Influence of water content on ion sorption and transport in cross-linked polymer membranes

Polymer membrane-based technologies have been successfully utilized for a variety of desalination applications, including reverse osmosis (RO), forward osmosis (FO), electrodialysis (ED), and others. In these applications, separation of water and ions is possible because of differences in their transport rates through the membrane. Generally, high water permeability through a membrane can be achieved by using more hydrophilic (i.e., highly water swollen) polymers. However, increasing water permeability often leads to a decrease in water/ion selectivity in hydrophilic polymers, rendering controlling water content of a polymer a key factor to achieve desirable ion separation properties. Thus, a fundamental understanding of ion transport in polymers with various water contents is essential for designing and optimizing polymer membranes. The relationship between ion size and sorption/diffusion properties in water swollen uncharged polymers was investigated as a model system for understanding ion sorption and transport in more complex systems (i.e., charged polymer networks). Alkali metal chloride (e.g., LiCl, NaCl, and KCl) sorption and diffusion coefficients in a series of cross-linked poly(ethylene glycol) diacrylate (XLPEGDA) polymer membranes were measured as a function of external salt concentrations ranging from 0.01-1.0 M. Generally, ion sorption and diffusion coefficients increase as polymer water content increases. Salt activity coefficients in the polymers were quantified to better understand the thermodynamic non-ideality of ions in polymer networks. The Flory-Rehner theory was used to predict water volume fraction in the polymers equilibrated with salt solution, based on salt sorption measurements. A series of cross-linked cation exchange membranes (CEMs) were synthesized with different water uptake values and similar fixed charge group concentrations to study the effect of water content on ion sorption. Equilibrium co-ion sorption data were interpreted using a thermodynamic model based on Donnan’s theory and Manning’s counter-ion condensation theory. The inhomogeneous morphology of the membranes was characterized by small angle X-ray scattering (SAXS). The Manning parameter was used as an adjustable constant to account for morphological heterogeneity. Finally, the classic Merten and Lonsdale transport model for reverse osmosis membranes was reformulated to explicitly demonstrate the effects of concentration polarization and solution phase thermodynamic non-idealities on salt transport. A framework presented here accounts for the concentration dependence of ion activity coefficients in salt solutions, which was not explicitly included in the classic model.