Unraveling Water and Salt Transport in Polyamide with Nuclear Magnetic Resonance Spectroscopy

Quantifying water and salt transport properties in polyamide is of growing importance as the use of reverse osmosis membranes grows in many industries. Here, using solid-state nuclear magnetic resonance (NMR) spectroscopy, we measure the translational diffusion coefficients using pulsed-field gradient NMR, examine ion dynamics with NMR relaxometry, and determine the activation energy barriers of hydrogen and sodium ions in ion-exchanged polyamide using variable-temperature NMR. We identify two predominant diffusion components within the spectra associated with bound and unbound hydrogen and sodium ions. We show that the diffusion coefficient of the bound hydrogen ions decreases by ∼50% while the unbound hydrogen diffusion coefficient remains constant as the salinity of the mixture doubles. Conversely, the bound sodium diffusion coefficient remained constant while the unbound sodium diffusion coefficient decreased by ∼40% as the salinity of the mixture doubled. Through examining the spin–lattice relaxation time (T1) we also report the associated activation energy for sodium and hydrogen. We believe these molecular-scale measurements can aid in extending physics-based models of salt and water transport in high-pressure reverse osmosis applications.