Adsorption dynamics of polyatomic molecules on planar surfaces.

This is a comparative study of the adsorption dynamics of increasingly longer polyatomic molecules on a planar surface. We perform kinetic Monte Carlo simulations of the gas uptake to identify the molecular mechanisms and parameters that govern the evolution of the adsorbed film to its final equilibrium state. We also develop an analytical model for the adsorption dynamics of mono-atomic gases that demonstrates a direct correlation between the strength of the gas-gas interaction and the adsorption rate dependence with coverage. When this is added to the effect of the orientational freedom of polyatomic molecules, we are able to explain the change in the adsorption rate dependence observed experimentally for longer molecules.

Equilibration processes during gas uptake inside narrow pores.

We analyze the adsorption kinetics of a gas in contact with the open ends of a narrow longitudinal pore, where gas transport along its interior occurs via single-file diffusion mechanisms. By implementing a Kinetic Monte Carlo simulation of the gas dynamics, we obtain the overall change in gas uptake inside the pore and the concentration profile of the adsorbed phase as the system evolves towards equilibrium. Typically, higher external pressure leads to faster kinetics as it happens for adsorption on open surfaces. However, when the pore is exposed to gas at very high pressures, blockage events near the ends of longer pores can slow down the overall adsorption, with desorption and internal diffusion eventually becoming the rate limiting processes. We determine the dependence of these phenomena on the amount of gas adsorbed, binding energy and length of the pore.

Adsorption kinetics of diatomic molecules.

The adsorption dynamics of diatomic molecules on solid surfaces is examined by using a Kinetic Monte Carlo algorithm. Equilibration times at increasing loadings are obtained, and explained based on the elementary processes that lead to the formation of the adsorbed film. The ability of the molecules to change their orientation accelerates the overall uptake and leads to competitive kinetic behaviour between the different orientations. The dependence of the equilibration time on coverage follows the same decreasing trend obtained experimentally for ethane adsorption on closed-end carbon nanotube bundles. The exploration of molecule-molecule interaction effects on this trend provides relevant insights to understand the kinetic behaviour of other species, from simpler molecules to larger polyatomic molecules, adsorbing on surfaces with different binding strength.

CF4 on carbon nanotubes: physisorption on grooves and external surfaces.

We present the combined results of a computer simulation and adsorption isotherm investigation of CF4 films on purified HiPco nanotubes. The experimental measurements found two substeps in the adsorption data. The specific surface area of the sample and the coverage dependence of the isosteric heat of adsorption of the films were determined from the measurements. The simulations, conducted for homogeneous bundles of close-ended tubes, also found two substeps in the first layer data: one corresponding to adsorption on the grooves and a second one, at higher pressures, corresponding to adsorption on the outside surface of the tubes. Our computer simulations are in very good agreement with the experimental data.

Lattice-gas Monte Carlo study of adsorption in pores.

A lattice-gas model of adsorption inside cylindrical pores is evaluated with Monte Carlo simulations. The model incorporates two kinds of sites: (a line of) "axial" sites and surrounding "cylindrical shell" sites, in the ratio 1:7. The adsorption isotherms are calculated in either the grand canonical or canonical ensembles. At low temperature, there occur quasitransitions that would be genuine thermodynamic transitions in mean-field theory. Comparisons between the Monte Carlo and mean-field theory results for the heat capacity and adsorption isotherms are provided.

Lattice model of gas condensation within nanopores.

We explore the thermodynamic behavior of gases adsorbed within a nanopore. The theoretical description employs a simple lattice gas model, with two species of site, expected to describe various regimes of adsorption and condensation behavior. The model includes four hypothetical phases: a cylindrical shell phase (S), in which the sites close to the cylindrical wall are occupied, an axial phase (A), in which sites along the cylinder's axis are occupied, a full phase (F), in which all sites are occupied, and an empty phase (E). We obtain exact results at T=0 for the phase behavior, which is a function of the interactions present in any specific problem. We obtain the corresponding results at finite T from mean field theory. Finally, we examine the model's predicted phase behavior of some real gases adsorbed in nanopores.

Ground state and thermal properties of a lattice gas on a cylindrical surface.

Adsorbed gases within, or outside of, carbon nanotubes may be analyzed with an approximate model of adsorption on lattice sites situated on a cylindrical surface. Using this model, the ground state energies of alternative lattice structures are calculated, assuming Lennard-Jones pair interactions between the particles. The resulting energy and equilibrium structure are nonanalytic functions of radius (R) because of commensuration effects associated with the cylindrical geometry. Specifically, as R varies, structural transitions occur between configurations differing in the "ring number," defined as the number of atoms located at a common value of the longitudinal coordinate (z). The thermodynamic behavior of this system is evaluated at finite temperatures, using a Hamiltonian with nearest-neighbor interactions. The resulting specific heat bears a qualitative resemblance to that of the one-dimensional Ising model.