Water diffusivity suppression and ethanol-over-water diffusion selectivity enhancement for ethanol/water mixtures in polydimethylsiloxane-zeolite membranes

The diffusion behavior of ethanol solutions in polydimethylsiloxane (PDMS) and zeolite-filled PDMS membranes at 298K was investigated. The partial effective diffusion coefficients of the ethanol and water permeants were determined experimentally using the gravimetric method in conjunction with concentration determination on remaining mixtures. The time-resolved sorption levels of ethanol/water binary mixtures in the membranes were measured and partitioned into individual solvent uptakes. These data were used to determine the diffusion coefficients of the individual solvents from the ethanol/water mixtures (10-95% ethanol concentrations). In the PDMS films, the faster-moving water molecules were slowed down and the ethanol diffusivity was accelerated. Both permeants exhibited similar diffusion coefficients (3-4×10 ^-10m ²/s), indicating a significant diffusion coupling effect of the co-permeants in the PDMS. In a PDMS-(30%) zeolite composite membrane, both permeant diffusivities for the binary mixtures decreased as compared to those in the PDMS. Meanwhile, the ethanol diffused faster than the water co-penetrant by a factor of 2.4-3.5 due to the suppression of water diffusion. Water molecules demonstrated a non-Fickian diffusion behavior, implying different transport mechanisms for the water diffusion process. The PDMS-zeolite membrane exhibited higher ethanol-over-water diffusion selectivities than those in the pristine PDMS membrane.