Revealing Hidden Dynamics of Hydrogel-Based Desalination with ²³Na Nuclear Magnetic Resonance

Today's fast-progressing water scarcity demands new energy-efficient water desalination technologies. Polyelectrolyte hydrogels are promising materials for this application as they can absorb water from saline solutions and reject salts. This process relies on the electrostatic imbalance between the charged groups in the hydrogel and the surrounding solution. However, current state-of-the-art methods for assessing desalination performance are primarily based on ionic conductivity measurements, which lack phase specificity, atomic-level insight, or spatial resolution, limiting research into the underlying microscopic mechanisms. We investigated salt-ion interactions in sodium polyacrylate hydrogels using ²³Na nuclear magnetic resonance (NMR) spectroscopy throughout the swelling process, which provides detailed microscopic information on the different compartments. ²³Na magnetic resonance imaging (MRI) is shown to map Na⁺ ion distributions across different environments, while polarization lifetime measurements are shown to probe ion dynamic regimes. Relaxation-edited imaging revealed the effects of the temperature and competing salts on ion behavior across phases. Additionally, multiple-quantum-filtered experiments selectively detected and characterized slow-tumbling Na⁺ ions, showing the presence of two dynamically distinct sodium pools. These findings highlight the power of integrated ²³Na NMR and MRI techniques for the in situ analysis of ion distribution and dynamics in hydrogels, providing a detailed tool for tuning and optimizing hydrogel desalination strategies.