A transition in diffusion behaviors of organic liquid mixtures in dense polymer membranes

Multi-component mass transfer frameworks, such as the Maxwell-Stefan approach, parameterized with the pure component diffusion and sorption behaviors of different substances in polymer materials have been used in predicting complex mixture permeation through polymer membranes. However, a change in the physical state of a polymer membrane (e.g., swelling) or the polymer dynamics (e.g., plasticization) is likely when the membrane is exposed to a high activity organic solvent mixture with strong chemical affinity for the polymer. These changes complicate the accurate estimation of diffusion coefficients for mixtures of solvents based on knowledge of pure component diffusivities within the polymer. Here, we present 120 organic solvent reverse osmosis (OSRO) permeation test results for hydrocarbon liquid mixtures with varying compositions in three different polymers and compare the experimental results to the predictions of a Flory-Huggins coupled Maxwell-Stefan model with two different diffusion modalities. The effect of the chemical affinity between the mixture and the membrane - estimated by the Hansen solubility difference - on the diffusion behaviors of individual solvents permeating through the membrane was investigated via the usage of this model. Our work demonstrates that polymer membranes undergo a reduction for the effectiveness of diffusion-based separation near a specific threshold level of solubility difference (8 MPa0.5) from the mixture and that this difference is consistent across several different membrane materials.