Molecular simulation of carbon nanotube membrane for Li+ and Mg2+ separation

Using molecular dynamics simulations, we study the transport of Li+ and Mg2+ through membranes formed from armchair type (8,8) to (11,11) single wall carbon nanotubes (SWNTs) under hydrostatic pressure, and investigate the potential of mean force(PMF), conductance and axial density distributions of ions in the carbon nanotubes, the results show that under 100 MPa, (9,9) and (10,10) SWNT are capable of extracting Li+; when adjusting pressure to 200 MPa, (8,8), (9,9) and (10,10) tubes can separate Li+ and Mg2+ with higher ion fluxes which also augment with increasing tube diameters. By calculating the potential mean force for ion translocation, we show that Mg2+ face greater energy barrier than Li+ when passing the carbon nanotube with same diameter, and free energy difference of ∼8 kJ/mol seem to be the threshold to effectively separate Li+ and Mg2+. Membrane incorporating narrow carbon nanotubes is found to be promising in Li+ and Mg2+ separation.