaCl, pure KCl and equimolar mixture of NaCl and KCl (50/50) aqueous solutions. Our results show that a membrane of uncharged nanotubes of the appropriate diameter can select K+ against Na+ without the presence of external electric field or surface charges. The key to the ion selectivity is that Na+-H2O clusters are more stable than K+-H2O clusters, and Na+ ions have higher desolvation energy than K+ ions. When both Na+ and K+ are present in the same solution, there is a competition between the two types of cations and the nanotubes select K+ over Na+.We also studied the effects of temperature and pressure to find the conditions, which permit Na+ to permeate through the nanotubes.