Ion Adsorption at Polymer Membranes: Surface Polarity Controls Interfacial Structuring in LiCl Solutions

Membrane distillation-crystallization (MD-MCr) has emerged as a promising route for recovering valuable salts and water from brine and wastewater streams; however, its industrial implementation depends critically on membrane robustness, scalability, and material chemistry, which together govern process selectivity and durability. Here, we employ constant chemical potential molecular dynamics (CμMD) simulations to probe the interfacial behavior of LiCl solutions in contact with polyvinylidene fluoride (PVDF) and polypropylene (PP) membranes across a range of concentrations. Concentration profiles reveal pronounced ion structuring near both polymer surfaces. PVDF exhibits gradual ion accumulation with increasing concentration, whereas PP displays distinct adsorption peaks only at higher salinity. Despite the absence of preferential ion adsorption and the resulting near-neutral interfacial charge distribution, adsorption isotherms and free-energy profiles reveal stronger affinity and greater interfacial heterogeneity for PVDF, arising from its polar fluorine groups. The deeper adsorption free-energy minima observed for PVDF indicate stronger ion-polymer interactions, consistent with the experimentally observed tendency of fluoropolymers to undergo wetting and structural modification upon prolonged exposure to lithium salts. These simulations, therefore, capture the earliest molecular-scale processes preceding membrane degradation. By linking polymer polarity, ion adsorption, and interfacial structuring, this work provides a molecular framework for understanding and tailoring ion-polymer interactions in membranes designed for lithium recovery and high-salinity separations.