Interfacial mechanisms governing rarefied gas transport in nanochannels: molecular insights beyond Knudsen theory

Rarefied gas transport in nanoporous media governs key processes in gas capture and separation, yet classical Knudsen theory neglects the molecular nature of gas–surface interactions. Using molecular dynamics simulations, we investigate the diffusion of rarefied CO 2 and propane (C 3 ), respectively, in 5 nm wide nanochannels with atomically resolved body-centered cubic surfaces of varying smoothness and interaction strength. We reveal two distinct transport regimes: (1) on rough surfaces with nearly constant TMAC, diffusivity is governed largely by collision time, while both CO 2 and C 3 exhibit a consistent relationship with gas–surface interaction; (2) on smooth surfaces with both TMAC and collision time affected by gas–surface interaction, C 3 shows stronger sensitivity than CO 2 despite identical molecular weights, due to differences in gas molecular structure and interaction parameters. Challenging conventional models, these results establish a molecular-level framework linking surface morphology, gas properties, gas–surface interactions, tangential momentum accommodation coefficient, and collision time in rarefied gas mobility. This mechanistic understanding provides a foundation for accurate predictions of gas transport in nanoporous materials, with potential applications in energy and environmental technologies.