System Size-Dependent Transport Properties in Materials of Nanoscale Dimension

Abstract

Fluid transport in finite-sized nanoporous materials is critically affected by apparent interfacial barriers, which severely restrict efficiency improvement on reduction in system size; however, the mechanism leading to this effect even in defect-free materials is unknown. Using large-scale atomistic simulations emulating carbon nanotube (CNT) and zeolite membranes up to a micrometer thick, we demonstrate that transport coefficients in finite nanomaterials are non-uniform and increase with distance from the external surface, attaining the bulk system value far from the surface. Attenuation of transport occurs in a finite entrance region of developing fluid flow, in which the fluid momentum decorrelates from that at the entry and whose extent depends on the texture of the pore surface. This effect disguises as interfacial resistance in nanomaterials and has remarkable effects on membrane selectivity. Thus, finite CNTs can be selective for H 2 over CH 4 , in contrast to the corresponding infinite tube that is selective for CH 4 , due to the smaller apparent interfacial resistance for H 2 . The simulations, covering a wide range of conditions, reveal a quantitative correlation between the apparent relative interfacial resistance and Maxwell reflection coefficient for CNT membranes. These insights will enable optimal design of ultra-thin membranes, sensors, and other applications using nanoscale materials.

Nonuniformity of Transport Coefficients in Ultrathin Nanoscale Membranes and Nanomaterials

“…As a result, the transport coefficient is position-dependent and only matches that of the bulk material beyond a sufficiently large distance, termed the entry length. 23 Hibbe et al 3 alluded to a thin layer of severely decreased diffusivity, consistent with this argument, albeit attributing pore blockage as the underlying cause. While this may be the case for nonideal materials, the existence of the entry region of nonuniform lower diffusivity for ideal materials is suggested by our findings.…”

“…14,15 However, simulations have shown that interfacial resistance is inherent even to ideal or defect-free materials 7,16−22 due to the symmetry breaking at the surface; nevertheless, the precise mechanisms that lead to the resistance have received limited attention. 23 Further, while its origin is unclear, this interfacial resistance is affected by the interphase interaction between the external environment and the confined fluid, as has been recently demonstrated by us for the gas−zeolite system, 7 with the internal interfacial resistance increasing in the presence of a polymer at the gas−solid interface.…”

“…Previous studies in our laboratory have successfully used this method in the quantification of the transport coefficient in finite systems. 7,23,42 Considering a finite system of length L, the collective coordinate n(t) is described as…”

“…For linear dependence of the flux on the chemical potential gradient (Δμ), the total internal transport resistance is given as 23…”

Molecular Dynamics Simulations on the Entrance of Methane and p-Xylene into ZSM-5 Zeolite

“…It is worth noting that the collective behavior of adsorbates on the surfaces, around the pore entrances, and within the pore channels can influence the adsorption, diffusion, and entering processes, − but under low pressure, these effects might be insignificant. On the other hand, revealing the decoupled entering process is also quite valuable.…”