Morishita Kazuya, Sachar Harnoor Singh, Duncan Tyler J, Zhang Zidan, Marioni Nico, Herrera Ashleigh, Asatekin Ayse, Ganesan Venkat
McKetta Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States.
Department of Chemical and Biological Engineering, Tufts University, Medford, Massachusetts 02155, United States.
ACS Appl Mater Interfaces. 2024 Oct 23;16(42):57888-57900. doi: 10.1021/acsami.4c13034. Epub 2024 Oct 14.
The separation of ions of similar charge is a crucial challenge in many applications, from water treatment to precious metal recovery. Membranes with cross-linked zwitterionic amphiphilic copolymer (ZAC-X) selective layers, which feature self-assembled, zwitterion-lined nanodomains for permeation, offer unique permselectivity between monovalent anions (e.g., Cl/F). This has motivated studies on the mechanisms of transport and selectivity in this family of materials. In this study, we conducted molecular dynamics simulations of aqueous salt solutions within zwitterion-functionalized nanopores to elucidate the influence of dipole orientation of the ZI ligands on anion diffusivities, partitioning, and permeabilities. Our model compares systems with contrasting ZI organization: surface-cation-anion (S-ZI-ZI, Motif A) and surface-anion-cation (S-ZI-ZI, Motif B). Our results reveal that Motif A exhibits less pronounced ion pairing due to a spatial separation in the radial profiles of cations and anions. Motif B demonstrates prominent ion pairing for smaller anions owing to their overlap with cation distributions. Further, our potential of mean force profiles reveals that anion partitioning increases with anion size in both ligand motifs, whereas Motif B exhibits significantly higher partitioning selectivity toward larger anions compared to Motif A. Our results for ion diffusivities show that the self-diffusivities of both anions and cations are lower for Motif B compared to Motif A. Such trends in anion partitioning and diffusivities can be explained by differences in the interactions and steric hindrance experienced by the anionic species in Motifs A and B. Finally, our results for anion permselectivity, obtained by combining partitioning and diffusivity, indicate that partitioning trends dominate over diffusivity trends. Consequently, anion permeability increases with anion size, and ligand Motif B yields much higher permselectivity toward larger anions compared to ligand Motif A.
在从水处理到贵金属回收的许多应用中，分离电荷相似的离子是一项关键挑战。具有交联两性离子两亲共聚物（ZAC-X）选择性层的膜，其特征在于具有自组装的、两性离子内衬的纳米域用于渗透，在单价阴离子（例如Cl/F）之间提供独特的渗透选择性。这激发了对该材料家族中传输和选择性机制的研究。在本研究中，我们对两性离子功能化纳米孔内的盐水溶液进行了分子动力学模拟，以阐明ZI配体的偶极取向对阴离子扩散率、分配和渗透率的影响。我们的模型比较了具有不同ZI组织的系统：表面阳离子-阴离子（S-ZI-ZI，基序A）和表面阴离子-阳离子（S-ZI-ZI，基序B）。我们的结果表明，由于阳离子和阴离子的径向分布存在空间分离，基序A表现出不太明显的离子配对。由于较小阴离子与阳离子分布重叠，基序B对较小阴离子表现出显著的离子配对。此外，我们的平均力势分布表明，在两种配体基序中，阴离子分配都随阴离子尺寸增加，而与基序A相比，基序B对较大阴离子表现出显著更高的分配选择性。我们的离子扩散率结果表明，与基序A相比，基序B中阴离子和阳离子的自扩散率都较低。阴离子分配和扩散率的这种趋势可以通过基序A和B中阴离子物种所经历的相互作用和空间位阻的差异来解释。最后，我们通过结合分配和扩散率获得的阴离子渗透选择性结果表明，分配趋势比扩散率趋势占主导。因此，阴离子渗透率随阴离子尺寸增加，并且与配体基序A相比，配体基序B对较大阴离子产生更高的渗透选择性。
