Paper title: A computational investigation of the properties of a reverse osmosis membrane
Journal: Journal of Materials Chemistry, 2010, 20, 7788?7799

Author(s): Zak E. Hughes* and Julian D. Gale
*Nanochemistry Research Institute, Department of Chemistry, Curtin University of Technology, P.O. Box U1987, Perth, WA, 6845, Australia

Summary:
The paper investigated the structure of the polyamide RO membrane, FT30, and its interaction with salt solution through molecular dynamics (MD) simulations. Using an MD approach at the atomic level, the system composed of a solvated polyamide membrane and bulk solution was setup to simulate the membrane interface. The behavior of water molecules and salt ions within the membrane and the bulk solution was also given emphasis.

The simulations done were able to calculate the diffusivities of both water and salts in the bulk solution and within membrane layer. The diffusion coefficients were observed to be an order of magnitude lower within the membrane than in the bulk solution. It was also observed that diffusion rates for all species in solution are approximately decreased by the same factor. This has an implication in the design of more efficient polyamide membranes. Consequently, it was inferred that by increasing the flow rate of water for a given pressure and maintaining the same salt rejection, energy consumption for the process might be reduced.

The individual interactions of ions with the membrane were also observed. In addition, the free energy surface associated with the passing of salt ions on the membrane-solution interface was determined using umbrella sampling methods. This reveals that resistance to salt diffusion into the membrane varies with the structure of water encountered by the ions as they permeate through the membrane. But even with the presence of this high variability in the free energy gradient, all of the simulations show a high resistance to ionic diffusion because of charge separation. This means that migration of Na+-Cl- ion-pairs did not lower the barrier to salt diffusion, proving the selectivity of the membrane for water.


Contribution and application:
This paper confirms the applicability of using MD simulations to study RO membranes in detail, something that is usually hard to perform through experiments. Being successful in studying polyamide membranes, it may be hypothesized that MD can also be used to study membrane surfaces of other types.



By: Hannah Ebro
hannah@gist.ac.kr