Molecular Dynamics Investigation of Ion Sorption and Permeation in Desalination Membranes

With the purpose of gaining insights into the mechanisms of ion uptake and permeation in desalination membranes, MD investigation of a model polyamide membrane was carried out. A relatively large membrane (45K atoms) was assembled, which closely matched real desalination membrane in terms of chemistry and water permeability. Simulations demonstrate that the mechanism of ion uptake distinctly differs from mean-field approaches assuming a smeared excluding Donnan potential. Ion sorption on charged sites in the membrane phase appears to be highly localized, due to electrostatic forces dominating over translational entropy. Moreover, sorption on partial atomic charges becomes possible as well, which greatly enhances salt (co-ion) uptake and weakens the effect of fixed charges on salt exclusion. This could explain high ion uptake measured in polyamide membranes for both co- and counterions and variations of ion sorption and permeation at low salt concentrations. On the other hand, present simulations greatly overestimate ion permeability, which could be explained by a more open structure than in real membranes, in which dense polyamide fragments may efficiently block ion permeation. Unfortunately, MD cannot analyze ion uptake and permeation in dense fragments containing too few ions, which calls for new approaches to studying barrier properties of polyamide.