Gas adsorption, diffusion, and exchange in one-dimensional nanotube systems by hyperpolarized xenon-129 NMR

One-dimensional (1D) nanotube materials hold great promise for a vast range of practical applications. Understanding the fundamental adsorption and transport properties of guest molecules in 1D nanotubes is essential to optimize their performance in such applications. At thermal equilibrium, the symmetric simple exclusion model for 1D channels too narrow for particles to pass one-another predicts the emergence of anomalous diffusion properties. Depending on the density and time-scale, single-file diffusion (SFD) may be observed, where the mean-squared displacement increases as t 1/2 rather than t, as in normal Fickian diffusion (ND). While there are numerous theoretical works on SFD, it is noted that SFD has not been investigated experimentally in detailed.;We have employed continuous-flow hyperpolarized 129Xe NMR to systematically investigate molecular adsorption, diffusion and exchange of Xe in two types of 1D nanotube systems: the self-assembled L-alanyl- L-valine (AV) dipeptide nanotubes and gallium based wheel-shaped nanotubes (Ga10 and Ga18), which have the internal channel diameter of 5.13 A, 8.1 A and 10.4 A, respectively. The Xe spectral line-shape in AV exhibits an axially symmetric chemical shielding anisotropy, whereas Xe adsorbed peaks in Ga10 and Ga 18 nanotubes demonstrate the isotropic NMR line-shapes, implying that Xe in the gallic-wheel nanotubes is less restricted than Xe in AV nanotubes. Xe occupancy at variable temperature and pressure can be determined from Xe chemical shift. The isosteric enthalpy of Xe adsorption in AV becomes increasing exothermic with increasing Xe occupancy. Moreover, Xe chemical shifts in Ga 10 and Ga18 nanotubes are almost independent of Xe occupancy over a wide range of Xe pressures at room temperature, indicating Xe-wall interaction is dominated in gallic-wheel nanotube system.;The selective saturation-recovery pulse sequence has been utilized to explore the Xe diffusion in AV and gallic channels. The kinetic model assuming diffusion-limited Langmuir adsorption and the distribution of desorption rate has been proposed. The data clearly showed that the mean-squared displacements of Xe in AV and Ga10 nanotubes are proportional to the square root of time, ⟨z2⟩ (t) ∝ t1/2, as in SFD. However, the mean-squared displacement of Xe in Ga18 nanotubes was observed to be proportional to the diffusion time, ⟨ z2⟩ (t) ∝ t, revealing that the SFD and ND time-scaling of Xe in 1D nanotube systems with different internal diameters can be evidently distinguished by the saturation-recovery hyperpolarized 129Xe NMR.;Xe exchange in the vicinity of the nanotube openings has been investigated by hyperpolarized 129Xe 2D exchange NMR (2D-EXSY) in AV and Ga10 nanotubes. Kinetic analysis of cross and diagonal-peak signals as a function of exchange time yielded the mean desorption rate, which was observed to decrease with increased Xe occupancy in AV channels. Furthermore, our kinetic model indicates that cross-peak amplitudes in the 2D spectrum are strongly attenuated under flow conditions. By incorporating a brief interrupting of the gas flow during the exchange period, we demonstrated that the cross-peak signals can be dramatically enhanced, thereby providing a way to probe slow exchange and diffusion processes in 1D nanotube systems. The results are relevant to potential applications of nanotubes, including gas storage, gas separations, catalysis, drug-delivery and nanofluidics.