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

Single-file diffusion (SFD) of CO/CH4 and CO/CO2 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/CH4 and CO/CO2 mixtures.