Ionization behavior, stoichiometry of association, and accessibility of functional groups in the active layers of reverse osmosis and nanofiltration membranes

We characterized the fully aromatic polyamide (PA) active layers of six commercial reverse osmosis (RO) and nanofiltration (NF) membranes and found that in contrast to their similar elemental composition, total concentration of functional groups, and degree of polymerization, the ionization behavior and spatial distribution of carboxylic (R-COOH) groups within the active layers can be significantly different. We also studied the steric effects experienced by barium ion (Ba²⁺) in the active layers by determining the fraction of carboxylate (R-COO^-) groups accessible to Ba²⁺; such fraction, referred to as the accessibility ratio (AR), was found to vary within the range AR = 0.40-0.81, and to be generally independent of external solution pH. Additionally, we studied an NF membrane with a sulfonated polyethersulfone (SPES) active layer, and found that the concentration of sulfonate (R-SO ₃^-) groups in the active layer was 1.67 M, independent of external solution pH and approximately three times higher than the maximum concentration (≈0.45 ± 0.25 M) of R-COO^- groups in PA active layers. The R-SO₃^- groups were found to be highly accessible to Ba²⁺ (AR = 0.95 ± 0.01).