Key factors influencing water diffusion in aromatic PA membrane: Hydrates, nanochannels and functional groups

• We investigate these factors influencing water diffusion in PA membranes.
• 8.75 Å is a critical value of radius for water channels in membranes.
• Hydration is the main reason that PA has higher rejection for salt than for water.

Desalination by aromatic polyamide (PA) composite membranes is currently the most important technology of solving the global freshwater crisis. However, understanding the main factors influencing water diffusion in the desalination process remains a major hurdle in designing new membrane materials. Here, we investigate the key roles of the hydrates, the nanochannels in a PA membrane and the functional groups of the membrane on water diffusion by molecular dynamic simulations. Results show that the membrane can strongly hinder the diffusions of water molecules via both geometrical obstruction and water–membrane interaction when the radius of the water diffusion channel in the membrane is less than 8.75 Å. Conversely, the membrane has little influence on water diffusion. In a dilute NaCl solution, the effect of salt ions on the overall diffusion behavior of water can be neglected because only a small quantity of water molecules form hydrates. Almost all Na+ and Cl−, on the contrary, can form hydrates, which could explain why the PA membrane has a higher rejection level to salt ions rather than to water in seawater.

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