Quantifying functional groups in the active layer of polyamide nanofiltration membranes via the dye adsorption method

Ionized amine group (R-NH₂) and carboxyl group (R-COOH) within the active layer of polyamide (PA) nanofiltration membranes result in the formation of positive (R-NH ⁺₃ ) and negative (R-COO⁻) functional groups, respectively, which determines membrane performance and is essential for membrane fabrication and modification. Herein, a facile dye adsorption/desorption method using Orange II and Toluidine Blue O dyes was developed to measure the densities of R-NH₂, R-NH ⁺₃ , R-COOH, or R-COO⁻ on surfaces of six PA membranes, and the correlation between the density of such groups and the zeta potential was established. The dye adsorption method was proven reliable due to its lower standard deviation, detection limit, and quantification limit values. Furthermore, the densities of R-NH ⁺₃ or R-COO⁻ under different pH values were measured, fitting well with results calculated from the acid-base equilibrium theory. Additionally, a correlation was established between the net surface density ([R-NH ⁺₃ ] − [R-COO⁻]) and the surface charge density (σ) calculated via the Gouy–Chapman model based on zeta potential results. The resulted correlation (σ/(mC·m⁻²) = (3.67 ± 0.08) × ([R-NH ⁺₃ ] − [R-COO⁻])/(nmol·cm⁻²) + (0.295 ± 0.08)) effectively predicts the σ value of the membrane. This study provides a facile and reliable dye adsorption method for measuring the density of R-NH₂, R-NH ⁺₃ , R-COOH, or R-COO⁻, enabling an in-depth understanding of membrane charge properties.