Adsorption of light alkanes and alkenes onto single-walled carbon nanotube bundles: Langmuirian analysis and molecular simulations

Grand canonical Monte Carlo (GCMC) simulations are employed to study the adsorption equilibrium properties of methane, ethane, ethylene, propane, and propylene onto homogeneous bundles of single-walled carbon nanotubes, at room temperature, from 10−4 bar up to 90% vapor pressure. Individual adsorption isotherms for the internal volume of a bundle and for its external adsorption sites are separately calculated for individual nanotube diameters in the range 11.0 Å ≤ D ≤ 18.1 Å. External adsorption is further decomposed into the contributions from its two main adsorption sites - external grooves and exposed surfaces of the peripheral tubes - based on a geometrical model for the average groove volume that takes into account the molecular nature of the adsorbate. Both intrabundle confinement and adsorption onto the grooves lead to type I isotherms, which are modeled with Langmuirian-type equations. Adsorption on the exposed surfaces of the peripheral tubes in a bundle gives rise to a type II isotherm, which is described by the BET model with a finite number of adsorbed layers. The linear combination of the Langmuir isotherm model for adsorption onto groove sites and the BET isotherm model produces a composite isotherm that is in good agreement with the GCMC isotherm for overall adsorption onto the external sites of a bundle. The influence of adsorbate molecular length and existence of an unsaturated chemical bond in its molecular skeleton are studied by monitoring the dependence of the Henry constant and zero-coverage isosteric heat of adsorption with the dispersive energy for the solid-fluid pair potential of each adsorbate. Our results show that the adsorptive properties are especially influenced by the presence of a double bond in the case of small molecules, such as the ethane/ethylene pair.