The ability to perform efficient desalination separations will be critical to addressing water scarcity issues that increasingly threaten global water supplies. Polymeric membrane-based separations (e.g., reverse osmosis) are energy efficient desalination processes because many polymers have intrinsic properties that facilitate water transport while restricting salt (i.e., ion) transport. Much remains unknown about how polymer chemistry specifically contributes to selective water and ion transport processes, and understanding the fundamental underpinnings that govern these transport processes is critical to engineering advanced membrane materials.
One strategy to design polymers that selectively suppress ion transport independently of water transport is to engineer thermodynamically non-favorable interactions between ions and the solvated polymer. The primary (i.e., shortest-range) interaction that contributes to ionic thermodynamic non-ideality in hydrated polymers are those between ions and their induced polarization charges, or the so-called ion solvation interactions. Because ion solvation (i.e., dissociation) is increasingly thermodynamically non-favorable in low dielectric constant media, reducing the dielectric constant and characteristic hydrated void space of desalination polymers may be a reasonable strategy to engineer materials that increasingly suppress salt transport independently of water transport.
This presentation presents ion sorption, swelling (i.e., mesh size), and dielectric constant data for a series of cross-linked styrene/acrylonitrile, methacrylate, and poly(ethylene oxide)-based polymer networks that may be useful as desalination membrane materials. The polymer network mesh size, which was used as a proxy measurement for the characteristic hydrated void space, was engineered by controlling synthetically backbone chemistry and degree cross-linking. The polymer dielectric constant was engineered by controlling polymer functionality. These data were used to inform the application of the Freger-Born model, which describes the influence of solvation interactions on ion sorption in hydrated polymers. Ultimately, we found that the ion sorption coefficient of the polymers decreased as the network mesh size and dielectric constant decreased. The relationship between the mesh size, dielectric constant, and the ion sorption coefficient was described quantitatively using the Freger-Born model, and this observation suggests that ion solvation interactions contribute significantly to ionic thermodynamic non-ideality in hydrated polymers. These observations further the fundamental understanding of the relationship between polymer chemistry and polymer ion sorption properties and may serve useful to guide engineering strategies for desalination polymers.
