Water Flow inside Polyamide Reverse Osmosis Membranes: A Nonequilibrium Molecular Dynamics Study

Among all desalination technologies, reverse osmosis (RO) technology is the most widespread technology to obtain the freshwater capacity. To further enhance the water flux of RO membranes, it is important to understand the molecular mechanism of water transport through the membrane. We apply a steady state nonequilibrium dynamics simulation (SS-NEMD) to study the behavior of water molecules flowing inside the RO membranes, by directly applying external forces on the water molecules to simulate pressure drops, since the mass transport resistance inside the membrane is what we mostly considered.

Before measuring the relationship between pressure drops and the pure water flux, we estimated the average density along z direction of simulation boxes and pore size inside the membrane, which is in good consistency with the experimental values. Then, six pressure drops were applied on water molecules to investigate the effect of pressure drops on the pure water flux (PWF). In each pressure drop, there is no obvious tendency of continuous increase or decrease in the PWF, we believe the simulation reached steady states. From the six pressure drops, it is obvious that the water flux increases almost linearly with pressure drops despite small fluctuations due to the random thermal motion. From that, we could calculate the water flux at the experimental condition. At last, we revealed the interactions between water molecules and functional groups. We find that, water molecules move faster around benzene rings than do around carboxyl or amino groups in the membrane.

In this work, the results confirm that RO membranes containing more benzene rings would reduce the mass transport resistance of water inside the membranes. These findings are expected to increase our understanding on the water transport behavior in RO membranes and also the rational design of advanced RO membranes with upgraded permeance.