Comparison of Structure and Dynamics of Polar and Nonpolar Fluids through Carbon Nanotubes

The behavior of fluids inside the nanotubes largely depends on the extent of confinements and polarity of the fluid under investigation. We employ extensive molecular dynamics (MD) simulations to make a comparative analysis of structure and dynamics of polar, hydrogen-bonded water and nonpolar methane molecules inside the carbon nanotube (CNT), with the intention to find out differences if any in structure, diffusion, and translocation of a polar and nonpolar fluid. We find that transport events occur in bursts of unidirectional translocation pulses in both of the cases. The present result therefore reveals that the pulse-like conduction of fluid molecules through the nanochannel does not have any relation with hydrogen bond forming ability of the fluid molecules. It may be attributed to the single-file arrangement of the fluid molecules inside the CNT. It is interesting to note that the empty–filled transitions inside CNT as observed in the case of water are not observed in the case of methane. The self-diffusivity, as well as rate of translocations along the nanotube axis, is found to be higher in the case of a nonpolar fluid (methane) than in the case of a polar fluid (water) for the same CNT diameter. The results obtained for methane are applicable to any other nonpolar, non-hydrogen-bonded fluid, as demonstrated by considering argon as another nonpolar fluid.