Dynamics of water and solute transport in polymeric reverse osmosis membranes via molecular dynamics simulations

The Ångström-scale transport characteristics of water and six different solutes, methanol, ethanol, 2propanol, urea, Na+, and Cl-, were studied for a polymeric reverse osmosis (RO) membrane, FT-30, using non-equilibrium molecular dynamics (NEMD) simulations. Results indicate that water transport increases with an increasing fraction of percolated free volume, or water-accessible open space, in the membrane polymer structure. The trajectories of solute molecules display Brownian motion and hop from pore to pore as they pass through the polymer chain structure of the membrane. The solute transport depends on both the Van der Waals size of the dehydrated solute and the electrostatic interaction of the solute with water and the membrane. For alcohol solutes, transport decreases for solutes with larger Van der Waals volume, which corresponds to less available percolated free volume, or solute-accessible space, within the membrane polymer structure. Urea has reduced transport compared ethanol, most likely due to more complex chemistry or polarity than the alcohol solutes, even though urea has a smaller Van der Waals volume than ethanol. Na+ and Clexperience the lowest transport, likely due to strong ion-water and ionion electrostatic interactions. NEMD simulations provide a unique opportunity to understand molecular level mechanisms for water and solute transport in polymeric RO membranes for water purification.