Thickness-dependent ion rejection in nanopores

Membrane-based desalination is playing a pivotal role in producing potable water. Tremendous efforts are made to reduce membrane thickness, e.g. by using ultrathin two-dimensional (2D) building blocks as the desalination layers, to enhance water permeance, however, reduction in membrane thickness to a certain threshold may lead to significant loss in ion rejection, which has generally been overlooked. Here, we perform non-equilibrium molecular dynamics simulations on water and ion transporting through carbon nanotube (CNT) membranes with various thicknesses. We reveal that there is an effect of membrane thickness on ion rejection, that is, salt rejection rises and then levels off with rising membrane thickness. Molecular analysis indicates that the dehydration of ions when transporting the pores is the origin of this thickness effect. Increase in membrane thickness results in stronger degree of dehydration and consequently enhanced ion rejections. When the membrane thickness is below a critical value (l_δ), the degree of dehydration and ion rejection keep rising and then maintain unchanged with further increased thickness, and l_δ is therefore, defined as the critical thickness for desalination. Compared to atomically thin membranes, a 2.34-nm-thick membrane can exhibit nearly doubled water permance while maintain 100% NaCl rejection. The effect of thickness-dependent ion rejection suggests us to enhance water permeation at no cost of ion rejection by using membranes with thicker desalination layers but larger pores, which is highly important in the design of next-generation membranes for desalination.