Important factors influencing molecular weight cut-off determination of membranes in organic solvents

In solvent resistant nanofiltration (SRNF), sensible selection of a membrane for a particular solvent/solute system is recognized as challenging. Prospective methods for suitability analysis of membranes include molecular weight cut off (MWCO) characterization. However, insufficient understanding of the interrelated effects of solvent, solute, membrane properties, and the applied process conditions often complicates interpretation of MWCO data. This study demonstrates and discusses such effects with respect to transport mechanism. To this end very different SRNF systems have been selected: a rigid porous membrane (hydrophobized zirconia) versus a rubbery dense membrane (polydimethylsiloxane); a low flux solvent (toluene) versus high flux solvent (n-hexane); and a stiff solute (polystyrene) versus a flexible solute (polyisobutylene). The results indicate that, for the applied conditions, the MWCO of the dense membrane is predominantly affected by solute–membrane and solvent–membrane interactions. For the rigid porous membrane a significant effect of applied pressure is observed, in particular for the flexible solute. The non-linear relation between flux and pressure and the variations in MWCO with pressure indicate combined effects of concentration polarization and shear induced deformation of the flexible solute. The results unmistakably show that the interpretation of MWCO is heavily dependent on the system under study.