Unravelling Water and Salt transport in Polyamide with Nuclear Magnetic Resonance Spectroscopy

Unraveling water and salt transport 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 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 46% while the free hydrogen diffusion coefficient remained constant as the salinity of the mixture increases from 1 M to 2 M. Conversely, the diffusion coefficient of bounded sodium did not change while the unbounded sodium diffusion coefficient decreased by 38% as the salinity of the mixture increases from 1 M to 2 M. Through examining the spin-lattice relaxation time (T1) at various temperatures we reveal that the sodium and hydrogen ion motion decreases with an increase in salinity, and we also report the associated activation energy. We believe these molecular-scale measurements can aid in extending the solution-diffusion model of reverse osmosis membranes.