Salt permeation mechanisms in charge-patterned mosaic membranes

Abstract

Charge-patterned mosaic membranes contain channels of both positive and negative charge, creating a preference for the transport of whole salts to neutral solutes. In filtration processes, this unique property results in the enrichment of the salt concentration in the solution that permeates through the charge-patterned mosaic membrane. While this concept has been known since the pioneering work of Söllner, the lack of robust, repeatable manufacturing processes for generating chemically-patterned mosaic membranes has obscured the fundamental origins of this novel transport mechanism. Here, we utilize inkjet printing techniques to precisely pattern the pore wall chemistry of track-etched membranes and characterize them using diffusion cell experiments. Critically, the template-assisted inkjet printing method offers the advantage of a direct comparison between the well-understood case of single-charge membranes and charge-patterned membranes that were manufactured using the same ink formulations and printing methodology. Concomitantly, a model that adapts the constraint of Donnan equilibrium to account for the local variations in the electrical potential that arise near charge-patterned surfaces is developed to elucidate the pivotal role of patterning in determining the properties of the mosaic membranes. Experimentally, we observe that the salt permeability increases with concentration for the single-charge membranes, and decreases with concentration for the charge-patterned membrane. Strong quantitative agreement between the model and experiments implies that the local distribution of ions within the double layer, and its effects on ion partitioning, results in the emergence of distinct transport properties when charged elements are patterned on a substrate.