Effect of hydrophilicity on ion rejection of sub-nanometer pores

Ion rejection by (sub)nanometer-sized pores enables membrane-based desalination. Narrowing the pore size is long considered as the primary choice to increase the ion rejection rate, which is unfortunately at much sacrifice of the water permeance. Using the uniform sub-nanometer pore channels in covalent organic frameworks (COFs) as the model system, we herein demonstrate that hydrophilicity of pore walls plays a significant role in determining ion rejections, and ion rejections can be significantly improved by enhancing hydrophilicity with less loss in water permeance. Via non-equilibrium molecular dynamics simulation, the augmented in-pore effect caused by significant pore-wall-involved hydration effect is discovered to dominate the ion rejections. Furthermore, the desalination mechanism of hydrophilic nanopores is proposed: strong hydrophilicity can make cations enter nanopores more easily than anions by compensation of ionic hydrations, but all ions are not able to transport inside nanopores due to the extreme in-pore effect; hence membranes consequently carry positive net charges after being saturated adsorption by ions and then in turn exclude cations by electrostatic repulsions. These findings and understandings on desalination mechanisms can also be applicable to other kinds of nanoporous materials and thus could provide guidelines for the experimental design of next-generation desalination membranes.