Transport Diffusion of Linear Alkanes (C₅–C₁₆) through Thin Films of ZIF-8 as Assessed by Quartz Crystal Microgravimetry

Abstract

We report uptake capacities and transport diffusivities, D, for each of eight linear alkanes (ranging from C 5 to C 16 ) in quartz crystal-supported films of solvent-evacuated ZIF-8. Analyses of the alkane uptake profiles revealed that the transport dynamics are governed by guest diffusion through metal−organic framework (MOF) (ZIF-8) crystallites rather than by rates of entry into films at the MOF/vapor interface. The obtained diffusivities range from just over 10 −18 m 2 /s to just under 10 −14 m 2 /s. Notably, minimum cross-sectional widths for all guests exceed the crystallographically measured width of ZIF-8's largest apertures and imply consistently with previous experimental and computational studies that apertures expand to accommodate guest uptake. On average, each additional carbon decreases the transport diffusivity of an alkane by twofold. Closer examination, however, reveals an odd−even effect such that linear alkanes having even numbers of carbons diffuse more rapidly than alkanes featuring one more or one less carbon atom. Thus, ZIF-8's differentiation of transport diffusivities for pairs of alkanes differing in length by only one carbon atom can be significantly greater than the aforementioned factor of 2. Elucidation of the microscopic basis for the odd−even behavior, however, awaits the outcome of molecular dynamics calculations that are beyond the scope of the present study. For compact, solvothermally prepared films, guest transport is dominated by 1D diffusion from the film/vapor interface and toward the underlying quartz crystal. For much lower density, electrophoretically deposited (EPD) films, crystallites behave nearly independently, and guest transport can be adequately modeled by assuming rapid permeation of macroscopic voids between crystallites, followed by entry and rate-limiting radial diffusion into isolated crystallites. One consequence is that EPD films can be much more rapidly infiltrated by molecular guests than can compact, solvothermally grown films. The combined results have potentially favorable implications for the development of kinetic separation schemes for closely related analytes.

Accessing accelerated molecular diffusion by nanopore alignment in a MOF thin film

“…39,40 This flexibility helps to expand the effective pore window (hexagonal) size, and thus, even larger molecules like C 12 -alkane can diffuse inside the ZIF-8 cage. 41 Hence, prediction of the selectivity, based on adsorbate size is difficult. Moreover, the molecular diffusivity does not follow a linear relationship with the adsorbate size, but depends on the adsorbate-adsorbent interaction.…”

“…We assume here that the diffusion was of 1D nature and use the following equation as explained by Wöll and coworkers previously (eqn ( 2)). 41,47…”

“…We assume here that the diffusion was of 1D nature and use the following equation as explained by Wöll and coworkers previously (eqn (2)). 41,47 where M t ( t ) is the uptake (μg) at time t , M ∞ is the uptake (μg) at saturation, D is diffusivity (m 2 s −1 ), and L is the film thickness. Note, that with the assumptions of i) 1D diffusion and ii) pinhole-free, homogenous ZIF-8 thin film, the estimated D values are not absolute and can be used for comparison.…”

Vapor-Phase Adsorption of Xylene Isomers and Ethylbenzene in MOF-74 Thin Films

“…A quartz crystal microbalance (QCM) is a high-sensitivity mass sensing device that can measure small changes in mass within the thin film including loading/unloading of guest species. , Such gravimetric sensing ability enables quantitative, nanogram-scale assessment of molecular adsorption/desorption processes in real time. Indeed, MOF thin films have been demonstrated to be compatible with QCM devices for conducting gas and vapor sorption studies. − …”

Construction of a Self-Assembled Polyelectrolyte/Graphene Oxide Multilayer Film and Its Interaction with Metal Ions

“…QCM is a technology that can be employed to monitor surface trace mass changes in the range of nanograms per cm 2 . It is widely used to study LbL self-assembly, biomolecular interactions, polymer chain conformations, , as well as adsorption, − degradation, antifouling, − and sensing , processes. The surface of the typically gold-coated QCM sensor can be modified easily and used as a substrate to construct a multilayer film using LbL self-assembly technology.…”