Surface area measurements with linear adsorbates: an experimental comparison of different theoretical approaches.

The specific area of a substrate was determined from the results of adsorption isotherms performed with a sequence of four alkanes, from methane to butane, using three different approaches. The data were first analyzed using the BET equation and the point B methods; these results were compared with those obtained using a new equation designed for examining the case of multisite occupancy. The new model specifically accounts for sites that are left uncovered in the case of adsorption by linear adsorbates. Of these three, only the last method gives essentially the same value for the specific surface area of the substrate when different adsorbates are used to measure it. The other two, more traditional, approaches give values of the specific surface area that decrease as the length of the adsorbate used increases.

Study of a butane monolayer adsorbed on single-walled carbon nanotubes.

We present the results of a study of first-layer butane films adsorbed on single-walled carbon nanotubes. We measured 12 isotherms between 180 and 311 K. Butane molecules bind more strongly than shorter alkanes to the nanotubes. We measured a value of 391 meV for the low-coverage isosteric heat of butane. This value is 1.21 times larger than that for butane adsorbed on planar graphite and 1.27 times larger than the value for ethane on nanotubes at comparable coverages. We also compared the characteristics of the adsorption isotherms for butane with those we determined for ethane at the same relative temperatures. This comparison allowed us to infer that there is a change in the adsorption behavior of linear alkanes which occurs as a function of increasing carbon chain length. While ethane isotherms display two substeps in the first layer (corresponding to adsorption on different groups of adsorption sites), one of these steps is significantly smeared for butane isotherms, becoming essentially impossible to resolve above 220 K.

Dependence of single-walled carbon nanotube adsorption kinetics on temperature and binding energy.

We present results for the isothermal adsorption kinetics of methane, hydrogen, and tetrafluoromethane on closed-ended single-walled carbon nanotubes. In these experiments, we monitor the pressure decrease as a function of time as equilibrium is approached, after a dose of gas is added to the cell containing the nanotubes. The measurements were performed at different fractional coverages limited to the first layer. The results indicate that, for a given coverage and temperature, the equilibration time is an increasing function of E/(k(B)T), where E is the binding energy of the adsorbate and k(B)T is the thermal energy. These findings are consistent with recent theoretical predictions and computer simulations results that we use to interpret the experimental measurements.

Argon adsorption on Cu3(benzene-1,3,5-tricarboxylate)2(H2O)3 metal-organic framework.

Using volumetric adsorption techniques, we have measured the adsorption of argon on Cu3(BTC)2(H2O)3, (BTC = benzene-1,3,5-tricarboxylate), a microporous metal-organic framework structure, at temperatures between 66 and 143 K. In addition to the experiments, we have used Grand Canonical Monte Carlo simulations to calculate the adsorption isotherm of argon at 87 K. Our experimental and theoretical results are compared to those of previous studies. The experiments were performed using a high density of points, allowing us to obtain, in detail, the isosteric heat's coverage dependence. Our values from the simulations are in reasonable agreement with those obtained in the experiments.

Higher coverage gas adsorption on the surface of carbon nanotubes: evidence for a possible new phase in the second layer.

We present adsorption isotherm results for Ne, CH4, and Xe on bundles of close-ended single-wall carbon nanotubes, for coverages above the completion of the first layer. We find a small, sharp, substep present in the second-layer data for Ne and CH4, and a weaker feature, that produces an isothermal compressibility peak, for Xe. The size and location of the feature allows its tentative identification as a new, second-layer, one-dimensional phase, in which the atoms sit atop high binding energy sites in the second layer.

Possible existence of a higher coverage quasi-one-dimensional phase of argon adsorbed on bundles of single-walled carbon nanotubes.

We present results of Ar adsorption isotherms at very low coverages in the first layer and, beyond monolayer completion, on bundles of close-ended single-walled carbon nanotubes. The low coverage results were used to determine the isosteric heat of adsorption and the binding energy of Ar in the groove sites in the first layer. The higher coverage results show evidence of the possible formation of a second-layer groove phase, beyond monolayer completion. Our results for higher coverages are compared with recent computer simulations for this system.

Existence of novel quasi-one-dimensional phases of atoms adsorbed on the exterior surface of close-ended single wall nanotube bundles.

We present results of adsorption measurements for Xe and Ar which confirm experimentally the formation of one-dimensional phases for these gases when adsorbed on the outer surfaces of close-ended single-wall carbon nanotube (SWNT) bundles. The existence of such phases had been predicted in recent computer simulations, but had remained, until the present work, unconfirmed. Experimental results for Xe and Ar on close-ended bundles of SWNT's are compared to, and found in substantial quantitative agreement with, those obtained in the computer simulation studies. The characteristics of the phases formed appear to be strongly influenced by steric effects.