The adsorption of gaseous molecules on single wall carbon nanotubes (SWCNT) have been investigated by many researchers [1,2]. This is due to the importance of the role of gas-SWCNT interaction in various areas as nanotubes production [3], sensors fabrication [1], fuel cells [2], and drug delivery [4]. In the present work, we study the adsorption of methane (CH4) and tetrafluoromethane (CF4) molecules on the inside and outside of zigzag SWCNTs. Our interest on the CH4 and CF4 rely on the fact that they are top symmetric molecules with different chemical behaviours. It is expected that new aspects of gas-nanotube interaction are revealed by comparison the adsorption behaviour of CH4 and CF4 molecules on the SWCNTs. Although several articles have investigated, theoretically, the adsorption of methane on SWCNTs and graphite, in the case of CF4 there is little attempt to investigate its interaction with SWCNTs. Investigation of the diffusion of methane inside SWCNTs employing Lennard-Jone potentials has been determined that the (11,0) tube has the optimal diameter to perfectly fit methane [5]. Bausclicher and Ricca studied the adsorption of methane on graphite and (9,0) SWCNT employing a cluster model and MP2 methodologies [6]. Their results were in good agreement with experiment. The work of Agraw et al. is one of the few works that considered the intern adsorption of methane and gave results that were in good agreement with experiment [7]. In the present work, we have studied the external and internal adsorptions of CH4 and CF4 on SWCNTs to understand the effect of orientation and size of SWCNT on the adsorption process of these molecules.
