(606b) Hydraulic Permeation-Induced Water Concentration Gradients in Ion Exchange Membranes

Permselective polymer membranes are essential components of water purification and electrochemical technologies that address the water-energy nexus (e.g., reverse osmosis, electrodialysis, fuel cells, batteries, CO₂ electrolysis). Ion exchange membranes (IEMs) are of particular interest in such applications because they contain fixed charge groups tethered to the polymer backbone that enable selective permeation of ions. In addition to controlling ion transport, IEMs must regulate permeation of neutral solvents (e.g., water and methanol) for efficient separations. Rational design of IEMs with enhanced performance requires a fundamental understanding of the mechanisms that govern penetrant transport in such materials. However, there are conflicting reports regarding the fundamental physics of water transport in IEMs, with some researchers postulating a pore-flow mechanism and others applying a solution-diffusion mechanism. For example, many have proposed a heterogeneous pore structure in swollen Nafion, the prototypical IEM in electrochemical processes, speculating that a pressure gradient through the membrane, rather than a concentration gradient, drives water transport under hydraulic permeation (e.g., reverse osmosis). To address this topic, we report measurements of water concentration profiles in pressurized films of Nafion and related materials. Several films were stacked together in a custom-built permeation cell, with the feed side pressurized to between 1000 and 2500 psi, and the permeate side held at atmospheric conditions. At steady state, the permeation experiment was stopped, the films were separated, and the water content of each film was measured, revealing a water concentration gradient through the stack thickness that increased with increasing feed pressure. The experimental data were in excellent agreement with theoretical predictions made using a model based on Fick’s law and polymer-solvent swelling theories (e.g., Flory-Huggins and Flory-Rehner). These results are consistent with the solution-diffusion model and cannot be explained by a pore-flow mechanism. The thermodynamic origins of pressure induced diffusion and the resultant implications on separation performance will be discussed.