(688a) Regioselective Competitive Adsorption of Water and Organic Vapor Mixtures On Heterogeneous Bundles of Single-Walled Carbon Nanotube: Experimental and Simulation Study

Sequential adsorption of water and organic vapor mixtures onto single-walled carbon nanotube (SWNT) bundles is studied experimentally and by grand canonical Monte Carlo (GCMC) simulation to elucidate the distinct interactions between the select adsorbates and the nanoporous structure of SWNTs. First, we developed a practical method for adsorption modeling of heterogeneity of SWNTs. Our method integrates experimental analysis with GCMC simulation of simple molecules, such as nitrogen, at 77 K. Using this method it is possible to estimate adsorption inside the nanotubes, adsorption on the external surface of the bundles, and adsorptive contributions from the impurities present in samples (Agnihotri et. al., J Phys Chem C, 2007). Using this methodology, we conducted a study on competitive adsorption of water and organic vapors to elucidate the distinct interactions between the select adsorbates and the nanoporous structure of SWNTs (Agnihotri et al, Langmuir, 2008). Experimental adsorption isotherms on SWNT bundles for hexane, methyl ethyl ketone, cyclohexane and toluene individually mixed in carrier gases that were nearly saturated with water vapor are compared with the GCMC simulated isotherms for hexane (chosen as a representative organic molecule) on only the external surface of the heterogeneous SWNT bundles. From the nearly perfect overlap between the experimental and simulated isotherms, it is concluded that until near saturation only the internal pore volume of pristine SWNT bundles fills with water. The adsorption of water vapor on the peripheral surface of the bundles remains insignificant, if not negligible, in comparison to the adsorption of water in the internal volume of the bundles. This is in contrast with the adsorption of pure hexane, which exhibits appreciable adsorption both inside the bundles as well as on their external surface. It is also suggested that during competitive adsorption, water molecules take precedence over small non-polar and polar organic molecules for adsorption inside SWNTs and leave unoccupied the hydrophobic external surface of the bundles for other more compatible adsorbates.