Adsorption equilibria of light organics on single-walled carbon nanotube heterogeneous bundles: thermodynamical aspects

Single-component adsorption equilibria of five different hydrocarbons (CH₄, C₂H₆, C₂H₄, C ₃H₈, and C₃H₆) on a high-purity sample of single-walled carbon nanotube heterogeneous bundles were measured using a static gravimetric method. The physicochemical properties and morphology of the bundles were thoroughly characterized to obtain information about the tube diameter distribution; the latter is shown to be well approximated by a Gaussian function with a mean diameter of 14.1 ± 1.4 Å. The adsorption measurements were performed at room temperature (T ≈ 300 K) and pressures from 2 × 10^-3 up to 80 bar corresponding to reduced temperatures, T/T_c, from 0.81 to 1.59 and relative pressures, p/p⁰, in the range 1.4 × 10^-5 to 0.89, where T _c and p⁰ are, respectively, the critical temperature and saturation vapor pressure of the adsorptive fluid. The excess adsorption isotherms exhibit an overall type-II shape, except that of supercritical CH ₄ which is type-I; none of the isotherms yield a hysteresis loop at the studied temperature. Henry constants, H, determined in the low-pressure region, show that the adsorbate's affinity for the solid increases with the number of carbon atoms in the molecule and decreases with the presence of an unsatured chemical bond: H(C₃H₈) > H(C ₃H₆) > H(C₂H₆) > H(C ₂H₄) > H(CH₄). The comparison of the experimental Henry constants to theoretical values obtained by molecular simulation of specific sites of the bundle shows that the experimental values lie between the simulated values for adsorption on the external surface of the bundle and those for intrabundle confinement.