Molecular insights into charged nanofiltration membranes: Structure, water transport, and water diffusion

While polyamide-based reverse osmosis (RO) and nanofiltration (NF) membranes are widely used for desalination and water purification, the influence of membrane porosity and charge on water transport remains to be fully understood at a molecular level. Here we use molecular dynamics (MD) to build 56 distinct piperazine-based NF membranes models, which cover a membrane density range of 0.78 gcm⁻³ to 1.08 gcm⁻³. These membrane models have various charge concentrations, corresponding to a pH range of 4–11. Results indicate that membrane charge is not monotonically correlated with the membrane density or the water transport. Instead, the water transport is mostly determined by the membrane's physical properties, specifically, the membrane density, with charged membrane end groups and counterions causing swelling of the membrane, which tends to increase flux. Additionally, the diffusion coefficient of water molecules within the membrane is strongly correlated with the membrane density. The diffusivity of water is independent of the transmembrane pressure, even under the large pressures employed in molecular simulations. Thus, the transmembrane pressure biases the direction of the random walk of water molecules through the membrane resulting in a water flux but does not alter their overall mobility within the membrane. These findings shed light on the relationship between membrane properties and water transport for charged membranes, as well as providing new insights into the structure of NF membranes at a molecular scale.