Adsorption in carbon nanotubes and computer simulation studies of wetting on alkali surfaces

Absorption in carbon nanotubes and computer simulation studies of the wetting properties of rare gases on alkali metals are presented. In the confining environment of nanotubes, adsorbed atoms exhibit behavior characteristic of 1, 2, and 3 dimensions as a function of thermodynamic parameters (number of particles N and temperature T), geometry (isolated tubes or nanotube bundles), and microscopic variables (relative size of atom vs. tube radius). Small atoms fit easily inside nanotubes and are strongly bound within them. At low coverage, atoms moving inside nanotubes are adsorbed in a cylindrical shell close to the tube walls, while above a threshold value, atoms start to populate the vicinity of the axis of the tube. In a nanotube bundle, interstitial channels between the tubes are even more favorable energetically for small atoms than the interior of the tubes. An extremely anisotropic condensed state is formed due to interactions between atoms in neighboring interstitial channels. Mixtures of 3He and 4He atoms in interstitial channels obey an analog of Raoult's law for ideal solutions. A similar behavior is expected for H₂ - D₂ mixtures.; At the opposite extreme of interaction strengths, alkali metal surfaces attract rare gases more weakly than any other surfaces. Grand Canonical Monte Carlo simulations of the wetting behavior of such systems are reported. In the case of the most weakly attractive surfaces, Cs, Rb and Li, nonwetting behaviors of Ne are found for all temperatures within about 2 K of the critical temperature, while a drying behavior is seen only near the Cs surface. At the slightly more attractive surface of Mg, a prewetting transition is found. A study of wetting of various rare gases adsorbed at alkali metal surfaces reveals a rich variety of behaviors at the triple temperature. The heuristic model of Cheng et al. is found to agree well with the general threshold for wetting at the triple point.