Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, USA.
Langmuir. 2013 Feb 12;29(6):1885-92. doi: 10.1021/la304574e. Epub 2013 Jan 30.
Membranes composed of multilayer poly(4-styrenesulfonate) (PSS)/protonated poly(allylamine) (PAH) films on porous alumina supports exhibit high monovalent/divalent cation selectivities. Remarkably, the diffusion dialysis K(+)/Mg(2+) selectivity is >350. However, in nanofiltration this selectivity is only 16, suggesting some convective ion transport through film imperfections. Under MgCl(2) concentration gradients across either (PSS/PAH)(4)- or (PSS/PAH)(4)PSS-coated alumina, transmembrane potentials indicate Mg(2+) transference numbers approaching 0. The low Mg(2+) transference numbers with both polycation- and polyanion-terminated films likely stem from exclusion of Mg(2+) due to its large size or hydration energy. However, these high anion/cation selectivities decrease as the solution ionic strength increases. In nanofiltration, the high asymmetry of membrane permeabilities to Mg(2+) and Cl(-) creates transmembrane diffusion potentials that lead to negative rejections (the ion concentration in the permeate is larger than in the feed) as low as -200% for trace monovalent cations such as K(+) and Cs(+). Moreover, rejection becomes more negative as the mobility of the trace cation increases. Knowledge of single-ion permeabilities is vital for predicting the performance of polyelectrolyte films in the separation and purification of mixed salts.
多孔氧化铝支撑体上的多层聚 4-苯乙烯磺酸盐（PSS）/质子化聚烯丙胺（PAH）膜组成的膜具有很高的单价/二价阳离子选择性。值得注意的是，扩散透析 K(+) / Mg(2+) 选择性>350。然而，在纳滤中，这种选择性仅为 16，表明膜缺陷存在一些对流离子传输。在 MgCl(2) 在（PSS/PAH)(4)-或（PSS/PAH)(4)PSS 涂层氧化铝两侧的浓度梯度下，跨膜电位表明 Mg(2+) 迁移数接近 0。聚阳离子和聚阴离子端接膜的低 Mg(2+) 迁移数可能源于其大尺寸或水合能导致的 Mg(2+) 排除。然而，这些高阴离子/阳离子选择性随着溶液离子强度的增加而降低。在纳滤中，膜对 Mg(2+) 和 Cl(-) 的渗透率的高度不对称会产生跨膜扩散电位，导致负排斥（透过物中的离子浓度大于进料中的离子浓度），对于痕量单价阳离子（如 K(+) 和 Cs(+)）低至-200%。此外，随着痕量阳离子的迁移率增加，排斥变得更加负。了解单离子渗透率对于预测聚电解质膜在混合盐分离和纯化中的性能至关重要。
