NaCl interaction with interfacially polymerized polyamide films of reverse osmosis membranes:: A solid-state 23Na NMR study

Na-23 nuclear magnetic resonance (NMR) spectroscopy of NaCl-exchanged polyamide (PA) films comparable to those of the active skin layer of many reverse osmosis (RO) membranes provides novel insight into the structural environments and dynamical behavior of Na+ in such films. Unsupported PA films were synthesized via interfacial polymerization of trimesoyl chloride in hexane and m-phenylenediamine in aqueous solution, and SEM, FT-IR, and C-13 NMR data demonstrate successful thin film polymerization. Compositional data confirm this conclusion and demonstrate equal Na and Cl incorporation during NaCl exchange from aqueous solution. The Na-23 NMR spectra for freshly made polymer samples exchanged in 1 M NaCl solution show significant relative humidity (RH) dependence. At near 0% RH, there are resonances for crystalline NaCl and rigidly held Na+ in the PA. With increasing RH, a resonance for solution-like dynamically averaged Na+ appears and above 51% RH is the only signal observed. The slightly negative chemical shift of this resonance suggests a dominantly hydrous environment with some atomic-scale coordination by atoms of the polymer. The greatly reduced Na-23 T-1 relaxation rates for this resonance relative to bulk solution and crystalline NaCl confirm close association with the polymer. Variable temperature Na-23 NMR spectra for a sample equilibrated at 97% RH obtained from -80 to 20 degrees C show the presence of rigidly held Na+ in a hydrated environment at low temperatures and replacement of this resonance by the dynamically averaged signal at temperatures above about -20 degrees C. The results provide support for the solution-diffusion model for RO membranes transport and demonstrate the capabilities of multi-nuclear NMR methods to investigate molecular-scale structure and dynamics of the interactions between dissolved species and RO membranes. (c) 2006 Elsevier B.V. All rights reserved.