Molecular simulations reveal gas transport mechanisms in polyamide membranes

JH Qian and RY Wang and HA Wu and FC Wang and M Elimelech, JOURNAL OF MEMBRANE SCIENCE, 731, 124056 (2025).

DOI: 10.1016/j.memsci.2025.124056

Membrane-based gas separation is an effective and energy-efficient technology widely applied in industrial processes. Polymeric membranes have been extensively utilized in large-scale gas separation processes due to their affordability, ease of fabrication, and design versatility. However, the underlying molecular-level transport mechanisms of gases within these membranes remain poorly understood, hindering the optimization of gas separation processes and membrane design. In this study, we employed non-equilibrium molecular dynamics simulations to investigate single gas permeation through aromatic polyamide (PA) membranes produced by interfacial polymerization of m-phenylenediamine (MPD) and trimesoyl chloride (TMC), and compared the results with the classic solution-diffusion behavior observed in gas transport through homogeneous liquid (water) films. Our simulation results reveal that gas transport in aromatic MPD-TMC PA membranes exhibits characteristics similar to those of nanoporous materials, including pronounced molecular sieving effects. Notably, smaller gases such as helium and hydrogen demonstrate markedly higher flux compared to larger gases like carbon dioxide and methane, which are unable to traverse the membrane within the simulation timescale. Moreover, gas molecules permeating through the PA membrane exhibit directional trajectories and a nonuniform distribution, in marked contrast with the random walk behavior assumed by the classic solution diffusion model and the observed behavior in water films. These unique phenomena of gas transport through aromatic PA membranes are attributed to the high rigidity of the crosslinked PA film, which results in a stable membrane pore structure that enables defined directional pathways for smaller gases. Our findings underscore the importance of incorporating pore structural characteristics-such as pore size distribution, connectivity, and porosity-into gas transport models to better predict and optimize gas separation with PA membranes.

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