Model calculations are presented which predict whether or not an arbitrary gas experiences significant absorption within carbon nanotubes and/or bundles of nanotubes. The potentials used in these calculations assume a conventional form, based on a sum of two-body interactions with individual carbon atoms; the latter employ energy and distance parameters which are derived from empirical combining rules. The results confirm intuitive expectation that small atoms and molecules are absorbed within both the interstitial channels and the tubes, while large atoms and molecules are absorbed almost exclusively within the tubes.Comment: 9 pages, 12 figures, submitted to PRB Newer version (8MAR2K). There was an error in the old one (23JAN2K). Please download thi

An overview is presented of the various phases predicted to occur when gases are absorbed within a bundle of carbon nanotubes. The behavior may be characterized by an effective dimensionality, which depends on the species and the temperature. Small molecules are strongly attracted to the interstitial channels between tubes. There, they undergo transitions between ordered and disordered quasi-one dimensional (1D) phases. Both small and large molecules display 1D and/or 2D phase behavior when adsorbed within the nanotubes, depending on the species and thermodynamic conditions. Finally, molecules adsorbed on the external surface of the bundle exhibit 1D behavior (striped phases), which crosses over to 2D behavior (monolayer film) and eventually 3D behavior (thick film) as the coverage is increased. The various phases exhibit a wide variety of thermal and other properties that we discuss here.

We explore the behavior of neon, xenon, and methane films adsorbed on the external surface of a bundle of carbon nanotubes. The methods used are classical: a ground state calculation, by grand potential energy minimization, and the grand canonical Monte Carlo (GCMC) method of simulation. Our results are similar to those found recently in a GCMC study of Ar and Kr. At low chemical potential (pressure) the particles form a quasi-one dimensional phase within the groove formed by two contiguous tubes. At higher chemical potential, there occurs a "three-stripe" phase aligned parallel to the groove (except for xenon). This is followed by monolayer and bilayer phases. The low temperature monolayer phase is striped; the number of stripes per nanotube is a quantized function of the adatom size. In the neon case, the bilayer regime also includes a second layer groove phase. Our results are compared with recent thermal and diffraction experiments. We find no evidence of a zig-zag phase reported recently.

Grand canonical Monte Carlo simulations have been performed to determine the adsorption behavior of Ar and Kr atoms on the exterior surface of a rope ͑bundle͒ consisting of many carbon nanotubes. The computed adsorption isotherms reveal phase transitions associated with the successive creation of quasi-one-dimensional lines of atoms near and parallel to the intersection of two adjacent nanotubes.

Amount of chemisorbed HzO, OH groups n d Figure 8. Relation between electrostatic field strength and surface H20 content on each of two kinds of surfaces: ( 0 ) (110) surface; (0) (100) + (101) surface.other hand, the F value of the (110) plane, F(,,,), remains constant at 1.54 X lo4 statvolt cm-l at evacuation temperatures from 25 to 150 OC. The F(llo, value then increases almost linearly with decreasing amount of surface hydroxyls, which also impliesthe (110) surface to be homogeneous. Strictly speaking, on the extremely dehydroxylated surfaces the F(',,) curve deviates slightly to higher values from the straight line, probably because af the presence of active sites or various kinds of surface defeds. In conclusion, the two kinds of surfaces, (110) and (100) + (101), are found to be homogeneous, respectively. Furthermore, it is interesting to see that the F(llo) value on the bare (110) surface is 3.26 X lo5 statvolt cm-', being remarkably larger than the average F value of the actual surface (Figure 4).In the bulk of the rutile crystal, the coordination numbers of Ti4+ and 02-are 6 and 3, respectively, so that the formal charges of Ti4+ and 02-are +2/3e and -2/3e per bond, respectively, where e is the electronic charge. On the other hand, in the bulk of the wurtzite crystal the coordination numbers of both Zn2+ and 02-are 4, so that the formal charges of Zn2+ are 02-is + 1 / 2 e and -1/2e per bond, respectively. Thus it can be expected that the F value of the bare (110) surface of rutile is larger than that of the bare (10x0) plane of ZnO. However, the experimental F values of rutile and ZnO are found to be almost equal, Le., 3.26 X lo5 and 3.38 X lo5 statvolt cm-l, respectively. Upon the analysis procedure of heat of adsorption stated above, it is assumed that the heat values of the organic adsorbates on the hydroxylated (110) surface are identical with those on the hydrated (100) + (101) surface, but in practice the former may be larger than the latter. With this point of view taken into account, the true F value of the (110) surface of rutile may be larger than the F(',,, value shown in Figure 8. Acknowledgment. I thank Professor Tohru Takenaka of Kyoto University for his kind advice and helpful suggestions and Professor Mahiko Nagao of Okayama University for much help in the heabof-immersion experiments. I also thank Professor Tetsuo Morimoto of Okayama University for his helpful discussions and constant encouragement throughout this work. RegistryLow-coverage isotherm data for the adsorption of 0 2 on exfoliated graphite are reported and analyzed by using the virial adsorption isotherm. The molecule-solid and the adsorbed molecule-molecule second virial coefficients are obtained for this system over a range of temperature. These data are compared with theory, and best-fit parameters for the site-site moleculesolid interaction potential are given. In addition, a comparative study of the molecule-molecule interactions for 02, N2, and CO is presented, based on the molecule-molecule virial coefficients rep...