Sachar Harnoor Singh, Zofchak Everett S, Marioni Nico, Zhang Zidan, Ganesan Venkat
McKetta Department of Chemical Engineering, The University of Texas at Austin, 200 E. Dean Keeton St. Stop C0400, Austin, Texas 78712-1589, United States.
Langmuir. 2024 May 7;40(18):9563-9578. doi: 10.1021/acs.langmuir.4c00286. Epub 2024 Apr 24.
Membranes incorporating zwitterionic chemistries have recently emerged as promising candidates for facilitating challenging ion-ion separations. Transport of ions in such membranes predominantly occurs in hydrated nanopores lined with zwitterionic monomers. To shed light on the physics of ion-ion selectivity underlying such materials, we conducted molecular dynamics simulations of sodium halide transport in model nanopores grafted with sulfobetaine methacrylate molecules. Our results reveal that in both functionalized and unfunctionalized nanopores smaller ions prefer to reside near the pore center, while the larger ions tend to reside near the pore walls. An enhancement in the selective transport of larger anions is observed within the unfunctionalized nanopores relative to that in salt-in-water solutions. Upon functionalization of the nanopores with zwitterions (ZIs), the disparities in the anionic distribution profiles within the pores coupled with differences in the anion-ZI interactions result in a slowdown of larger anions relative to smaller anions. Increasing the ZI grafting density exacerbates these effects, further promoting the selective transport of smaller anions. Our results suggest that selectivity toward large anions can be realized by using nanoporous membranes with ZI content that is high enough to facilitate ion/water partitioning into the pores while preserving the characteristic tendency of the unfunctionalized pores to facilitate faster transport of the larger anions. On the other hand, selectivity toward smaller anions can be achieved by targeting ZI content within the pores that is high enough to significantly slow down the transport of large anions but not high enough to hinder the partitioning of ions/water molecules into the pore due to steric effects.
包含两性离子化学结构的膜最近已成为促进具有挑战性的离子-离子分离的有前途的候选材料。此类膜中离子的传输主要发生在由两性离子单体排列的水合纳米孔中。为了阐明此类材料背后的离子-离子选择性的物理原理，我们对在接枝了甲基丙烯酸磺基甜菜碱分子的模型纳米孔中卤化钠的传输进行了分子动力学模拟。我们的结果表明，在功能化和未功能化的纳米孔中，较小的离子更喜欢驻留在孔中心附近，而较大的离子则倾向于驻留在孔壁附近。相对于在水盐溶液中，在未功能化的纳米孔中观察到较大阴离子的选择性传输有所增强。在用两性离子（ZIs）对纳米孔进行功能化后，孔内阴离子分布轮廓的差异以及阴离子-ZI相互作用的差异导致较大阴离子相对于较小阴离子的传输减慢。增加ZI接枝密度会加剧这些影响，进一步促进较小阴离子的选择性传输。我们的结果表明，通过使用具有足够高ZI含量的纳米多孔膜可以实现对大阴离子的选择性，该含量足以促进离子/水分配到孔中，同时保留未功能化孔促进较大阴离子更快传输的特征趋势。另一方面，通过将孔内的ZI含量设定为足够高，以显著减慢大阴离子的传输，但又不至于高到由于空间位阻效应而阻碍离子/水分子进入孔中，可以实现对较小阴离子的选择性。
