Molecular Dynamic Simulation Of Salt Rejection And Water Transport Through DWCNT

Reverse-osmosis (RO) is one of the important technology for large-scale seawater desal-ination, where RO membrane with high water flux and salt rejection is required. There has been an increasing interest in Carbon nanotube (CNT) because of its high performance in wa-ter transport and ion selection. Doping CNTs into RO membrane is an effective way to im-prove the performance. Compared to single-walled CNTs (SWCNTs), double-walled CNTs (DWCNTs) have higher mechanical strength and better thermal and chemical stability, thus DWCNT was adopted in this work. We performed molecular dynamics (MD) simulation to study the structures and dynamics of water molecules and ions in several systems composed of the DWCNTs with different sizes. The neutral DWCNT of armchair type was used to build the RO membrane, and the sodium chloride aqueous solution of concentration 0.5 mol·L-1 was used to mimic seawater. The chiral index of the inner-wall was fixed at (8,8). A constant force was imposed on the salt aqueous solution to produce osmotic pressure. In the study, we inves-tigated salt rejection and water flux in the monolayer RO membrane systems with the DWCNTs of different sizes, and calculated the potential of mean force, hydrogen bonding lifetime and dipole moment for water molecules. Additionally, three water models (TIP3P, TIP4P and SPC/E) were adopted to study the effect of water model on the simulation results.It was found that inter-layer spacing has effect on the nano-structure and dynamics of water molecules (i.e. hydrogen bonding lifetime, potential of mean force and dipole moment distribution) in the channel. It was shown that the permeabilities of water molecules and ions vary with the size of CNT. Our work indicated that the water could be separated completely from the sodium chloride aqueous solution in small-sized DWCNTs (inter-layer spacing was less than 0.680 nm), but the water flow was smaller than others. By contrast, large-sized DWCNTs with the inter-layer spacing greater than 0.815 nm could not prevent salt ions pene-trating into the bulk water, but the water flow in it was greater than other DWCNTs. Mid-dle-sized DWCNTs with the inter-layer spacing of 0.680～0.815 nm combined the both ad-vantages that the water flow closes to the large-sized DWCNTs and ions rejection reaches 100%. It was found that DWCNTs with an inter-layer spacing of 0.815 nm gave the optimum balance between water flux and salt rejection. Salt rejection, water transport and hydrogen bonding lifetime of DWCNTs could be affected by different water models. However, these properties of DWCNTs in RO membrane existed similar trends in all simulations. It turns out that TIP3P water model was suitable for RO simulation because of the high water permeabili-ties and shorter hydrogen bonding lifetime.The results from MD simulations provide molecular insight into the application of DWCNTs in desalination, which facilitates the improvement of the reverse-osmosis mem-branes with high performance in water transport and salt rejection.