# Specific ion effects on ion transport in charged polymer membranes

## TL;DR

The study explains how specific ions affect transport in charged polymer membranes, using ion softness as a key factor for designing selective membranes.

## Contribution

The study introduces ion softness from HSAB theory as a novel predictor of ion transport in charged polymer membranes.

## Key findings

- Solvent-mediated ion interactions drive specific ion effects in charged polymer membranes.
- Ion softness, or hydration shell malleability, is a key predictor of transport properties.
- HSAB theory explains the mechanism of solvent-mediated ion interactions in these membranes.

## Abstract

Selective separation of like-charged ions is a central challenge in applications such as critical mineral recovery. Electrochemical membrane–based separations offer promising pathways to address this need, but a limited fundamental understanding of ion transport in charged polymer membranes hampers the development of highly selective materials. This study elucidates the role of specific ion effects (SIEs) in ion transport through charged polymer membranes by integrating experimental measurements of ion mobility and in situ ion/ion and ion/water interactions in model charged polymer membranes with results from molecular dynamics simulations on analogous systems. We demonstrate that solvent-mediated ion interactions drive pronounced SIEs within the studied membranes, with the ion softness, i.e., the malleability of ion hydration shells, from hard/soft acid/base (HSAB) theory emerging as a key predictor of transport properties. HSAB theory also explains the observed mechanism of solvent-mediated ion interactions. Our findings offer a mechanistic framework for designing membranes with tailored ion selectivity, potentially enabling efficient separations of chemically similar ions.

Specific ion effects in charged polymers are governed by ion softness, i.e., the malleability of the ion hydration shell.