Charge Density Can Enhance Both Transport and Ion Exclusion in Simulated Polystyrenesulfonated Membranes

Abstract

Bound charge density is a critical design parameter for tuning water and ion diffusivity in polyelectrolyte membranes. Higher charge density results in increased water uptake and improved diffusivity (transport). However, the impact of bound charge density and consequent water uptake on ion exclusion is crucial for designing membranes with uncompromised selectivity. In this molecular simulation study, we investigate sulfonated polystyrene–polymethylbutylene (PSS–PMB) membranes at different sulfonation levels to explore the effects of bound charge density on water and ion transport and salt exclusion. Remarkably, the equilibrium water uptake per sulfonate group and pore size remain constant irrespective of sulfonation, while the pore morphology transforms significantly. At lower sulfonation levels, the tortuous pores are locally one-dimensional, while higher sulfonation results in locally two-dimensional pores and consequently a 2-fold increase in molecular diffusivity. This morphological change also increases ion concentration at the pore centers, resulting in improvement in salt exclusion of up to 50%.