关键词: 亲水性能, PVC纳滤膜, PDA中间层, 界面聚合, 荷电性

Abstract: Negatively charged nanofiltration (NF) membranes, while widely employed in water treatment, face a fundamental performance limitation known as the permeability-selectivity trade-off. To overcome this challenge, this study introduces an interfacial reconstruction strategy utilizing a polydopamine (PDA) interlayer, designed to concurrently enhance the water permeability and solute selectivity of thin-film composite (TFC) NF membranes. A polyvinyl chloride (PVC) substrate was fabricated via phase inversion. First, the casting solution was prepared by the phase inversion method, consisting of 0.5% polyvinylpyrrolidone (PVP), 5% polyethylene glycol 3500 (PEG 3500) and 20% polyvinyl chloride (PVC), with DMAC as the solvent. After casting, the film was immersed in a deionized water coagulation bath to obtain the unmodified PVC base membrane (M0). Subsequently, PDA modification was performed on the surface of the M0 membrane: it was immersed in a 0.2 g/L dopamine solution (pH = 8.8) and shaken for 1.5 hours. After cleaning and heat treatment, the PDA-modified base membrane (M1) was obtained. Finally, with M0 and M1 as the base membranes respectively, two composite nanofiltration membranes—N1 (without a PDA layer) and N2 (with a PDA intermediate layer)—were synthesized via the interfacial polymerization reaction between piperazine (PIP) and trimesoyl chloride (TMC). The surface properties of the membranes were characterized using techniques such as water contact angle measurement, surface roughness analysis, and Zeta potential testing, and their pure water flux and MgCl2 rejection performance were evaluated. The results demonstrate that the PDA interlayer effectively engineered the membrane's surface properties. A significant enhancement in hydrophilicity was observed, with the water contact angle decreasing from 60.02° to 23.78°. The surface roughness increased markedly, as indicated by an 88.3% and 92.7% rise in the average (Ra) and root-mean-square (Rq) roughness, respectively, which contributes to an enlarged effective filtration area. Furthermore, the surface charge property was altered, evidenced by a shift in the isoelectric point from 8.1 to 5.2, confirming a reinforcement of surface electronegativity. These synergistic modifications resulted in superior separation performance. Compared to the control N1 membrane, the N2 membrane with the PDA interlayer achieved a pure water flux of 17.23 L/(m2·h), alongside a 45.32% increase in MgCl2 rejection. This study validates that the PDA interfacial reconstruction strategy successfully mitigates the classic trade-off in NF membranes by synergistically tailoring hydrophilicity, surface charge, and morphology. The pronounced improvement in hydrophilicity is strongly correlated with the induced charge reversal. The mechanism is attributed to the catechol/quinone redox equilibrium inherent to PDA, which facilitates the formation of a surface dipole layer capable of simultaneous proton transfer and electron conduction, leading to negative charge accumulation. This strategy provides a novel and effective pathway for designing advanced high-performance negatively charged NF membranes, with promising potential for applications in high-salinity wastewater treatment, water softening, and selective separation processes.

Key words: hydrophilic performance, PVC nanofiltration membrane, PDA middleware layer, interfacial polymerization, charge performance