(704e) Unraveling Water-Ion Dynamics in Reverse Osmosis Membranes with Nuclear Magnetic Resonance Spectroscopy

The development of highly permeable thin-film composite (TFC) polyamide membranes with high salt rejection properties has allowed reverse osmosis (RO) to become the leading technology in desalination. The-state-of-art Reverse Osmosis (RO) membranes are being used industrially for desalinating high salinity (seawater) and lower salinity (brackish water) where unwanted ions are retained by the membrane resulting in the production of drinking water. Most commercial RO membranes possess an aromatic polyamide active layer which is made via interfacial polymerization of m-phenylene diamine (MPD) in the aqueous phase and trimesoyl chloride (TMC) in the organic phase. As the demand has grown for selective separation of single solute from water, more focus is needed on understanding the water-ion dynamics at the membrane interface. The exact nature of ion and water transport in polymeric membranes is often ambiguous and depends on the complex interplay between polymer structure and dynamics that facilitate transport. Experimentally, the Nuclear magnetic resonance (NMR) technique has a great potential to be used as an emerging technology for characterizing the structure and mobility of solutes in polymeric membranes. We have investigated the dynamics-transport of water and sodium ions in complex ionic mixtures of a polyamide polymeric membrane using solid-state NMR spectroscopy. Water and sodium ion diffusion measurements are made using pulsed-field gradient NMR diffusometry. The energy barriers for water and ion transport within the active layer of the RO membrane are assessed using magic angle spinning (MAS) and static NMR spectral line shape analysis. Through monitoring ¹³C, ²³Na, and ¹H NMR nuclei, quantitative insights into the diffusion of water (¹H), sodium mobile ion (Na⁺), and polymer membrane dynamics (¹³C) are extrapolated. Furthermore, the spin-lattice (T₁) relaxation time of sodium and water ions is investigated to obtain quantitative insight into how sodium and water ions rotational movements occur in the absence and presence of other competing seawater cations such as potassium. Room temperature spectra and spin-lattice relaxation times indicated the presence of both mobile and rigidly held ionic species in the polyamide at different levels of relative humidity. These species are attributed to a highly immobile, "bounded" (or aggregated) sodium/hydrogen and a significantly more mobile sodium/hydrogen for which concentration is temperature, ionic, and polymer dependent. NMR relaxation measurements detect how the water rotation is modified by sodium and potassium cations in the polyamide membrane. Results show that at low relative humidity and in the presence of competing ions, there are sodium ion resonances for crystalline sodium ions and rigidly held sodium ions on the surface. With increasing relative humidity and decreasing competing ions, the resonance suggests solution-like dynamic rotational movements of sodium ions in polyamide. The results confirm the change in activation energy barrier for sodium and hydrogen ion permeation in polyamide upon the presence of potassium ions.