(226d) Using High-Throughput Membrane Characterization to Elucidate How Ligand Length Affects Solute Transport

There is a growing demand for materials that can separate solutes of similar size and charge, such as critical minerals. Current processes such as liquid-liquid extraction (LLE) apply molecular recognition principles which leverage ligand-solute coordination to facilitate separations. Unfortunately, LLE requires copious amounts of harmful solvents and large processing facilities to achieve the desired separation. Self-assembled copolymer membranes are a promising avenue for obtaining high purity products with reduced negative environmental impacts. Additionally, these membranes provide a framework for post-synthetic modifications to incorporate molecular recognition ligands. In this regard, ligands can be covalently bonded to the pore wall without compromising membrane structural integrity and chosen based on their potential to facilitate selective separations.

In this study, we aim to understand how ligand-solute interactions affect solute transport and membrane performance. We use diafiltration to rapidly characterize membrane materials and establish relationships between nuanced changes in pore wall chemistry and membrane performance. Using a copolymer membrane scaffold, ligands possessing spacer arms with 0, 2, 4, 6, or 10 ether repeat units and a carboxylic acid pendant group were covalently bound to the pore wall through a copper(I)-catalyzed azide-alkyne cycloaddition “click” reaction. For all membranes, neutral solute rejection experiments demonstrate that the functionalization process resulted in a slight decrease of pore size from 2 nm to 1.5 nm. This indicates that the addition of a ligand decreased the size of the hydrophilic channels that solutes diffuse through. Independent of ligand length, the functionalized membranes displayed similar concentration-dependent rejection profiles; the rejection of NaCl decreased from 90% to 10% as the feed concentration increased from 1 mM to 100 mM. These results are distinct from a non-functionalized membrane, where rejection was 0% due to the lack of a fixed charge. Experiments were repeated at pH 4 and pH 8, just below and above the pKa of a carboxylic acid. Regardless of pH, no change was seen within the rejection profile. To investigate the strength of solute-ligand interactions outside of electrostatic interactions, experiments were repeated using KCl due to the tendency of potassium to complex with ether oxygens. The rejection profiles of NaCl and KCl are distinct from one another, with KCl rejection ranging from 90% to 30%. Similar trends are observed for divalent ions, Na₂SO₄ and K₂SO₄, however, with higher rejection values due to the presence of the fixed charge. These data sets suggest the importance of solute-ligand coordination to separate solutes with the same valency, while electrostatic interactions play a more significant role in separations between solutes with different valences.