Single-File Diffusion of Gas Mixtures By High Field Diffusion NMR

Molecules restricted to diffuse through unidimensional channels so narrow that they cannot pass each other exhibit 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. This property opens a possibility for highly selective separations if one component in the mixture exhibits normal diffusion and another SFD. Inducing SFD in catalytic systems can also be beneficial since single-file conditions offer better control over catalytic reactions. These applications are expected to involve more than one type of sorbate molecules in the single-file channels.  However, currently there are no experimental studies that explore molecular SFD of gas mixtures.

Here, we report the observation of single-file diffusion of pure CO and CH₄ as well as their mixture in Ala-L-Val (AV) nanochannels 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 similar experimental conditions the single-file mobility factor of CH₄ was found to be smaller than that of CO in the nanochannels containing only one type of gas molecules. At the same time, the single-file mobilities were the same for both gases in the nanochannels loaded with the gas mixture. The single-file mobility in the mixture sample was intermediate to those obtained in the single-sorbate samples. These results will be discussed in the context of relevant molecular dynamics simulation studies that explore this phenomenon of SFD of gas mixtures.