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Researchers using ultrabright x-rays reveal the molecular structure of membranes used to purify seawater into drinking water

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Structural study of polyamide barrier layers in reverse osmosis membranes


The molecular structures of polyamide barrier layers in reverse osmosis membranes, made by interfacial polymerization of meta-phenylene diamine and trimesoyl chloride under different reaction and post-treatment conditions, were characterized by grazing incidence wide-angle x-ray scattering (GIWAXS). The molecular backbone packing is consistent with two different aromatic molecular packing motifs (parallel and perpendicular) with preferential surface induced orientation. The results suggest that the perpendicular, T-shaped, packing motif (5 Å spacing) might be associated with optimal membrane permeance, compared with the parallel packings (3.5 to 4.0 Å spacings).

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Water adsorption in aromatic reverse osmosis polyamide barrier layers observed by GIWAXS


To further explore the inter-molecular structure of water vapor adsorption in polyamide layers, we have carried out GIWAXS measurements on freestanding PA films at variable RH. It was found that the intensity of the first peak at ~1.60 Å-1 is relatively insensitive to RH, whereas the intensity of the second peak at ~1.83 Å-1 increases monotonically with RH. At saturated humidity, the intensity became 23% larger than that of the in the dry sample. The peak position seems to suggest this intensity increase results from water bound to the PA scaffold. In contrast, bulk water has a scattering peak ~ 2.0 Å-1 Our results further showed that the solvent treatment of freestanding PA films modified the characteristics of water adsorption behavior. For instance, a freestanding PA film rinsed either with benzyl alcohol (BA) or tetrahydrofuran both exhibited a 50% increase in the peak intensity at 1.83 Å-1, suggestive of extra water adsorption.

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