(23c) Quasi-One-Dimensional Adsorption of Alkanes on Carbon Nanotubes Observed from Experiments and Simulations

Adsorption of alkanes on carbon nanotubes has been studied experimentally using temperature programmed desorption (TPD) and theoretically using molecular simulations. Linear alkanes, ranging from n-pentane to n-nonane and one branched alkane, 2,2,4-trimethylpentane, have been studied. The TPD spectra for each of the alkanes display four peaks. The highest temperature peak is assigned to molecules adsorbed in the internal sites of the individual nanotubes. The next highest peak is assigned to adsorption in groove sites, where two tubes meet on the external surface of a bundle. The assignment of these peaks is made based on analysis of the experimental data and by comparison with simulation data. Integration of the TPD peaks gives a linear trend with carbon number for the relative space occupied by a molecule for each of the n-alkanes in the groove site. However, the peak area for the branched alkane does not follow this trend, occupying much less space than n-octane, which has the same number of carbons. Computer simulations show this same trend for molecules adsorbed in the groove site. These results demonstrate the one-dimensional nature of the groove site for adsorption, since the space occupied by a molecule is proportional to the chain length, not to the number of carbons in the chain. Similar work for the highest temperature peak also gives a linear trend for the relative space occupied with the carbon number for linear alkanes in the interior sites. However, the experimental data for 2,2,4-trimethylpentane cannot be interpreted, due to peak overlap. Simulation data show that the branched alkane falls on the same line as the linear alkanes for interior site adsorption. The interior site, therefore, is quasi-one-dimensional, since the volume occupied by a molecule is proportional to the number of segments, not to the length of the molecule.