Acetonitrile confined in carbon nanotubes, part I: Structure, dynamic and transport properties

Abstract

In the first part of our study, here an all atom molecular dynamic study on the effect of confinement on structure, dynamic and transport properties of acetonitrile is presented. For this purpose, Single Walled Carbon Nanotubes (SWCNTs) were filled with the acetonitrile by employing isothermal-isobaric ensemble followed by canonical ensemble molecular dynamics simulations for investigating interactions between the acetonitrile and SWCNTs. Several interesting features of the acetonitrile were identified as the diameter of CNTs becomes smaller. First, two distinct regions were identified i.e., a core region along the longitudinal direction dominated by rarefaction effects and an interface shell with relatively high density of fluid. Volume of rarefied region decreases with larger values of tube diameters. Secondly, interfacial mobility in the vicinity fluid-CNT interface favors axial mean squared displacements of the acetonitrile molecules. Analyses also show that radial mobility of the molecules strongly depends on the size of the core region and diameter of the tube whereas the axial self-diffusion coefficient varies almost exponentially with the tube diameter. Thirdly, a preferred coordination between each pair of the C (methyl group carbon), C (nitrile group carbon) and N atoms, and an ordering in the vicinity of wall were observed contrary to those of larger tubes. Fourthly, we observed that the E2g mode frequencies of SWCNT dominates those of C1-C2-N bending and C1-C2 stretching modes. The frequencies of both SWCNTs and those of the fluid are the same in these modes. Lastly, the shear viscosity diminishes with the diameter of the tube. © 2020 Elsevier B.V.