M.Sc Thesis

M.Sc StudentMutat Tali
SubjectAtomistic Simulation of Diffusion and Separation of Small
Molecules in Carbon Nanotubes
DepartmentDepartment of Physics
Supervisors DR. Joan Adler
Full Thesis textFull thesis text - English Version


The transport of gas mixtures through inorganic membranes, such as carbon nanotubes, is a subject of great current interest in chemical industries. The understanding of the molecular transport in carbon nanotubes is still largely incomplete. This is due to the fact that a wide variety of factors, such as the nanotube structure and diameter, the molecule dimensions and the temperature of the gas, contribute to the overall transport properties. Therefore, computational methods, such as molecular dynamics or Monte Carlo simulations are used  to determine transport mechanisms inside nanotubes.

We apply the molecular dynamics method to investigate the self diffusion of CH4 under macroscopic equilibrium through single wall carbon nanotubes (SWNTs). For this purpose, an empirical many-body classical potential, (the reactive bond order (REBO) potential), was chosen for modeling the covalent bonding between the carbon atoms of the SWNT as well as the tetrahedral structure of the diffusive molecules. Values of self diffusion coefficient, Ds, decrease with increased loading. A non-equilibrium molecular dynamics method is implemented to study transport diffusion of CH4 through SWNTs. The driving force of transport diffusion is a gradient in density of molecules between the edges of the SWNT. The diffusive molecules are treated as uniform spheres that interact with each other and with the carbon atoms of the rigid SWNT via a Lennard-Jones potential. Values of the transport diffusion coefficient, Dt, increase monotonically with increasing density gradient until the flux rate of molecules reaches saturation. The diffusivities: Dt and Ds are larger for smaller nanotube diameters - a higher curvature creates a smoother potential energy surface for the CH4 - wall interaction.

We present transport probability results for CH4, N2 and H2 diffusing as pure components or in binary and triple mixtures through narrow SWNTs as a function of counter diffusion of molecules transporting in the opposite direction. The transport probability decreases as the counter diffusion increases. The transport probabilities of CH4 and N2 were considerably diminished due to mutual inhibition, whereas the transport probability of H2 diffusing in a binary or ternary mixture was found to be only slightly lower than the values for H2 diffusing as a single component. The model based on single-file transport theory yields predictions of the transport probability in good agreement with the simulation results of CH4 and N2. The differences between the characteristics of the diffusive gases are found to be critical for the separation efficiency of a SWNT.