Considering growing efforts to understand and improve the solute-specific selectivity of nanofiltration (NF) membranes, we explored the ion-specific effects that govern the charge and performance of a loose polyamide NF membrane that is commonly used for solute-solute separations. Specifically, we systematically evaluated the zeta potential of the membrane under different conditions of pH, salinity, and ionic composition, and correlated the obtained data with membrane performance tested under similar conditions. Our results identify the pK_a of both carboxylic and amine groups bonded to the membrane surface and suggest that the highly polarizable chloride anions in the solution adsorb to the polyamide, increasing its negative charge. We also show that monovalent cations of different “stickiness” can neutralize the negative membrane charge to different extents due to their varying tendency to sorb to the polymer matrix or screen the fixed carboxyl groups on the membrane surface. Notably, our correlation between zeta potential measurements and permeability experiments indicates the substantial contribution of solution ions to Donnan exclusion in NF membranes.

考虑到人们越来越努力地了解和提高纳滤 (NF) 膜的溶质特异性选择性，我们探索了控制常用于溶质-溶质分离的松散聚酰胺 NF 膜的电荷和性能的离子特异性效应。具体来说，我们系统地评估了膜在不同pH、盐度和离子组成条件下的zeta电位，并将获得的数据与在类似条件下测试的膜性能相关联。我们的结果确定了与膜表面键合的羧基和胺基的 pKa _，并表明溶液中高度极化的氯阴离子吸附到聚酰胺上，增加其负电荷。我们还表明，不同“粘性”的单价阳离子可以不同程度地中和膜负电荷，因为它们吸附到聚合物基质或筛选膜表面上固定羧基的倾向不同。值得注意的是，我们的 zeta 电位测量和渗透性实验之间的相关性表明溶液离子对 NF 膜中的唐南排除有重大贡献。