Mary J. Bojan

Uptake of gases in bundles of carbon nanotubes

Model calculations are presented that 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 that 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.

Quasi-one- and two-dimensional transitions of gases adsorbed on nanotube bundles

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 (bundle) 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.

Computer simulation of physisorption on a heterogeneous surface

Abstract The technique of molecular dynamics was used to determine the structures of krypton physisorbed on a simple heterogeneous surface. The solid was modelled as an infinite set of straight square-walled grooves cut into an otherwise flat surface. The computed local densities show that the atoms adsorb as one-dimensionally ordered rows near the walls of the grooves. This order decays with increasing distance from the walls. Also, the atoms adsorbed on the steps exhibit little order at the simulation temperature of 110 K.

Phases of neon, xenon, and methane adsorbed on nanotube bundles

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 zigzag phase reported recently.

Computer Simulation Studies of the Storage of Methane in Microporous Carbons

Abstract Simulated isotherm and energies of adsorption are reported for methane in a number of model porous solids at 300 K. The solids are made up of graphite basal planes arranged to make either parallel-walled slit pores or pores of triangular cross section. The limiting low coverage behavior was characterized by direct calculations of Henrys law constants and average gas–solid energies for the pore systems considered. The isotherms were evaluated for pressures ranging up to 50 atm by utilizing the Widom particle insertion algorithm. The simulations and calculations were carried out for a range of pore sizes and, in the case of the triangular cross-section, for a range of apex angles in the isosceles triangles considered. Methane storage capacities of model solids were evaluated for values of the porosity based on two different choices of pore wall thickness. Although it is shown that adsorption is not limited to monolayer formation under these conditions, capacities obtained are not sufficiently larg...

Simulation studies of sorption in model cylindrical micropores

Abstract Simulations of the thermodynamic properties of simple gases in straight-walled cylindrical micropores are discussed. To begin, various models used for the gas atom/pore wall interaction energy are presented. These include the hard-wall cylinder, the interaction in a completely smooth but attractive/repulsive cylinder that is obtained by integrating over Lennard–Jones sitewise interactions, and those generated by summing over Lennard–Jones atoms in the solid surrounding the cylinder. Simulations of such systems are described, particularly those carried out using the Grand Canonical Monte Carlo algorithm since these yield sorption isotherms as well as energies. The isotherms generally show condensation/evaporation vertical steps and, for reasonable starting conditions, hysteresis loops. The variations in these characteristic parts of the isotherms with temperature, pore diameter and gas/pore wall interaction strength and heterogeneity will be discussed.

WETTING TRANSITIONS OF NE

We report studies of the wetting behavior of Ne on very weakly attractive surfaces, carried out with the grand canonical Monte Carlo method. The Ne-Ne interaction was taken to be of Lennard-Jones form, while the Ne-surface interaction was derived from an ab initio calculation of Chizmeshya et al. [J. Low Temp. Phys. 110, 677 (1998)]. Nonwetting behavior was found for Li, Rb, and Cs in the temperature regime explored (i.e.,

Threshold criterion for wetting at the triple point

Tl42 \mathrm{K}).

Computer simulations of the adsorption of xenon on stepped surfaces

Drying behavior was manifested in a depleted fluid density near the Cs surface. In contrast, for the case of Mg (a more attractive potential) a prewetting transition was found near

COMPUTER SIMULATIONS OF THE WETTING PROPERTIES OF NEON ON HETEROGENEOUS SURFACES

T=28 \mathrm{K}.