(759a) From Normal to Single-File Diffusion of Gas Molecules in Nanotube Systems by NMR

Studies of sorbate diffusion in systems of one-dimensional nanochannels are important due to significance of such studies for a number of applications including catalysis, separations, and nanofluidics. Confinement to one-dimensional channels can give rise to anomalous transport. One of the most fascinating and best known types of anomalous transport in channels is single-file diffusion, viz. diffusion under conditions when sorbate molecules cannot pass one another in narrow channels. Here we report results of microscopic studies of self-diffusion and tracer exchange of gas molecules in aluminosilicate nanotubes and in self-assembled L-alanyl-L-valine (AV) nanotubes. Spherical gas molecules with similar sizes (tetrafluoromethane and xenon) were chosen as sorbates for these studies. Diffusion studies were performed by C-13 and Xe-129 pulsed field gradient (PFG) NMR method under conditions of application of large (up to 30 T/m) gradients and high (17.6 T) magnetic field. Investigations of tracer exchange between nanotubes and the surrounding gas phase were performed using continuous flow hyperpolarized Xe-129 NMR spectroscopy.  Depending on the relationship between the nanotube diameter and sorbate size, application of the two unique NMR techniques allowed observation of different types of dynamics corresponding to normal and single-file diffusion. The distinct signatures of both types of diffusion in the obtained experimental data will be discussed in detail. The experimental results for tetrafluoromethane diffusion in aluminosilicate nanotubes will be compared with the corresponding results of MD simulations.