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

Abstract

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-{\text{NH}}_{\text{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-{\text{NH}}_{\text{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-{\text{NH}}_{\text{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-{\text{NH}}_{\text{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-{\text{NH}}_{\text{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-{\text{NH}}_{\text{3}}^{+}, R-COOH, or R-COO⁻, enabling an in-depth understanding of membrane charge properties.

Graphical abstract

Keywords

carboxyl group / amine group / dye-adsorption method / charge property

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Authors’ contributions

Li-ping Yue: original draft writing, software, methodology, investigation, formal analysis, and data curation; Fan-xin Kong: original draft writing, supervision, methodology, investigation, and funding acquisition; Jin-fu Chen: methodology and data curation; Ai-guo Zhou: investigation and data curation; Guang-dong Sun: resources and methodology. All authors have read and agreed to the published version of the manuscript.

Declaration of competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors acknowledge the financial support provided by the National Natural Science Foundation of China (Grant Nos. 52270014 and 51708556).

Online appendix

Electronic supplementary material (ESM) can be found in the online version at https://doi.org/10.1007/s11706-024-0706-8 and https://journal.hep.com.cn/foms/EN/10.1007/s11706-024-0706-8 that includes Table S1‒S2 and Figs. S1–S4.

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