Ion Selective Membrane Enabled by Specific Ion Interactions in Cationic Polyelectrolyte Coacervates

Coacervates have garnered attention as promising materials due to their spontaneous and energy-efficient phase separation, stimuli responsiveness, and dynamic compartmentalization. Specifically, simple coacervation, a phase separation of a single polymer species, offers a unique platform in which the physicochemical properties of coacervates can be tuned by ion-specific interactions. Recent work exhibited the phase separation behavior of cationic polyelectrolytes depending on the water affinity of counter anions: anions with dissimilar water affinity to polycation remain soluble in water due to long-range Coulombic repulsion between cationic moieties, whereas anions with comparable water affinity promote attractive cation-π interaction, leading to precipitation of polyelectrolytes [1]. In this study, we exploit the ion-specific phase behavior of the coacervates to design an ion-selective membrane in which low-affinity anions permeate easily, while high-affinity anions are effectively excluded. Experimental results reveal that the permeability coefficient ratio of SO₄^2- to PF₆^– is approximately 70, and that of Br^⁻ to Cl^⁻ is around 7. This approach is distinct from conventional ion separation mechanism such as Donnan exclusion, size exclusion, or dielectric exclusion. Our findings will enhance the understanding of the ion-specific effects on coacervation and can hold promise for advancing ion separation technologies.