Mixed binary alkali halide salt transport in PEO systems*

Salt permeability coefficients are important properties of membranes for industrial applications such as water purification and energy generation; however, there are few systematic studies probing the differences in permeability as a function of external solution composition. Understanding how ion transport is affected by the presence of other ions can help advance the design of membranes tailored with specific ion affinities. Here, we use a model cross-linked poly(ethylene glycol) diacrylate membrane to examine how permeability coefficients of binary alkali halide salt mixtures vary as a function of external salt mole fraction. We see that, at constant ionic strength, mixed salt permeability is largely governed by mixed salt partitioning. Furthermore, starting from the Nernst-Planck framework, we have derived a model for predicting permeability coefficients based off a thermodynamic partitioning model and the Mackie-Meares model for diffusion. This new model, with no adjustable parameters, shows great agreement in conditions with minimal ion-ether oxygen coordination (i.e., Na⁺, Cl^-, and Br^-), and shows slight overpredictions in cases with strong cation-oxygen binding (i.e., K⁺).