Decoupling Polymer Rigidity and Penetrant Size Effects on Transport in Hydrated Membrane Systems using Coarse-Grained Simulations

The growing demand for critical minerals like lithium for electric vehicles and renewable energy has created an urgent need for scalable, energy-efficient separation methods. Membrane-based processes are a promising alternative to conventional approaches, with lower energy costs and tunable selectivity. However, their performance depends on molecular-level transport mechanisms that remain poorly understood. While water uptake is known to enhance ion mobility, the role of polymer rigidity and its interplay with the penetrant size has received less attention. In this work, we employ coarse-grained molecular dynamics (CGMD) simulations to explore how polymer rigidity, hydration, and penetrant size govern transport in hydrated membranes. By simulating uncharged penetrants, we eliminate coulombic interactions and isolate size-dependent diffusivity through the membrane. Our results show that polymer rigidity imposes significant free volume constraints, often outweighing hydration effects on mobility. We extend classical free volume models to account for polymer dynamics and water content, achieving strong agreement with our data. These findings provide a framework for designing selective membranes for sustainable energy, water purification, and critical mineral recovery.