Transport of water and ions at the nanoscale has attracted extensive attentions from wide research areas due to its potential applications in nanoscience and nanotechnology, such as sensing and separation, energy conversion, sea water desalination, detection and drug delivery. With substantial efforts in experimental and numerical investigations, the understanding of fluid transport at the nanoscale has achieved considerable progress in recent years. However, many unclear mechanisms and controversial results for water and ion transport at the nanoscale still need to rationalized, such as the ultrafast ion transport in carbon nanotubes (CNTs), and new transport phenomena are waiting to be discovered. Aiming at these objectives, we present numerical and theoretical investigations of...[ Read more ]

Transport of water and ions at the nanoscale has attracted extensive attentions from wide research areas due to its potential applications in nanoscience and nanotechnology, such as sensing and separation, energy conversion, sea water desalination, detection and drug delivery. With substantial efforts in experimental and numerical investigations, the understanding of fluid transport at the nanoscale has achieved considerable progress in recent years. However, many unclear mechanisms and controversial results for water and ion transport at the nanoscale still need to rationalized, such as the ultrafast ion transport in carbon nanotubes (CNTs), and new transport phenomena are waiting to be discovered. Aiming at these objectives, we present numerical and theoretical investigations of water and ion transport in CNTs and slit nanochannels.

First, we studied ion effects on water diffusion in CNTs by performing molecular dynamics (MD) simulations. The diffusion coefficient of water molecules in the presence of cations (Na⁺ and K⁺) and anions (F^-, C1^-, and Br^-) is found highly nonlinear with ion concentration and distinct for different ions. For positively charged systems, as the ion concentration is varied, water diffusion coefficient assumes a maximum under the competition between the number and orientation changes of free OH bonds and the effects of ionic hydration. For negatively charged systems, however, water diffusion coefficient decreases monotonically with increasing ion concentration for F^-. For C1^- and Br^-, water diffusion coefficient reaches the minima at certain ion concentrations and then gently increases. The different behaviors of water diffusion coefficient in the presence of different anions are caused by the stability change of water hydrogen bonds due to ionic hydration.

Second, a method for flow control in nanochannels based on changing external electric field strength is proposed and verified by simulation results. We investigated the water motion of CsF solutions under external electric fields in slit nanochannels. It is found that the water flow strongly depends on the channel surface properties. In channels of low surface energy, the flow direction of water can be altered by varying the strength of the electric field E. Under weak electric fields, water molecules, on average, move in the opposite direction to E mainly due to the motion of F^- ions. However, the flow direction is changed when the electric field becomes sufficiently strong. The flow direction change is caused by the migration of F^- ions toward the surface as the electric field is strengthened, which makes the transport of Cs⁺ dominant.

Third, to explore the mechanisms for fast ion transport in CNTs, we combine MD simulation and theoretical analyses to investigate ion transport under external electric fields through 1.36 nm-diameter CNTs with lengths ranging from 9.8 to 984 nm. It is shown that the ion-water liquid chain inside CNT can break up into small liquid clusters under certain conditions. In the liquid breakup regime, ionic mobility and conductance can be greatly enhanced due to the reduction of water density and the constraint of hydrogen bonds inside CNTs, and it confirms the two-order-magnitude enhancement reported by previous experimental studies. A simple criterion based on Poisson-Boltzmann equation for the liquid breakup inside CNT is developed and it is well consistent with simulation results. These results provide new insights for the future development of CNT-based nanofluidic devices.