Molecular geometry effect on gas transport through nanochannels: Beyond Knudsen theory

JH Qian and H Wu and FC Wang, APPLIED SURFACE SCIENCE, 611, 156613 (2023).

DOI: 10.1016/j.apsusc.2022.155613

Gas transport through nanochannels has gained increasing attention in membrane-based gas separations. Knudsen theory is the cornerstone to quantify the gas flux in the regime of free molecular flow, and gas molecules are usually treated as mass points and distinguished solely by molecular weight. However, recent experiments have shown that gases with similar molecular weight exhibit a remarkable difference in the flow rate, which cannot be interpreted by Knudsen theory. In this study, we ascertain the molecular geometry effect on the transport of various gases through nanochannels. Gas molecules with a complex geometry are more likely to experience multiple reflections on the surface, leading to the diffuse scattering and a reduced flux. During the collision, only the normal translational kinetic energy acts as a positive contribution to the successful reflection, while the vibrational, rotational and tangential translational kinetic energies are all ineffective in this process. The ratio of this ineffective energy to the initial kinetic energy is suggested as the criterion whether the gas can disengage from the wall after each collision. The proposed molecular geometry effect offers a new perspective to extend Knudsen theory for broader applications. Our findings also deepen the understanding of gas separation by nanoporous materials.

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