Customizing the surface charge of thin-film composite membranes by surface plasma thin film polymerization

Solute-surface interactions are critical in membrane science and dominate a number of diffusion and selectivity parameters. In water treatment particularly, the charge on the membrane has been shown to affect ion transport selectivity as well as fouling mechanisms. The development of advanced surface technologies that allow for potential customization of the surface charge for specific applications, without compromising the essential performance of the membrane, is therefore desirable. In this paper, a novel plasma polymerization strategy was applied to commercial reverse osmosis membrane in order to tune the surface charge. Two monomers, maleic anhydride and vinylimidazole, were plasma polymerized onto the membrane resulting in a modification of the surface energy with resultant isoelectric points of approximately pH 3 and pH 7 respectively. This required only a short 5 min plasma polymerization treatment in each case. Thus, in addition to enhancing the water permeation by up to 10%, in comparison to the reference membranes, the overall charge of the membranes was shifted from highly negatively charged upon maleic anhydride polymerization to highly positively charged upon vinylimidazole polymerization. A comprehensive morphological and chemical analysis was performed to correlate the changes to the presence of functional groups and the alteration of the surface texture. Short treatments were found to smooth the surface whilst enriching the surface with either carboxylic or amine/amide groups. This work opens new avenues to engineer advanced membranes with improved performance and selectivity.