Hydration, porosity and water dynamics in the polyamide layer of reverse osmosis membranes: A molecular dynamics study

This study employs molecular dynamics to gain insight into the mechanisms underlying the formation, hydration and functioning of aromatic polyamide membrane used for water purification by reverse osmosis. A polyamide membrane structure was successfully generated with a chemical composition and degree of crosslinking that closely match recent experimental data. The density of dry polymer was found to be slightly lower than in previous studies but closer to recent experimental measurements. The volume expansion and mass gain upon hydration show marked variations due to the presence of a significant fraction of permanent voids, revealed by dynamically averaged water concentration maps. The mass hydration and porosity were also larger than previously obtained, but water self-diffusivity was similar to previous simulations, presumably, since the larger porosity was offset by the higher cross-linking. Using a radial distribution function of water within polyamide, strong next-neighbor correlations and random long-distance water-water correlations could be differentiated, which could be assigned to small network and large aggregate pores. Unfortunately, this dual porosity picture could not be linked to the experimentally found bimodal distribution of the pKa values of the COOH groups, since the distribution of the local water content around COOH was found to be unimodal.