  The Angstrom-scale transport characteristics of water and six different
solutes, methanol, ethanol, 2-propanol, urea, Na+, and Cl-, were studied for a
polyamide reverse osmosis (RO) membrane, FT-30, using non-equilibrium molecular
dynamics (NEMD) simulations. Results indicate that water transport increases
with an increasing fraction of connected percolated free volume, or
water-accessible open space, in the membrane polymer structure. This free
volume is enhanced by the dynamic structure of the membrane at the molecular
level as it swells when hydrated and vibrates due to molecular collisions
allowing a continuous path connecting the opposite membrane surfaces. The
tortuous paths available for transport of solutes result in Brownian motion of
solute molecules and hopping from pore to pore as they pass through the polymer
network structure of the membrane. The transport of alcohol solutes 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. However, the Van der Waals size of the dehydrated
solutes is generally not a good measure to predict solute transport or
rejection. Urea has reduced transport compared to ethanol, most likely due to
more complex chemistry, even though urea has a smaller Van der Waals volume
than ethanol. Na+ and Cl- experience the lowest transport, likely due to strong
ion-water and ion-ion electrostatic interactions.
