Single-File Diffusion of Gas Mixtures in Nanochannels of the Dipeptide l-Ala-l-Val: High-Field Diffusion NMR Study

Abstract

Single-file diffusion (SFD) of CO/CH 4 and CO/CO 2 mixtures as well as the corresponding pure gases in channels of polycrystalline L-Ala-L-Val dipeptide was investigated by pulsed field gradient (PFG) NMR. The measured SFD mobilities of the mixtures, where each component was found to exhibit identical diffusion behavior, were compared with the SFD mobilities of the corresponding pure gases at the same or comparable total gas concentrations. Both studied mixtures were observed to diffuse faster than the slowest pure component forming the mixture. This observed behavior is in stark contrast to the trend often reported in the case of normal diffusion in microporous materials where the addition of a faster diffusing component to a slower diffusing component does not change significantly the diffusivity of the slower diffusing component. Molecular clustering in the studied single-file channels is proposed to explain the observed relationship between the mixture and one-component mobilities and to reconcile the experimental SFD data with the predictions of a random walk model reported earlier. For sufficiently large diffusion times, this random walk model predicts a transition from the SFD to the mechanism of center-of-mass diffusion, which is characterized by concerted movements of all molecules in each channel. This transition to center-of-mass diffusion was not observed experimentally for CO/CH 4 and CO/CO 2 mixtures.

Pulsed field gradient NMR diffusion measurement in nanoporous materials

“…Hence, PFG NMR diffusion measurement with, e.g., 13 C and 129 Xe, may sometimes be preferred to 1 H PFG NMR (see, e.g. [49,50], and Baniani et al this issue). These nuclei offer, with the increase of their electron shell, the additional advantage of facilitating high-resolution measurement, with the option to selectively determine the diffusivity of each individual component in mixtures [38,[50][51][52]] to which we shall refer in more detail in Sects.…”

Diffusion in quasi-one-dimensional channels: A small system n, p, T, transition state theory for hopping times

“…Single file diffusion occurs in a variety of natural and engineered systems, including ion and water transport through biological membranes [8][9][10][11], diffusion in molecular sieves such as zeolites [12][13][14], carbon nanotubes [15], * richard.bowles@usask.ca or metal organic frameworks [16,17], and charge-carrier diffusion in one dimensional channels [18]. At the molecular level, pulsed field gradient nuclear magnetic resonance (NMR) techniques have been used to study SFD in adsorbate molecules confined in zeolites [19][20][21] and gas mixtures confined to polycrystalline dipeptide channels [22]. However, the observation of SFD by NMR is subject to some uncertainty in systems where alternative diffusion mechanisms could be responsible [23,24].…”

Diffusion in nanopores: inspecting the grounds

“…The opposite situation is encountered under the conditions of single-file diffusion (Kukla et al 1996;Dutta et al 2016) where molecules contained in channel pores are unable to pass each other. In such a situation, Ɖ ii drops to zero so that, with (15), also the selfdiffusivity vanishes, irrespective of the fact that the transport and corrected diffusivity may assume finite values.…”