Alkali halide salt partitioning in PEGDA membranes under mixed salt conditions

Recent efforts in membrane science have focused on incorporating ion-specific interactions into polymeric materials to bias the partitioning or diffusivity of one ionic species over others. While many notable advances have been made in this area, most studies are limited to single salt partitioning and diffusivity experiments, which may or may not accurately reflect the behavior observed in the complex mixed salt environment that these separations are employed in. In this work, we study how the presence of multiple salts influences salt partitioning into poly(ethylene glycol) diacrylate (PEGDA), a model membrane material exhibiting ion-specific interactions. We observe that at a fixed external total salt concentration, increasing the mole fraction of a non-complexing salt (e.g., LiCl) leads to an increase in the partition coefficient of that salt, in accordance with single salt partitioning experiments. Conversely, increasing mole fraction of a complexing salt (e.g., KCl) leads to a decrease in the partition coefficient of that salt. We attribute the trends for the complexing salt to a saturation of ethylene oxide binding sites within the membrane with increasing mole fraction of the complexing salt. To rationalize these experimental observations, molecular dynamics simulations are employed to reveal changes in cation binding with changing mole fraction. Our results provide fundamental insights into how partitioning trends in mixed salt environments differ from single salt environments, with important implications for industrial applications of membranes to ion-specific separations.