(28d) Applicability of Water-Activated Carbon Isotherm Models to Water-Carbon Nanotube Isotherms

The applicability of established water-activated carbon interaction theory to water-carbon nanotube interactions remains to be fully explored. We have conducted sensitive experiments of adsorption kinetics and equilibrium of water on purified and treated single-walled carbon nanotubes (SWNTs) procured from 5 commercial suppliers. Experiments were conducted using a customized set-up that consisting a gas generation system, high sensitivity gravimetric balance (detection limit = 0.1 µg) and a data acquisition system. Adsorption was measured in an open-system in which a carrier gas of a known quantity of vapor continuously flows over the sample while the sample weight is being monitored in real time. Isotherms for each sample (3 ? 4 mg) were obtained point-by-point at approximately 20 intermediate concentrations between 2% to 95% relative humidity (RH) by administering adsorption kinetics and then extracting equilibrium capacity at each data point. Triplicate runs were conducted for statistical purposes. The isotherms for most samples were type V in shape. This is similar to water-activated carbon isotherms. This isotherm characteristic arises from water adsorbing onto functional groups and water adsorption by conventional pore filling. We have applied several isotherm models, such as the D.S. model and D.D Do model to isotherms obtained for all SWNT samples. These models account for the semi-chemisorption characteristic of water, and were originally developed for water adsorption in activated carbon. The D.D.Do describes water adsorption by deconvoluting experimental adsorption into separate contributions from adsorption on functional groups and micropore filling. The micropore volume by water adsorption was identical to that by standard N2 (77 K) adsorption for all samples. It is emphasized that the application of LDF kinetics model and isotherm models other than the DS equation to water adsorption in carbon nanotubes is not tractable in the present literature. These are common water-activated carbon models and their applicability to water-nanotube system remains unexplored. Additionally, our high-sensitivity analytical techniques can provide can provide an insightful examination of the realism of the methodology and inter-molecular potentials commonly used in molecular modeling water and nanotubes, which has become the most common tool for studying adsorption of water.