Sandeep Agnihotri

Application of water-activated carbon isotherm models to water adsorption isotherms of single-walled carbon nanotubes.

The objective of this study is to understand the interactions of water with novel nanocarbons by implementing semiempirical models that were developed to interpret adsorption isotherms of water in common carbonaceous adsorbents. Water adsorption isotherms were gravimetrically determined on several single-walled carbon nanotube (SWNT) and activated carbon samples. Each isotherm was fitted to the Dubinin-Serpinsky (DS) equation, the Dubinin-Astakov equation, the cooperative multimolecular sorption theory, and the Do and Do equations. The applicability of these models was evaluated by high correlation coefficients and the significance of fitting parameters, especially those that delineate the concentration of hydrophilic functional groups, micropore volume, and the size of water clusters. Samples were also characterized by spectroscopic and adsorption techniques, and properties complementary to those quantified by the fitting parameters were extracted from the data collected. The comparison of fitting parameters with sample characterization results was used as the methodology for selecting the most informative and the best-fitting model. We conclude that the Do equation, as modified by Marban et al., is the most suitable semiempirical equation for predicting from experimental isotherms alone the size of molecular clusters that facilitate adsorption in SWNTs, deconvoluting the experimental isotherms into two subisotherms: adsorption onto hydrophilic groups and filling of micropores, and quantifying the concentration of hydrophilic functional groups, as well as determining the micropore volume explored by water. With the exception of the DS equation, the application of other water isotherm models to SWNTs is not computationally tractable. The findings from this research should aid studies of water adsorption in SWNTs by molecular simulation, which remains the most popular tool for understanding the microscopic behavior of water in nanocarbons.

Regioselective competitive adsorption of water and organic vapor mixtures on pristine single-walled carbon nanotube bundles.

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 select adsorbates and the nanoporous structure of SWNTs. 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, as a representative organic, on 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 and on their external surface. It is also suggested that during competitive adsorption, water molecules take precedence over small nonpolar and polar organic molecules for adsorption inside SWNTs and leave unoccupied the hydrophobic external surface of the bundles for other more compatible adsorbates.

Aging of Nanocarbons in Ambient Conditions: probable metastability of carbon nanotubes

We studied the physicochemical properties of several commercially available single- and multi-walled carbon nanotubes (SWNTs and MWNTs) and fullerenes stored in normal ambient conditions for 24 months. We found that SWNTs exhibit a trend of decreasing surface area and pore volume up to 7-15 months but then stabilized, no longer being impacted by sample age or outgassing temperatures. Using X-ray Photoelectron Spectroscopy, we also observed a trend of decreasing surface oxygen in all samples from the beginning with much lower % oxygen observed after 12-15 months of aging under ambient conditions. The surface oxygen then stabilized for the duration of this study. There was also evidence that the total structural-defect concentration, estimated from Raman spectroscopy, was somehow lowered during the aging process. The decrease in surface oxygen is an unexpected phenomenon because most other carbons, such as activated carbons or carbon molecular sieves, either oxidize or remain unaffected by age. We believe that nanocarbons are meta-stable materials (in pseudo-thermodynamic equilibrium), and that their aging in ambient conditions makes them more thermodynamically stable with fixed properties. This new information about the properties of nanocarbons should be further explored as it can help resolve some of the conflicting reports such as those about the environmental impacts of nanomaterials.