Polymeric nanofiltration of binary water-alcohol mixtures: Influence of feed composition and membrane properties on permeability and rejection

In this paper, fluxes and rejections for solvent resistant nanofiltration membranes are studied for binary mixtures (water-methanol, water-ethanol and methanol-ethanol). It is shown that transport through polymeric nanofiltration membranes is influenced both by membrane characteristics and by properties of solvents and solutes. With respect to solvent permeability, hydrophilicity and porosity of the membrane surface appeared to be the membrane characteristics with the largest influence. Viscosity and polarity of the mixture were the solvent properties with the largest influence. Partial permeabilities were calculated to evaluate the mutual influence of the permeation of two components. Solute rejection was found to be the result of a combination of three effects. Firstly, the pore size determines steric hindrance during solute transport: the larger the membrane pores, the lower the resistance against permeation and the lower the solute rejection. Secondly, the solute-solvent affinity affects solute transport: a high affinity causes solvation (of the solute), which leads to an increase of the effective molecular size of the solute and thus higher rejection. Finally, interaction between the membrane and the solvent may lead to solvation of the pore wall (swelling), causing a reduction of the effective pore diameter and thus an increase of the solute rejection. Experimental results are confirmed by the comparison of the Stokes-Einstein diameters in water, methanol and ethanol with the effective pore diameter of the membranes used.