Hydration of a polyamide reverse-osmosis membrane

Over the last decades only a few atomistic simulations were performed to investigate water transport through polyamide reverse osmosis membranes at the molecular scale. In most reported simulations water uptake by the membrane was fixed on the basis of scarce experimental data available in the literature but it has never been predicted directly from explicit molecular simulations of water transport across the membrane/water interface. This was attempted in the present work by using a technique based on non-equilibrium molecular dynamics simulations. The prediction of the membrane water uptake is indeed of the utmost importance for (i) validating an atomistic model of a polymer membrane and (ii) getting an accurate picture of the molecular mechanisms ruling water transport through the membrane. Simulations performed in the present work led to a water content inside a cross-linked fully aromatic polyamide membrane around 24 wt% in very good agreement with reported experiments. We further showed that this high water uptake results from favorable interactions between the polyamide membrane and water molecules, which compensates the decrease in the number of hydrogen bonds between water molecules inside the membrane. Eventually, it was shown that water molecules confined in the membrane formed an interconnected hydrogen-bond network made of cyclic and linear aggregates.