Understanding Monovalent Cation Diffusion in Negatively Charged Membranes and the Role of Membrane Water Content

Membranes capable of differentiating between similarly charged ions could enable applications such as resource recovery from naturally occurring waters and industrial wastewaters. Understanding the factors that govern ion transport in these materials is crucial for designing such membranes. This study investigates the impact of membrane water content on the diffusion of monovalent cations in negatively charged membranes by using absolute reaction rate theory. The ion activation energy and entropy of diffusion in the membrane both increase substantially when most of the water is structurally bound. The increase in activation energy of diffusion is predicted by a model incorporating Coulombic interactions between the membrane fixed charges and counter-ions. The activation entropy of diffusion in the low water content membranes increases with increasing size of the hydrated cations, suggesting possible rearrangement in the primary hydration shells of strongly hydrated cations, such as Li⁺ and Na⁺, during diffusion. These results suggest that polymer tortuosity, Coulombic interactions, and water structure govern monovalent cation diffusion in negatively charged membranes.