Isolating the effects of gate layer permeability and sorbent density on the performance of solute-selective polymeric ion pumps†

The growing demand for solute selective separations necessitates the development of materials and processes capable of separating species of similar charge and size. Polymeric ion pumps, which are composite membranes composed of a gate layer situated on top of a sorbent layer, have the potential to address this opportunity. The gate and sorbent layers are designed to undergo changes in permeability and affinity, respectively, in response to an external stimulus. Subsequently, cyclic changes in the stimulus promote the selective transport of a target solute. Within this study, a mathematical model and numerical solver were developed to investigate the effect of the gate layer resistance on the performance of polymeric ion pumps. Notably, imperfect gate layers lead to solute diffusing back into the feed solution, reducing but not irrevocably hindering membrane performance. In the limit of high sorbent densities, the fraction of solute diffusing into the receiving solution is determined by the relative resistances of the gate and sorbent layers while the total flux of the target solute increases in proportion to the sorbent density. The analysis highlights that current materials possess the necessary properties to fabricate polymeric ions pumps with performances that exceed conventional membrane systems. Enhancing the performance of polymeric ion pumps will rely on identifying material combinations that respond coherently to rapidly changing stimuli.