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

Simulation results are reported for the adsorption of pure pentane (C-5) 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 nC(5)< iC(5)< neoC(5) 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 nC(5)>= iC(5)> neoC(5) as a result of the configurational entropy effect. In smaller SWNTs, depending on the diameter, only nC(5) adsorbs, or no adsorption at all occurs. The entropy effects are found to lead to a large adsorptive separation among the C-5 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. (c) 2006 American Institute of Physics.