ATOMISTIC SIMULATION OF WATER AND SALT TRANSPORT IN THE REVERSE-OSMOSIS MEMBRANE FT-30

Atomistic computer simulations of water and salt (NaCl) transport in t he polyamide discriminating layer of the reverse osmosis membrane FT-3 0 are reported. We find that water transport occurs by a ''jump'' diff usion process, similar to the diffusion of simple, dissolved gas molec ules in amorphous polymer glasses. As expected, lower water mobilities are observed at higher polymer densities. Cross-linking in the polyme r matrix leads to an increase in density, which results in a decrease in water mobility, in accordance with experiment. We also observe a lo wer mobility for Cl- in the hydrated polymer, relative to Na+, which w e attribute in part to the larger number of polar groups on the polyme r chain that participate in solvating the anion. That the anion limits salt transport in our model FT-30 structures is consistent with exper imental observations. Finally, although we find that the presence of s alt reduces water mobility in the polyamide, in accordance with experi ments; contrary to previous views, this effect is not related to a cha nge in polymer density. Based on estimates of the salt partition coeff icient and the diffusion coefficient of salt in the membrane, we concl ude that high salt rejection in FT-30 is due in large part to the larg e difference in water and salt mobilities within the polyamide discrim inating layer. (C) 1998 Elsevier Science B.V.