Thermodynamics of quasi-one-dimensional deposits on carbon nanobundles

The low-temperature thermodynamics of helium adsorbed in the outer grooves of carbon nanobundles is investigated theoretically in a lattice-gas model with the use of the Green's function formalism. The proposed model describes both the formation of a one-dimensional (1D) condensate on the bottoms of the grooves and also the formation of two secondary chains in the groove (a three-chain structure) and thus is adequate for interpreting the behavior of the adsorbate over a rather wide range of coverages in the initial stage of deposition. The temperature dependence of the density of the deposit is obtained for the primary chain and secondary chains. The energy, heat capacity, and heat of adsorption are found as functions of temperature, and the total density of the 1D adsorbate is obtained for several different values of the binding energy with the substrate for atoms deposited in the primary and secondary positions. The adsorption isotherms (the total density of adsorbate as a function of external pressure) are calculated for various temperatures. Experimentally measured adsorption isotherms are presented for He-4 deposited on bundles of single-walled carbon nanotubes. The measurements were made in the temperature range 2-15 K. Those experiments and previously published data on the adsorption isotherms of methane are interpreted using a theoretical model developed in this paper. (c) 2005 American Institute of Physics.