Accessibility and ion exchange stoichiometry of ionized carboxylic groups in the active layer of FT30 reverse osmosis membrane

We have experimentally determined the concentration of Ba²⁺ that associates with the accessible ionized R-COO^- groups in the polyamide active layer of the FT30 reverse osmosis membrane in the pH range 3.42-10.30. Ba²⁺ concentrations in the active layer ([Ba ²⁺]) were measured using the ion-probing/Rutherford backscattering spectrometry procedure reported in our previous work. We found that at all but the lowest experimental pH 3.42, [Ba²⁺] was lower than the corresponding total concentrations of R-COO^- groups; their difference was consistent with steric and charge effects determining the accessibility and association, respectively, of Ba²⁺ to R-COO^- groups. Accordingly, we propose two descriptors, the accessibility ratio (AR) and the neutralization number (NN), to account for the observed difference. AR, the fraction of R-COO^- groups accessible to Ba²⁺ ions, and NN, the average number of RsCOO^- groups neutralized per Ba²⁺ ion, were determined experimentally performing Ag⁺-Ba²⁺ ion-exchange tests. The resulting AR = 0.40 indicated that on average only 40% of ionizable carboxylic groups were accessible to Ba²⁺. [Ba ²⁺] values calculated using R-COO^- concentrations and the AR and NN concepts were in agreement with experimental [Ba²⁺] results.