Nanofluidic Behaviors of Water and Ions in Covalent Triazine Framework (CTF) Multilayers

Abstract

Covalent triazine frameworks (CTFs) hosting arrays of highly ordered sub‐2‐nm pores are expected to exhibit unusual nanofluidic behaviors, which may enable important applications such as desalination. Herein, nonequilibrium molecular dynamics simulations are applied to investigate transport of water and ions inside two typical CTFs—CTF‐1, and CTF‐2—having intrinsic pores of 1.2 and 1.5 nm, respectively. Their monolayers exhibit extremely high water permeance but weak ion rejection. CTF multilayers are then investigated. Transport resistances composed of interior and interfacial contribution are correlated with stacking numbers of CTF monolayers to develop equations of predicting water permeance. It is revealed that both the stacking fashion and the number of CTF monolayers forming multilayers significantly influence permeation and ion rejection. Staggered multilayers exhibit much higher ion rejection than eclipsed ones. Staggered CTF‐2 multilayers completely reject ions because the interlayer paths between two adjacent staggered monolayers allow only water molecules to pass through. Importantly, it is predicted from the equations that few‐layered staggered CTF‐2 multilayers, which can be relatively easily produced by experimental methods, exhibit 100% NaCl rejection and up to 100 times higher permeance than commercial reverse osmosis membranes, implying their great potential as building blocks to prepare next‐generation desalination membranes.

Pressure-Dependent Ion Rejection in Nanopores

“…To accurately describe the ΔPdependent ion rejections, the intervals of ΔPs are smaller when the changes of ion rejections are faster, which are 5, 25, and 100 MPa with ΔPs of 10−100, 100−350, and 400−1000 MPa, respectively. Enhanced ion rejections with decreased ΔPs were also reported in many other materials, for example, graphene, 12 graphyne, 16 and COFs, 32 the staking layered structure of which afford one-dimensional straight pores such as CNTs.…”

Single-Layered Nanosheets of Covalent Triazine Frameworks (CTFs) by Mild Oxidation for Molecular-Sieving Membranes

“…Interestingly, the rejection rate of acid fuchsin with a smaller size (1.17 nm × 1.13 nm × 1.13 nm) than 1.4 nm also exceeded 90%. This phenomenon might be attributed to the offset or fully staggered pores originating from the random stack of CTF nanosheets, resulting in a reduced pore size. , The membranes exhibited a moderate rejection rate of methyl orange (56.9%). This is mainly because the methyl orange molecule possesses a much narrower dimension (1.13 nm × 0.42 nm × 0.61 nm) compared with that of acid fuchsin, resulting in easier penetration through the membrane.…”

Engineering Polymeric Nanofluidic Membranes for Efficient Ionic Transport: Biomimetic Design, Material Construction, and Advanced Functionalities

“…114,115 Stacking in an offset eclipsed fashion can further reduce the equivalent pore size of COFs. 116 The pore wall is key since it sets chemical basis for the realization of functions and determines local microenvironment and interface for interactions with ions and molecules. 36 The chemistry of pore walls can be designed by choosing monomers with corresponding functional groups or postsynthetic modification on a pre-established network.…”