Grazing Incidence\nWide-Angle X‑ray Scattering\nof Water Adsorption in Polyamide Barrier Layers of Reverse Osmosis\nMembranes

To understand the relationship between the intermolecular\nstructure\nof aromatic polyamide (PA) scaffold and the water molecules in the\nbarrier layers of reverse osmosis (RO) membranes, a grazing incidence\nwide-angle X-ray scattering (GIWAXS) study was carried out on freestanding\nPA thin films at varying relative humidity (RH) conditions. The scattering\nresults were analyzed by an interference scattering model, containing\na phase factor between a PA chain and an adsorbed water molecule.\nThis model yielded good fits to the GIWAXS profiles where the water\nadsorption was found to vary linearly with RH. Atomistic molecular\ndynamics (MD) simulations were also performed to complement the experimental\nstudy. The simulations revealed that a rapid condensation layer initially\nformed on the PA film surface, followed by the slow water molecule\ndiffusion inside the PA membrane. Sparse adsorbed water, isolated\nin subnanopores of the PA film adjacent to the polar atoms, even in\nvery low quantities, modifies the X-ray scattering. Atomistic simulations\nat the microscopic scale provide partial support for several X-ray\nscattering findings.