How interactions among solutes, solvents, and confining membrane surfaces control separation performance is an intriguing question. Lacking a molecular level understanding of such interactions in complex liquids perpetuates the intractable challenge of simultaneously achieving high membrane permeance and selectivity in separation. Highly crystalline covalent organic frameworks (COFs) have demonstrated ultrahigh permeance, high selectivity, and stability in harsh organic solvents. Using reactive force field molecular dynamics modeling and direct experimental comparisons of a carboxylated COF (C-COF), we demonstrate that unprecedented separation performance can be accomplished by the well-aligned, highly crystalline pores. Significant differences between designed and apparent pore size can also be achieved by changing solvents for a given solute. Combined with differences of solvated solute size in different solvents, selectivity for a given solute can change from 58% to 90% by simply changing the solvent environment. Another environmental parameter, pH, can also result in significant changes in the solvated solute size, which can result in selectivity variation from 23% to 98% for the same solute/solvent pair. Such experimental observations were modeled using reactive force field (ReaxFF) molecular dynamics (MD). The agreement between experimental and ReaxFF results for protic solvent filtration demonstrate that ReaxFF models can be used to predict new COF membrane performance and guide the design of future COF materials for various separation applications.

Molecular interactions among carboxylated covalent organic framework, solute, and solvent dictate membrane separation performance

ACS Spring 2022

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