Water behavior in mesoporous materials as studied by NMR relaxometry

Area of Science:

• Materials Science
• Physical Chemistry
• Nuclear Magnetic Resonance (NMR) Spectroscopy

Background:

• Mesoporous materials with hexagonal structures are crucial in various applications.
• Understanding water behavior within these confined spaces is essential for optimizing their performance.
• Nuclear Magnetic Resonance (NMR) relaxometry offers a powerful tool to probe molecular dynamics and interactions.

Purpose of the Study:

• To characterize water dynamics and interactions within two-dimensional hexagonal mesoporous materials.
• To differentiate between various water populations based on their proximity and interaction with the pore surface.
• To elucidate the role of paramagnetic relaxation and water reorientational motions in confined environments.

Main Methods:

• Utilized NMR relaxometry, specifically measuring longitudinal relaxation time (T1) and transverse relaxation time (T2), relaxation in the rotating frame (T1ρ).
• Conducted measurements across a wide frequency range (5 kHz to 400 MHz) using variable-field spectrometers.
• Employed H2O and HOD (heavy water) to distinguish between intermolecular and intramolecular relaxation contributions.

Main Results:

• Identified two distinct water populations within the pores based on low-frequency intramolecular relaxation data: surface-interacting water and a second layer.
• Paramagnetic relaxation at high frequencies also revealed two water types: relatively free water and bulk (interparticular) water.
• The proportions of these water types were quantified and found to be consistent with their assignments.

Conclusions:

• NMR relaxometry effectively distinguishes multiple water populations in mesoporous materials.
• Water molecules exhibit distinct behaviors based on their interaction strength with the pore surface and confinement effects.
• The findings provide insights into the structure-property relationships of water in confined geometries.