Reverse osmosis membranes made of crosslinked aromatic polyamide are among the most successful desalination membranes. One of the reasons is because these membranes combine high water flux with high salt rejection, and are easy to synthesize. However, they are prone to fouling and chemically weak against chlorine. Reinforcing these membranes with carbon nanotubes (CNTs) is one approach to improve their performance. Indeed, experimental results have shown that nanotubes improve both the antifouling properties and the chlorine resistance of these membranes, however, the exact mechanism is still far from complete understanding. In this work, we have used first principles chemical calculations and classical molecular dynamics to simulate CNTs reinforced polyamide membranes. We focused on the effect of molecular orientation on the water transport mechanism, chlorine resistance and fouling. For the membrane model, we designed an empirical algorithm that considers some of the stages such as monomer adsorption on the CNT, or cases such as a rink like structures. We simulated a step-growth addition where monomers were added and a topologically correct structure was built in a similar fashion to a polymerization (see Figure 1). After building the membrane we relaxed the structure. We considered GAFF force field for the polymer structure, and the Lennard-Jones (LJ) and Coulomb type interactions between m-phenylendiamine (MPD) and trimesoylchloride (TMC). A particle mesh Ewald method was used to calculate Coulomb interactions. The SPC/E model for water with the SHAKE algorithm was used in the permeability simulations. Simulation of the water-solvated membrane system was performed in the NPT ensemble at 300 K and 1 atm. a graphene sheet was used as a virtual piston for pressure control. The orientation of the polymer aromatic moieties shows the effect of CNTs on the polymer structure. The simulations show that CNTs effectively reduce membrane hydration and network mobility, avoiding the formation of water pockets, having a positive impact on the membrane desalination performance.

Molecular orientation and water transport in carbon nanotube reinforced aromatic polyamide membranes

254th National Meeting (2017)

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