Boron removal by water molecules inside covalent organic framework (COF) multilayers

Removal of small neutral solutes (SNSs) by reverse osmosis membranes is crucial but challenging. Common consideration of narrowing pore sizes works very little as SNSs have close sizes to the one of water molecules. The most serious drawback of narrowing pore sizes is the reduction of permeance, which dramatically enhances the energy input per volume of permeate water. In this work, an alternative strategy of driving water molecules to reject SNSs is proposed. Using non-equilibrium molecular dynamics simulations, we reveal that inside the nanopores of TpTG multilayers, stable imine-linked covalent organic frameworks, water molecules will preferentially adsorb via the hydrogen bonding interaction. In that way, water molecules in TpTG multilayers other than TpTG themselves can completely reject SNSs, e.g. boric acid. The permeance of TpTG multilayers is excellent as the nanopores are fulfilled with fluidic water. The findings in this work will inspire researchers with an alternative way to design the membranes for SNS removal.