Fluxes and rejections for nanofiltration with solvent stable polymeric membranes in water, ethanol and n-hexane

The solvent flux and rejection of 2,2′-methylenebis-(6-tert-butyl-4-methyl phenol) was studied in water, ethanol and n-hexane for four solvent-stable polymeric nanofiltration membranes (N30F, NF-PES-10, MPF-44 and MPF-50). Solvent fluxes were analysed with the Machado model to determine hydrophobicity/hydrophilicity of the membranes. The results are consistent with N30F, NF-PES-10 and MPF-44 being hydrophilic membranes, and MPF-50 being a hydrophobic membrane. The solvent flux for MPF 50 increases with decreasing polarity of the solvent: for n-hexane, a flux as high as 1600l/m2h was obtained at a 20 bar transmembrane pressure. Much lower fluxes were found with ethanol; water fluxes were almost zero. For the other three membranes, the flux decreases in the order water>ethanol>n-hexane, i.e. with decreasing polarity. Pretreatment of the membranes by immersion for 24hours in the solvent used for the flux measurements reduced the ethanol flux and n-hexane flux for MPF-50, but increased the solvent fluxes for the hydrophilic membranes. According to the Machado model, a possible explanation is that changes in the membrane structure cause a decrease of the hydrophobicity or hydrophilicity of the membranes. Rejections of 2,2′-methylenebis-(6-tert-butyl-4-methyl phenol) (insoluble in water) in ethanol and n-hexane were always below 10%. Rejection of maltose in water was much higher for all membranes, although maltose is a component with a comparable molecular weight (insoluble in ethanol and n-hexane). The enhanced mobility of polymeric chains in organic solvents might cause these low rejections by increasing the effective pore size available for transport through the membrane. Moreover, in aqueous solution a component such as maltose may have a water shell increasing the effective size of the molecule; this effect is absent in organic solvents. The difference in rejection was also obtained with components with higher molecular weight: raffinose in water and DL-alpha-tocopherol hydrogen succinate in ethanol and n-hexane.