Pressure-driven Li+/Mg2+ selective permeation through size-sieving nanochannels: The role of the second hydration shell

Non-equilibrium molecular dynamics simulations were carried out to investigate the size-sieving mechanism of the permselective transport of Li+ and Mg2+ through one-dimensional nanochannels modelled by carbon nanotubes. Ion transmembrane permeation was driven by a pressure gradient of 100 MPa for mimicking the commonly used pressure-driven processes. It was found that the ions experienced a dehydration of the second hydration shell for squeezing into the nanochannels, during which the first hydration shell was well retained. The second hydration shell was clearly found to play an important role in the pressure-driven ion transmembrane permeation. Due to the different affinities of Li+ and Mg2+ to the surrounding hydration water, the Li+ and Mg2+ ions exhibited distinct abilities in the dehydration of the second hydration shells, producing excellent Li+/Mg2+ permselectivity of 2.63. The novel understanding of the mechanism of Li+/Mg2+ transmembrane permeation will efficiently guide the development of high-performance membrane for Li+/Mg2+ separation.