Ion sieving plays a critical role in diverse fields, including water desalination, environmental pollution remediation, valuable resource recovery, and the development of next-generation ion batteries. Over the past few decades, advanced membrane materials have been introduced, and ion transport mechanisms have been extensively studied. However, most research has focused on aqueous systems, leaving the behavior of dissociated ions in organic-aqueous mixtures largely unexplored. This gap presents opportunities for improvements in industries such as food and beverage processing, resource recovery, and pharmaceutical manufacturing. Therefore, this research aims to develop high-performance selective ion exchange membranes using conjugated microporous polymers (CMPs) and to investigate ion transport phenomena in complex solvent systems.

While advancements in membrane material development have expanded the potential for ion separation, controlling specific ion transport remains a significant challenge. One key aspect of this challenge lies in understanding and manipulating the hydration structure of metal ions, which plays a pivotal role in ion sieving. This study focuses on controlling ion separation performance by tailoring the hydrated diameter of metal ions. By adjusting solvent compositions, the effective size of metal ions can be regulated. Using this strategy, we engineered a conjugated microporous polymer (CMP) membrane as the analytical platform. Ion separation tests demonstrated that performance can be finely tuned by controlling the ethanol (EtOH) concentration. Notably, the CMP membrane exhibits remarkably enhanced monovalent-ion permeation rates in EtOH/water mixtures, outperforming many conventional ion-exchange membranes. A mathematical model, supported by empirical equations and metal ion diffusion tests, validated the relationship between hydrated metal ion sizes and the pore size of the CMP membrane. These findings highlight a novel approach for tailoring permselectivity in metal ion sieving and underscore the potential of CMP membranes in complex solvent systems.