Drastic-yet-distinct alterations in rarefied gas transport of CO2 and propane in nanochannels by finely-tuning surface characteristics

Rarefied gas transports with similar molecular weights (such as CO₂ and propane) render similar Knudsen diffusivity in nanochannels. Nevertheless, by using molecular dynamics (MD) simulations, we find that the Knudsen theory breaks down for rarefied CO₂ and propane transport in β-cristobalite nanochannels with width of 5 nm under ambient conditions (298 K and 1 atm): CO₂ self-diffusivity is only half of that of propane. The drastic differences in their self-diffusivity are due to the penetration of CO₂ into the three-dimensional hexagonal ring structures on β-cristobalite surface, resulting in substantial CO₂ rotations and curved topological accessible plane, which are detrimental to its diffusion. In contrast, propane cannot penetrate into pore surface. On the other hand, by finely-tuning surface properties (the size of surface Oxygen atoms), we observe drastic-yet-distinct alterations in their self-diffusivities: the enhancement in CO₂ self-diffusivities is more than 8-fold of that for propane (290 % v.s. 35 %). This is achieved by prohibiting CO₂ penetration and consequently limiting its rotations, thereby largely promoting its transport. On the other hand, the bending structure of propane, coupled with its larger size, always prevents its penetration into regular or tuned (pseudo) surface. Our study indicates that the collective effects of fluid and surface characteristics are instrumental to rarefied gas transport in nanochannels which are largely overlooked in conventional diffusion models and previous experimental as well as simulation studies. This work offers novel insights into rarefied gas transport mechanisms and the development and optimization of advanced materials for gas capture and separation.