Quantification of overcompensated cations in layer-by-layer membrane by Orange yellow II

The charge density of the nanofiltration (NF) membrane separation layer is critical for ion separation because the charge-based Donnan exclusion plays a critical role. To quantified the charge density has been an important but difficult task. We recently discovered the excellent performance of poly(styrene sulfonate) (PSS)/ poly (allylamine hydrochloride) (PAH) layer-by-layer (LBL) NF membranes for Mg²⁺/Li⁺ separation. The overcompensated amine groups (R-NH₃⁺) in PAH were assumed to be the main contribution, but lacked a quantification of the charge density for further proof. Here, a simple quantitative experimental method by Orange yellow II staining was reported for the first time to measure the overcompensated amine groups in the separation layer. The sulfonic acid group (R-SO₃^-) in Orange yellow II stoichiometrically adsorbed to the excessive R-NH₃⁺ at pH = 3 by electrostatic interaction, and desorbed at pH = 12. Thermodynamics and kinetics analysis of the sorption process demonstrated a Langmuir monolayer sorption and the sorption was controlled by sorption reaction not diffusion. The pore size distribution and rejection measurement confirmed that Orange yellow II sorption did not disrupt the original PSS/PAH layer. A linear relationship was observed between the charge density and the coating bi-layers or PAH layers. The accurate, easy assessment of charge density of LBL NF membrane provides future possibility for understanding overcompensation during LBL assembly and a bottom-up understanding of the charges to the NF performance in separation.