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Carboxylate anions of various chain lengths are important molecules for many applications such as CO2 reduction, membrane-based bioreactors, etc. Also, carboxylate anions are ubiquitous in biological molecules such as amino acids, fatty acids, etc. Therefore, understanding the transport behavior of carboxylates of different chain lengths in polymer materials is important both as a fundamental phenomenon but also for designing materials for applications. Here, we characterized transport behavior by measuring the permeability (P), and total partition coefficient (K) for a series of polymer membranes for four model carboxylate salts—sodium salts of formate (NaOFm), acetate (NaOAc), propionate (NaOPr), and butanoate (NaOBu)—at varied upstream salt concentrations (0.1–1 M) or a series of polyethylene glycol diacrylate (PEGDA)-based membranes with 1) varying pre-polymerization water content; 2) varying uncharged side chain comonomer (polyethylene glycol methacrylate, PEGMA), and 3) varying charged comonomer)2-acrylamido-2-methyl-1-propanesulfonic acid, AMPS). Also, diffusivity values of the four salts through the membranes have been calculated based on the solution diffusion model equation (Pdouble bondK × D), experimentally obtained permeability, and total partition coefficients. For a majority of these membranes, NaOFm's permeability is much higher than the other three carboxylate salts (NaOAc, NaOPr, and NaOBu) seemingly due to the lower chain length and thereby smaller hydrated diameter. In terms of total partition coefficient, a size-based trend is not observed. For example, NaOBu's total partition coefficient (K) is generally the largest among the four, and at higher upstream salt concentrations (1 M), the values of the total partition coefficients of the four salts converge. From this we conclude that the carboxylate salt transport through these PEGDA-based non-porous dense membranes to be primarily driven by kinetics and not sorption. 

Understanding multi-component transport through polymer membranes is critical for separation applications such as water purification, energy devices, etc. Specifically for CO2 reduction cells, where the CO2 reduction products (alcohols and carboxylate salts), crossover of these species is undesirable and improving the design of ion exchange membranes to prevent this behavior is needed. Previously, it was observed that acetate transport increased in copermeation with alcohols for cation exchange membranes consisting of poly(ethylene glycol) diacrylate (PEGDA) and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) and that the inclusion of poly(ethylene glycol) methacrylate (PEGMA) (n = 5, n represents the number of ethylene oxide repeat units) could suppress this behavior. Here, we further investigate the role of PEGMA in modulating fractional free volume and transport behavior of alcohols and carboxylates. PEGDA-PEGMA membranes of varied membranes are fabricated with both varied pre −polymerization water content at constant PEGMA (n = 9) content and varied PEGMA content at two pre −polymerization water contents (20 and 60 wt.% water). Permeability to sodium acetate also decreases in these charge-neutral PEGDA-PEGMA membranes compared to PEGMA-free films. Therefore, incorporation of comonomers such as PEGMA with long side chains may provide a useful membrane chemistry structural motif for preventing undesirable carboxylate crossover in polymer membranes. 

In many applications of hydrated, dense polymer membranes—including fuel cells, desalination, molecular separations, electrolyzers, and solar fuels devices—the membrane is challenged with aqueous streams that contain multiple solutes. The presence of multiple solutes presents a complex process because each solute can have different interactions with the polymer membrane and with other solutes, which collectively determine the transport behavior and separation performance that is observed. It is critical to understand the theoretical framework behind and experimental considerations for understanding how the presence of multiple solutes impacts diffusion, and thereby, the design of membranes. Here, we review models for multicomponent diffusion in the context of the solution-diffusion framework and the associated experiments for characterizing multicomponent transport using diffusion cells. Notably, multicomponent effects are typically not considered when discussing or investigating transport in dense, hydrated polymer membranes, however recent research has shown that these effects can be large and important for understanding the transport behavior.