Molecular weight resolution of solvent resistant nanofiltration (SRNF) membranes

Engineering membranes for molecular separations in organic media is still a big challenge, especially when two closely resembling solutes have to be separated by allowing one to pass with the solvent and the other to be rejected. The so-called resolution of the membrane then becomes critical. Resolution reflects the difference in rejection values of the two (or more) solutes for that solvent-membrane system. Recently developed crosslinked poly(vinylidene difluoride) (XL-PVDF) and well-known crosslinked polyimide (XL-PI) membranes were prepared and tested for solvent resistant nanofiltration (SRNF). Rejections of 7 dyes (i.e. Rose Bengal (RB), Acid fuchsin (AF), Rhodamin B (RhB), Sudan Black (SB), Zinc tetraphenylporphyrin (ZTPP), and 55,10,15,20-Tetrakis(3,5-ditert-butyl phenyl)porphyrin (TBPP) in a broad range of solvents, i.e. a set of polar solvents (water, ethanol, isopropanol), and a set of non-polar ones (butyl acetate, chloroform, xylene and toluene) were studied. As anticipated, the molecular weight cut-offs (MWCOs) of both membrane-types varied with the applied solvent type. Butyl acetate and chloroform filtrations respectively yielded the sharpest MWCO for XL-PVDF and XL-PI. The varying MWCO-curve per solvent is explained by the changing affinity of solvents and solutes for the membrane, as indicated by swelling and sorption experiments respectively.

Automated summary

Engineering membranes for molecular separations in organic media remains a difficult task, particularly when separating similar solutes by allowing one to pass and rejecting the other. The resolution of the membrane becomes critical in such cases. Crosslinked poly(vinylidene difluoride) (XL-PVDF) and crosslinked polyimide (XL-PI) membranes were prepared and tested for solvent resistant nanofiltration (SRNF). The molecular weight cut-offs (MWCOs) of the membranes varied with the type of solvent used.