Molecular Dynamics Simulation Of Diffusion Of Simple Molecules On Carbon Nanotubes

The diffusion of water through carbon nanotubes as compared to other porous materials has gained considerable attention recently due to the potential applications.the experiments have shown that the permeability of water through carbon nanotubes is much higher than the anticipated.for example,the permeability of water on the membranes of carbon nanotubes with diameters of less than 2 nm exceeds values calculated from continuum hydrodynamics models by more than three orders of magnitude.Majumder et.al.,reported some experimental results with even more enhancement.they shown that the measured water permeability on multiwalled carbon nanotubes membranes of 7 nm diameter pores is about 4-5 orders of magnitude larger than that from Hagen-Poiseuille hydrodynamics predictions.It is believed that the high diffusivity on carbon nanotubes is generally attributed to the smoothness of inner walls and hence molecular reflections from walls are specular other than diffusive,which mode the molecular diffusion in most of cases generally adheres to.The molecular dynamics simulations on dilute simple gases?e.g.Ar,N₂,CH₄?confined in carbon nanotubes showed that the diffusivities on these sub-nanometer diameter tubes are as the same magnitude as that in gaseous states at ambient conditions,but two orders of magnitude larger than the predicted diffusivities from Knudsen models.Qualitatively,for the diffusion problems of dilute gases inside CNTs,simulations could agreed with the experiments like Holts's measurements.This indicates that molecular dynamics simulations are good tools to understand such a system.Calculating the diffusivity of water on CTNs,however,encounters much challenges.due to distinct diffusion mechanism as compared to non-polar molecules.due to the strong bonding of hydrogen bonds between water molecules,water molecules experience concerted cluster movement through the tubes,which give rise the difficulties in the theoretical simulations.So far,we are still lacking a reasonable theoretical understanding for the diffusion,and for the reasons of what will be motioned later,the calculations from molecular simulation also fail to reproduce the experimental measurements in the correct order of magnitude.In the paper,based on the model of the Brownian motion,we propose a new approach to directly estimate transport diffusivity instead of the conventional procedure of calculating transport diffusivities from simulated self-diffusivity.the molecular dynamics simulation results from our model can give much high transport diffusivity,which is in good agreement in the orders of magnitude with the previous experiments.