Chapter 11. HP Xe-129 NMR Characterization of Single-File Diffusion in Nanotubes

Abstract

In its simplest embodiment, single-file diffusion (SFD) is a manifestation of correlated random displacements in a one-dimensional lattice gas subject to inter-particle exclusion interaction. When mutual exchange of positions in the lattice is prohibited, particles retain their sequential order on the lattice and the random-walk (RW) process is inhibited. SFD has been studied intensively in laboratory channel–particle constructions with macroscopic dimensions. Extension of such studies to molecular single-file systems is motivated by significant potential applications, including catalysis and separations. Defect-free single-file nanochannel/molecule systems with monodisperse cross-sectional dimensions are most likely to yield results that can be interpreted with the help of dynamical models. Equally important is the measurement technique. It should have sufficiently high sensitivity, dynamic range, and time-resolution to track molecular displacements over the widest possible time window with the highest possible time resolution in materials which are typically optically opaque. The many advantages of Xe-129 NMR for characterization of porous media are well-documented in the literature. A key property for single-file transport studies is the dependence of the Xe-129 chemical shielding tensor to pore size, shape and orientation. Additionally, the available hyperpolarization (HP) enhancement of Xe-129 nuclear spins by spin exchange optical pumping with an alkali metal can increase the sensitivity of high field Xe-129 NMR by over four orders of magnitude. These characteristics make Xe-129 well-suited for Xe single-file diffusion studies. HP Xe-129 NMR has been applied to a plethora of novel microporous materials. This chapter focuses on recent advances in the HP Xe-129 NMR methodology diffusion studies in materials consisting of one-dimensional nanochannels. The HP Xe-129 signal enhancements enable diffusion dynamics to be followed in an observation time window that can span more than 3 orders of magnitude. While pulsed field gradient NMR probes displacements predominantly far from channel openings, HP Xe-129 NMR signals in the channel-adsorbed phase result from exchange and diffusion near file openings. The two techniques are thus complementary and when combined yield a more complete picture of diffusion in the system. Representative results obtained in several types of nanochannel materials are presented. Successes and limitations of the HP Xe-129 NMR approach are discussed. Since realistic molecular dynamics simulations currently do not provide access to the time or length-scales of interest in real single-file systems, fundamental studies will continue to rely mainly on high-quality experimental data. The capabilities of HP Xe-129 NMR based techniques, some of which are highlighted in this chapter, show high potential to advance our understanding of molecular transport in real single-file materials.