Indexed on: 12 Oct '16Published on: 12 Oct '16Published in: ACS Applied Materials & Interfaces
Nanofiltration membranes (NFMs) are widely used in saline water desalination, waste water treatment and chemical product purification. However, conventional NFMs suffer from broad pore size distribution, which limits their applications for fine separation, especially in complete separation of molecules with slight difference in molecular size. Herein, defect-free composite NFMs with narrow pore size distribution are fabricated using a contra-diffusion method, with dopamine/polyethylenimine solution on the skin side and ammonium persulfate solution on the other side of the ultrafiltration substrate. Persulfate ions can diffuse through the ultrafiltration substrate into the other side and in-situ trigger dopamine to form a co-deposited coating with polyethylenimine. The co-deposition is hindered on those sites completely covered by the polydopamine/polyethylenimine coating, while promoted at the defects or highly permeable regions because it is induced by the diffused persulfate ions. Such a "self-completion" process results in NFMs with highly uniform structures and narrow pore size distribution as determined by their rejection to neutral solutes. These near electrically neutral NFMs show high rejection to divalent ions with low rejection to monovalent ions (MgCl2 rejection = 96%, NaCl rejection = 23%), majorly based on steric hindrance effect. The as-prepared NFMs can be applied in molecular separation such as isolating cellulose hydrogenation products.