A universal metric for classifying gas transport regimes in nanoconfined media

Abstract

Gas transport in nanoconfined media is fundamental to applications such as gas separation, catalysis, and shale gas extraction. While transport mechanisms in idealized rigid pores or simple fluids are well understood, classifying gas transport in complex soft matter and highly viscous liquids remains challenging. Here, we introduce a quantitative, physically grounded framework for classifying gas transport regimes based on the intrinsic dependence of gas diffusivity on molecular mass. Using molecular dynamics simulations, we systematically examine how gas diffusion coefficients scale with molecular mass across a broad range of nanoconfined media. We define a diffusivity–mass scaling exponent (α) that serves as a mechanistic fingerprint of the transport regime: α values near zero correspond to random diffusion, whereas values approaching −0.5 indicate transport regimes dominated by rigid pore confinement, such as Knudsen, surface, or hopping diffusion. This metric enables the quantitative identification of gas transport mechanisms and captures critical regime transitions of gas nanoflow that have previously been difficult to classify. Further analysis reveals that the molecular mass dependence arises from variations in characteristic step length, governed by molecular momentum and gas–medium interactions. The proposed mass-scaling framework provides a unified and objective criterion for identifying gas transport mechanisms in nanoconfined systems, laying the foundation for a general theory of nanoscale gas transport and enabling more reliable prediction and design of gas-transport materials.