Shape versus inverse-shape selective adsorption of alkane isomers in carbon nanotubes

Abstract

Simulation results are reported for the adsorption of pure pentane (C5) isomers and their ternary mixture in a series of open-ended armchair-type (m,m) single-walled carbon nanotubes (SWNTs). Inverse-shape selective adsorption occurs in the order of nC5<iC5<neoC5 only in the (7,7) SWNT as a result of the length entropy effect. In the larger (20,20) and (10,10) SWNTs, shape selective adsorption occurs in the order of nC5>or=iC5>neoC5 as a result of the configurational entropy effect. In smaller SWNTs, depending on the diameter, only nC5 adsorbs, or no adsorption at all occurs. The entropy effects are found to lead to a large adsorptive separation among the C5 isomers from their mixture. Using the ideal-adsorbed-solution theory with data on the adsorption of only the pure isomers, we predict mixture adsorption. The agreement between predictions and simulations deteriorates with decreasing diameter of the SWNT.