Charge Density\nCan Enhance Both Transport and Ion\nExclusion in Simulated Polystyrenesulfonated Membranes

Bound charge density is a critical\ndesign parameter for\ntuning\nwater and ion diffusivity in polyelectrolyte membranes. Higher charge\ndensity results in increased water uptake and improved diffusivity\n(transport). However, the impact of bound charge density and consequent\nwater uptake on ion exclusion is crucial for designing membranes with\nuncompromised selectivity. In this molecular simulation study, we\ninvestigate sulfonated polystyrene–polymethylbutylene (PSS–PMB)\nmembranes at different sulfonation levels to explore the effects of\nbound charge density on water and ion transport and salt exclusion.\nRemarkably, the equilibrium water uptake per sulfonate group and pore\nsize remain constant irrespective of sulfonation, while the pore morphology\ntransforms significantly. At lower sulfonation levels, the tortuous\npores are locally one-dimensional, while higher sulfonation results\nin locally two-dimensional pores and consequently a 2-fold increase\nin molecular diffusivity. This morphological change also increases\nion concentration at the pore centers, resulting in improvement in\nsalt exclusion of up to 50%.