(397bc) Assessing the Potential Permeability and Salt Rejection of Membranes Incorporating Carbon Nanotubes

The incorporation of carbon nanotubes (CNTs) into water filtration membranes has been suggested as a potential route to improve the efficiency of water filtration and desalination processes. Experimental studies have shown that water and gases are able to move through CNTs at rates considerably higher than expectations derived from continuum hydrodynamic theories.Simulations have also suggested that CNTs may be able to prevent the passage of salts and large solutes, which when coupled with rapid transport could provide an ideal basis for new filtration membranes.

While considerable progress has already been made in fabricating these materials, a number of questions have to be answered if this technology is to be realised. Using molecular dynamics simulations we have addressed the following issues:

1. Why does water pass so quickly through CNTs?
2. What is the theoretical maximum membrane permeability that could be achieved?
3. Why are salts rejected by some CNTs and how can this rejection be improved?
4. How does CNT diameter and length influence water transport and salt rejection.
5. How does chemical functionalisation influence transport and rejection?

Rapid water flux is seen to result from the near frictionless walls of the CNT which in turn is due to their physical smoothness and lack of localised electrostatic interactions. Salt rejection arises in narrow CNTs as a consequence of the energetic costs of ions having to dehydrate in order to fit inside the pores. An upshot of this is that salt rejection is strongly related to the CNT diameter. Appropriate chemical modification of the CNTs can improve salt rejection by simple charge repulsion or attractive binding interactions. However any chemical modification is likely to reduce the water transport rate by increasing the friction of the walls. Together these results indicate some of the design principles that should be considered when fabricating membranes incorporating carbon nanotubes for water filtration applications.