Single-File Diffusion of Gas Mixtures in Dipeptide Nanochannels: High Field Diffusion NMR Study

Molecular diffusion in one dimension inside non-intersecting channels so narrow that they forbid mutual passage of molecules is referred to as single-file diffusion (SFD). An important consequence of SFD is a significant slowdown in the growth of MSD with time in comparison to that for normal Fickian diffusion. The potential use of this property in highly-selective separations and controlled catalysis has motivated studies of SFD in mixtures of different types of molecules using molecular-level simulations.^1-4 Our recent communication reported, for the first time, experimental observation of SFD of a gas mixture.⁵

Here, we report the observation of single-file diffusion of CO/CH₄ and CO/CO₂ mixtures as well as the corresponding pure gases in L-Ala-L-Val channels by C-13 pulsed field gradient (PFG) NMR. High magnetic field gradients up to 23 T/m were used to study diffusion in a broad range of diffusion times. Under the single file conditions, both studied mixtures were observed to diffuse faster than the slowest pure component forming the mixture. This 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 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.

References:

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3. Rao, S. M.; Coppens, M.-O., Predicting Mixture Diffusion in Zeolites: Capturing Strong Correlations with a Simplified Theory and Examining the Role of Adsorption Thermodynamics. J. Phys. Chem. C 2012, 116 (51), 26816-26821.

4. Chen, Q.; Moore, J. D.; Liu, Y.-C.; Roussel, T. J.; Wang, Q.; Wu, T.; Gubbins, K. E., Transition from single-file to Fickian diffusion for binary mixtures in single-walled carbon nanotubes. J. Chem. Phys. 2010, 133 (9), 094501.

5. Dutta, A. R.; Sekar, P.; Dvoyashkin, M.; Bowers, C. R.; Ziegler, K. J.; Vasenkov, S., Relationship between single-file diffusion of mixed and pure gases in dipeptide nanochannels by high field diffusion NMR. Chem. Commun. 2015, 51 (69), 13346-13349.