Fundamentals of ion sorption and transport in charged and ligand functionalized membranes

Sujanani, Rahul
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Polymeric membranes that selectively control water and ion transport rates are critical to various separation technologies addressing the Water-Energy Nexus. For example, highly selective ion transport across membranes is essential to processes including, reverse osmosis, nanofiltration, electrodialysis, and fuel cells. Developing membranes with enhanced ion transport and selectivity properties could improve the efficiency of existing membrane-based technologies and facilitate the design of new processes for emerging applications (e.g., resource recovery). A detailed fundamental understanding of ion transport in ion selective membranes could permit rational design of new membrane materials with desired separation properties. However, many molecular interactions underpinning ion sorption, diffusion, and selectivity in such materials are poorly understood. This dissertation focused on fundamental studies of aqueous ion sorption and transport in two classes of ion selective membranes: ion exchange membranes (IEMs) and 12-Crown-4-Ether (12C4) ligand functionalized membranes. IEMs contain charged groups tethered to the polymer backbone that enable selective permeation of ions based on charge/valence. Ion sorption properties of commercial IEMs equilibrated with various salts (i.e., NaCl, MgCl₂, Na₂SO₄, and MgSO₄) were characterized experimentally and compared with theoretical predictions to determine the utility of existing models (i.e., Manning’s counter-ion condensation theory and Donnan equilibrium theory), and gaps therein, to describe thermodynamics in charged polymers. These studies demonstrated the importance of counter-ion condensation and ion pairing (a topic rarely considered in hydrated membranes) on ion sorption in IEMs, leading to the development of a new thermodynamic framework that accounts for both phenomena. In some cases, good quantitative agreement was observed between theoretical predictions and experimental data, using no adjustable parameters. Given that commercial membranes exhibit limited selectivity between ions of the same valence (e.g., Li⁺ vs. Na⁺), the influence of fixed ligand sites on ion transport in hydrated polymers was also investigated. Permeability, solubility, and diffusivity of LiCl, NaCl, and MgCl₂ were measured in a series of 12C4 functionalized membranes. In some cases, 12C4 membranes exhibited an unusual, LiCl/NaCl permeability selectivity higher than previously reported values among hydrated polymers under single salt conditions (i.e., ~2.3). These results provided an improved fundamental understanding of host-guest interactions in hydrated polymers and potential guidelines for designing ion selectivity in membranes.