Revealing the interfacial and interior effects on transport behaviors of fluids through ultra-thin covalent organic framework nanosheets

Covalent organic frameworks (COFs) have received considerable attention in nanofiltration due to their tunable structures. While reducing the thickness to decrease the transport resistance at a nanometer scale, some factors would significantly reduce the desalination performance of COFs. Herein, the transport behavior as a function of layer number was investigated for TpPa-(OMe)2 COFs with nonequilibrium molecular dynamics simulations. With an increasing in the layer number, the ion rejection is enhanced from 50 % to nearly 80 % and then remains constant despite further increasing in the number of layers. From PMFs, it is discovered that the interfacial part makes a major contribution to the rejection. At this situation, while the thickness of the COF is smaller than the diameter of the ion hydration shell, the Mg2+ only overcome a less energy to strip off the small part of water molecules in hydration shell to enter the pore, leading a smaller ion rejection.