Adsorption (2009) 15: 1–12 DOI 10.1007/s10450-009-9154-0 Behavior of ethylene and ethane within single-walled carbon nanotubes. 1-Adsorption and equilibrium properties Fernando J.A.L. Cruz · Erich A. Müller Received: 20 March 2008 / Revised: 13 January 2009 / Accepted: 9 February 2009 / Published online: 28 February 2009 © Springer Science+Business Media, LLC 2009 Abstract Endohedral adsorption properties of ethylene and ethane onto single-walled carbon nanotubes were investi- gated using a united atom (2CLJQ) and a fully atomistic (AA-OPLS) force fields, by Grand Canonical Monte Carlo and Molecular Dynamics techniques. Pure fluids were stud- ied at room temperature, T = 300 K, and in the pressure ranges 4 × 10 −4 <p< 47.1 bar (C 2 H 4 ) and 4 × 10 −4 < p< 37.9 bar (C 2 H 6 ). In the low pressure region, isotherms differ quantitatively depending on the intermolecular poten- tial used, but show the same qualitative features. Both po- tentials predict that ethane is preferentially adsorbed at low pressures, and the opposite behavior was observed at high loadings. Isosteric heats of adsorption and estimates of low pressure Henry’s constants, confirmed that ethane adsorp- tion is the thermodynamically favored process at low pres- sures. Binary mixtures of C 2 H 4 /C 2 H 6 were studied under several (p, T ) conditions and the corresponding selectivities towards ethane, S , were evaluated. Small values of S< 4 were found in all cases studied. Nanotube geometry plays a minor role on the adsorption properties, which seem to be driven at lower pressures primarily by the larger affinity of the alkane towards the carbon surface and at higher pres- sures by molecular volume and packing effects. The fact that the selectivity towards ethane is similar to that found earlier on carbon slit pores and larger diameter nanotubes points to the fact that the peculiar 1-D geometry of the nanotubes provides no particular incentive for the adsorption of either species. F.J.A.L. Cruz · E.A. Müller ( ) Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK e-mail: e.muller@imperial.ac.uk Keywords Adsorption · Molecular simulation · Carbon nanotubes · Ethylene · Ethane · Grand canonical Monte Carlo · Molecular dynamics 1 Introduction Separations of close boiling point mixtures, such as the ethylene/ethane mixture, are amongst the most energy- intensive unit operations in chemical and petrochemical plants, where they usually involve low-temperature distil- lation techniques. For this type of separations, encompass- ing structurally and/or energetically similar molecules, al- ternative techniques are actively sought, particularly for small-scale applications such as those used in the phar- maceutical, biomedical and aerospace industry (Noble and Agrawal 2005). While adsorption using nanoporous mate- rials is posed to be considered as an option, our current molecular-level knowledge of the physical phenomena in- volved in the adsorption and diffusion of fluids in confined nanospaces still needs improvement. Molecular simulation has played a key role in under- standing the intricate details of the cooperative adsorption of ethane and ethylene. To date, two types of adsorbents have been proposed for the separation of this kind of mixtures; zeolites (Al-Baghli and Loughlin 2006; Da Silva and Ro- drigues 1999) and π -complexation sorbents (Yang 2003). The separation using zeolites is kinetic in nature, i.e. the separation is based on the distinct diffusivities of the species. On the other hand, π -complexation sorbents are designed by doping conventional adsorbents with cations (usually Cu + or Ag + ) that have a high affinity with the double bond present in alkenes. While selectivity in these latter adsor- bents is high, regeneration of the adsorbent is an issue if continuous processing is required. In this context, the quest