Pulsed-field-gradient measurements of time-dependent gas diffusion

Area of Science:

• Physical Chemistry
• Chemical Physics
• Nuclear Magnetic Resonance Spectroscopy

Background:

• Gas diffusion is crucial in various chemical and physical processes.
• Accurate measurement of diffusion coefficients, especially in complex environments, remains a challenge.
• Pulsed-field-gradient NMR (PFG-NMR) is a powerful tool for studying molecular motion.

Purpose of the Study:

• To demonstrate pulsed-field-gradient NMR techniques for measuring time-dependent gas diffusion.
• To measure the diffusion coefficient of xenon in a free gas mixture.
• To perform the first measurement of time-dependent gas diffusion within a porous medium.

Main Methods:

• Application of standard Pulsed Gradient Spin Echo (PGSE) NMR technique and its variants.
• Utilized both thermally polarized and laser-polarized xenon gas.
• Employed modified NMR pulse sequences: Pulsed Gradient Echo (PGE) and Pulsed Gradient Multiple Spin Echo (PGMSE).

Main Results:

• Reproducible measurement of xenon diffusion coefficient (5.71 x 10^-6 m^2 s^-1 for pure xenon at 1 atm), matching previous non-NMR data.
• Successful measurement of time-dependent, restricted gas diffusion in a porous medium (glass beads), aligning with theoretical predictions.
• PGMSE technique demonstrated superiority for efficient measurement of laser-polarized noble gas diffusion across various diffusion times.

Conclusions:

• PFG-NMR techniques are validated for precise gas diffusion measurements.
• Demonstrated the capability of PFG-NMR to study diffusion in restricted geometries.
• PGMSE emerges as a highly effective method for advanced noble gas diffusion studies.