Multiwalled carbon nanotube membranes for water purification

For the development of advanced membrane technologies, a good understanding of the materials properties and their transport mechanisms, as well as the realization of innovative functional materials with improved properties, are key issues. Due to their attractive permeability characteristics, various nano-structured RO membranes were proposed with the incorporation of carbon nanotubes. The growth in computational power has now made possible simulating tubes with characteristics closer to the real material, providing novel insights in the water flux to tube structure relationship. However, the understanding of the effect of the different number of walls, remains challenging and as a result, a deeper understanding is needed of how these changes can affect the performance of the membrane at the molecular level. To address this issue, molecular dynamic simulation (MD) is well known to be a powerful tool to enhance the understanding of nanoscale systems. This theoretical work provides new insights on the effect of walls in Multi Walled Carbon nanotube membranes, as model case of a CNT nanocomposite RO membrane, for desalination applications. Specifically, two types of vertically aligned multi Walled Carbon Nanotube membranes, MWCNT (6,6) and MWCNT (8,8), were analyzed theoretically by means of non-equilibrium Molecular Dynamics simulations, in order to study the influence of the number of walls on permeation of reverse osmosis simulations. A comparison of the two membranes formed by using differently sized tubes give us the estimation of the level of desalination and efficiency. The carbon nanotube membranes were modeled using two graphene sheets and single walled (SW), double walled (DW), three walled (TW) for both (6,6) and (8,8) configurations, and six walled (6W) for (6,6). Simulations were conducted with a hydrostatic pressure difference to investigate the ion rejection and water conductance in each membrane system. MWCNT (6,6) are very selective to ions whilst MWCNT (8.8) were more permeable to water. The different behaviour was justified in terms of the different size of entrance of CNT with a cooperative effect due to numbers of walls of CNT and the hydrophobic effect of the graphene layers. A good agreement is found if we compare our system with some functionalized SWCNT (8,8) (Corry, 2011) and with that of protein aquaporin-1 (Zhu et al., 2004). As the number of walls is augmented, the water conductance followed the same trend with a general performance greater than commercial reverse osmosis membranes (Corry, 2011). Under the conditions of our simulations, it appears that the MWCNT (8,8), particularly with DW (8,8) and TW(8,8) could offer an improvement over current generation membranes in terms of water conductance with a relatively good compromise between water conductance and salt rejection.