LAUSR

Two models describe solvent transport through swollen, nonporous membranes. The pore-flow model, based on fluid mechanics, works for porous membranes, whereas the solution-diffusion model invokes molecular diffusion to treat nonporous membranes. Both approaches make valid arguments for swollen polymer membranes, but they disagree in their predictions of intramembrane pressure and concentration profiles. Using a fluid-solid model that treats the solvent and membrane matrix as separate phases, we show both classical models to be valid, to represent complementary approaches to the same phenomenon, and to make identical predictions. The fluid-solid model clarifies recent reverse osmosis measurements; provides a predictive and mechanistic basis for empirical high-pressure limiting flux phenomena, in quantitative agreement with classic measurements; and gives a framework to treat nonporous but mechanically heterogeneous membrane materials. Description Unite and expand for better models Transport through swollen polymer membranes has traditionally been described by either a pore flow or a solution-diffusion model. The former works best for porous membranes, in which transport is controlled by size exclusion and electrostatic effects, whereas the latter works best for nonporous membranes, in which transport is driven by solubility and diffusivity. Hegde et al. show that is possible to unite these two classical models using a fluid-solid model that treats the solvent and the matrix as separate phases (see the Perspective by Geise). This approach captures the predictions of the earlier models and allows for the determination of both the intramembrane pressure and the concentration profiles. It also allows for the modeling of mechanically heterogeneous membranes, and thus may be a useful tool in membrane design. —MSL A fluid-solid model for flow in polymer membranes expands upon and captures the predictions of earlier models.