Adsorption of nitrogen in carbon nanotube arrays

Grand canonical Monte Carlo molecular simulations have been carried out for adsorption of nitrogen at 77 K on square arrays of open and closed single-wall carbon nanotubes with diameters of 6-30 Angstrom and nanotube separations of 4-30 Angstrom. Exohedral spaces for arrays of closed nanotubes with small tube separations are microporous. For large separations, two-stage adsorption occurs, corresponding to monolayer formation followed by a condensation step. Filling of the exohedral space is dominated by geometrical factors that allow the calculation of adsorbate molecular packing factors, which are close to that for liquid nitrogen. The amounts adsorbed in arrays of open nanotubes are higher than for closed tube arrays, as expected. The highest increase is for arrays with small tube separations and large tube diameters, where endohedral adsorption dominates; for wider tube separations the increase in amount adsorbed is less because exohedral adsorption becomes important. Arrays of single-wall carbon nanotubes have the potential to develop very high adsorptive capacities, to which the interstitial spaces between the nanotubes make significant contributions. The adsorptive capacity can be optimized when exohedral adsorption dominates, that is, with arrays of narrow single-wall nanotubes with wide spacings between them.