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 298 K 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 (3−4×10−10 m2/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.