Reverse osmosis has become the leading method of converting seawater into potable water due to its lower energy costs compared to other methods. Despite the technical importance of the barrier layer, an understanding of the relationship between molecular level structure and function has yet to emerge. To determine the molecular level structure of the barrier layer, we have carried out Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) studies on well-defined thin polyamide (PA) layers. These were prepared at the oil/water interface using solutions containing m-phenylenediamine (MPD) and trimesoyl chloride (TMC). The GIWAXS patterns exhibit diffuse scattering features (see left figure) characteristic of an amorphous structure with several characteristic length scales. Further, the anisotropy shows the existence of preferential surface induced alignment. Specifically, the q-dependent scattering profiles depend on the azimuthal angle "chi" (see right figure). Detailed fitting analysis to the scattering profiles shows that they are well-described by the sum of several Lorentzian profiles along with a sloping background term. The characteristic lengths scales, determined from the peak positions of the Lorentzian profiles, correspond to about ~3.8 and ~5 Å molecular spacings. The former is associated with the “pi-pi” packing configuration (parallel packing of aromatic rings) whereas the latter is associated with the “T-shaped” packing configuration (perpendicular stacking of aromatic rings). We show that citric acid post-treatment promotes the T-shaped packing configuration.

Molecular structure of aromatic reverse osmosis polyamide barrier layers prepared at the oil/water interface

2019 Spring National Meeting

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