Effect of selective layer thickness on pervaporation of composite poly(vinyl alcohol)–poly(sulfone) membranes

Effects of the selective layer thickness of flat composite membranes were studied on pervaporation. Selective layers of different thicknesses were prepared from poly(vinyl alcohol), and the porous support layer was prepared from poly(sulfone). The microstructure, surface roughness and hydrophobicity of the selective PVA layers were determined from SEM, AFM and contact angle measurements, respectively. For each composite PVA–PSf membrane, dehydration of ethanol–water mixtures was conducted over a range of ethanol concentrations (10–85 wt%) in the feed solution. Results of the total flux and selectivity were obtained and statistically analyzed as a function of selective layer thickness. In general, the outcome agrees with the solution-diffusion (S-D) theory: the total flux is significantly affected by the PVA layer thickness, while the selectivity remains nearly unaffected. Using the S-D theory, the mass transfer resistance of the selective layers was calculated and found to increase with thickness. The relatively small change observed in the selectivity has been related to changes in surface properties of the crosslinked membrane, especially when the selective layer thickness is small.