(582c) The Collective Effects of Fluid and Surface Characteristics on Rarefied Gas Transport in Smooth Crystalline Nanoslits: A Molecular Dynamics Study

The knowledge about gas transport under nano-confinement is of great importance in many engineering applications, such as carbon capture and storage (CCS). In particular, low-pressure gas transport (i.e., the so-called rarefied gas transport) in nanoporous media is crucial to CO₂ capture and separation.

Knudsen number (K_n) which is a ratio of the mean free path of a fluid molecule to a characteristic length of the flowing medium is often used to describe flow types under confinement. At typical CO₂ capture and separation conditions (where K_n>10), fluid flow under nanoconfinement can be regarded as molecular flow, which can be described by the Knudsen diffusion theory, assuming complete diffuse reflections of ideal gas molecules on rough surfaces. According to the Knudsen equation, the gas diffusivity D_k under cylindrical confinement (with the diameter of d) at the temperature of T can be given as

Dk=d/3*SQRT(8*R*T/Pi/M)

in which M is the gas molar mass. For given temperature and nanoconfinement, the Knudsen diffusivities of CO₂ and propane are nearly identical, as their molecular weights are very similar.

Holt et al. (Science 2006, 312 (5776), 1034-1037) reported that gas (including CO₂ and hydrocarbon) fluxes exceed the predictions from the Knudsen theory by more than one order of magnitude in sub-2 nm carbon nanotubes (CNTs). They attribute such enhancement to specular reflection thanks to the smooth surfaces of CNTs, which is also reported by molecular dynamic (MD) simulation studies. In addition, previous studies reported that gas molecules exhibit significant slippage on smooth crystalline surfaces thanks to specular reflection, but much-dampened slippage on rough amorphous surfaces. However, Qian et al. (Carbon 2021, 180, 85-91) reported that even on smooth surfaces, a subtle distinction in surface atomic-level roughness can result in remarkable differences in helium transport. On the other hand, Qian et al. (Applied Surface Science 2023, 611, 155613) observed that specular reflection on the smooth crystalline surface is also dependent on fluid molecular characteristics. They found that gas molecules with more complex geometries and configurations are inclined to have more diffuse reflections. Interestingly, despite nearly identical molecular weight, CO₂ has a higher diffusivity than C₃H₈, as C₃H₈ consumes more initial kinetic energy during gas-surface collisions. Actually, it has been illustrated that fluid molecules have favored orientations and configurations even on smooth crystalline surfaces. Particularly for β-cristobalite, the linear molecular structure of CO₂ may even penetrate into the gaps among surface O atoms, while other gas molecules with complex structures and large sizes (such as C₃H₈) might not. Thus, it is compelling to study the influence of the orientations and configurations of gas molecules near specific crystalline surfaces (such as β-cristobalite) on gas transport. Although the effects of surface and fluid molecular characteristics on gas transport have been investigated respectively before, how specific surface and gas molecular characteristics collectively influence gas diffusion behaviors on confined crystalline slits still remains elusive.

In this study, we use MD simulations to study rarefied CO₂ and C₃H₈ transport behaviors in β-cristobalite slits (with width of 5 nm) under the standard temperature and pressure (298 K and 1 atm). We artificially adjust the parameter σ in Lennard-Jones potential of the surface O atoms, to compare CO₂ and C₃H₈ diffusion behaviors respectively in original β-cristobalite and pseudo β-cristobalite (with larger σ) slits. Overall, both CO₂ and C₃H₈ have an adsorption layer on the slit surface. In the original β-cristobalite slit, CO₂ adsorption is more significant, as the linear molecular structure allows it to align perpendicularly to the surface, even penetrating into the gaps among surface O atoms. Such perpendicular orientation is energetically favored near the hexagonal center of the surface structure. In contrast, the bending structure of C₃H₈, coupled with its larger size, prevents its penetration into surfaces. Consequently, CO₂ topological accessible planes are much more curved than that of C₃H₈, which significantly hinders CO₂ specular reflection. Therefore, the self-diffusivity of CO₂ is lower than that of C₃H₈. In the pseudo β-cristobalite slit, however, CO₂ adsorption is dramatically reduced, because larger σ of surface O atoms blocks the space, where CO₂ can enter and penetrate on the original β-cristobalite surface, while CO₂ self-diffusivity is greatly enhanced. On the other hand, C₃H₈ self-diffusivities are nearly identical in original and pseudo β-cristobalite slits. It suggests that CO₂ transport behaviors are more susceptible to the alterations of surface characteristics.

This study elucidates the importance of collective effects of specific fluid and surface characteristics as well as their specific interactions on rarefied gas transport behaviors in crystalline nanoslits, which provides important insights into optimization of functional materials for gas capture and separation.