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A molecular dynamics investigation of water and ion transport through model carbon nanotubes Liu, Lin


In this dissertation, we investigate water and ion transport through carbon nanotubes using molecular dynamics simulations. Specifically, we examine how different water models influence the simulated conduction rates. We consider three common water models, which are TIP4P/2005, SPC/E, and TIP3P, and observe that water flow rates through the same nanotube are strikingly different amongst the different water models. Also, the water flow rate dependence on temperature fits an Arrhenius-type equation over a temperature range from 260 to 320 K. We provide evidence that there are two factors which determine the conduction rate: the bulk fluid mobility, and the molecular structure of confined water. For narrow nanotubes, for example, a (6,6) nanotube, where water only forms a single-file configuration, the first factor can largely account for the flow rate differences. In this case, we show that the conduction rate correlates with the diffusion coefficient of bulk water. Our simulation results are well described by continuum hydrodynamics as well. The factor of bulk fluid mobility is still important in the water conduction through intermediate-size nanotubes, such as a (9,9) nan- otube. Also, the formation of complex configurations within such nanotubes can impede the transport rate by influencing the mode of water conduction. The ordered structure occurring within nanotubes can also explain the differences between simulation results and continuum hydrodynamics predictions. Hence, both factors decide the water conduction rates through intermediate-size nanotubes. Moreover, we demonstrate that the ion flow rate depends on the viscosity of the bulk solution, as well as the water structure within the nanotubes, together with the ion size. In particular, at lower temperatures complex water configurations act to impede ion transport while still allowing water to flow at a significant rate. In general, our efforts on this issue are of importance for future simulation studies investigating water and ion conduction through nanoscopic channels. This dissertation might also prove useful in designing more efficient nanoscopic conduits for future experimental studies.

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