Narrowing pore size distribution of polyamide nanofiltration membranes via introducing polyamino acid for high precision ion sieving

Nanofiltration (NF) membranes with narrow pore size distribution are of significant importance for realizing high-precision separation of ions-ions in addressing the solutions with complex components or high concentrations, while remaining a challenging task. In this work, we report polyamide (PA) NF membranes with narrow pore size distribution which were synthesized using interfacial polymerization of piperazine (PIP) and trimesoyl chloride (TMC) regulated by poly-γ-glutamic acid (γ-PGA), and the mechanism of γ-PGA regulating pore size distribution was discussed. The γ-PGA tailored the diffusion and uniform horizontal distribution of PIP at the oil-water interface, which was confirmed by the combination of experiments and molecular dynamics simulations. High crosslinking degree and uniform fluctuation morphology were therefore bestowed upon the synthesized PA membranes. The mean pore size was tailored from 0.48 nm to 0.44 nm with geometric standard deviation ( σ p ) reducing from 1.39 to 1.30. As a result, the membrane can achieve high separation factor not only for Li + /Mg 2+ as high as 92.5, but also for Cl − /SO 4 2− of up to 67.40, which is relatively rare among reported PA membranes. When facing the mixed salt solution with a concentration of 10 g L –1 , the separation factor of Li + /Mg 2+ still reached 35.6, demonstrating the unique advantage of utilizing size-sieving effect as the main mechanism for ion-ion separations.