Atomistic simulation of water and salt transport in the reverse osmosis membrane FT-30

Atomistic computer simulations of water and salt (NaCl) transport in the polyamide discriminating layer of the reverse osmosis membrane FT-30 are reported. We find that water transport occurs by a “jump” diffusion process, similar to the diffusion of simple, dissolved gas molecules in amorphous polymer glasses. As expected, lower water mobilities are observed at higher polymer densities. Cross-linking in the polymer matrix leads to an increase in density, which results in a decrease in water mobility, in accordance with experiment. We also observe a lower mobility for Cl− in the hydrated polymer, relative to Na+, which we attribute in part to the larger number of polar groups on the polymer chain that participate in solvating the anion. That the anion limits salt transport in our model FT-30 structures is consistent with experimental observations. Finally, although we find that the presence of salt reduces water mobility in the polyamide, in accordance with experiments; contrary to previous views, this effect is not related to a change in polymer density. Based on estimates of the salt partition coefficient and the diffusion coefficient of salt in the membrane, we conclude that high salt rejection in FT-30 is due in large part to the large difference in water and salt mobilities within the polyamide discriminating layer.