This paper studies the thermodynamic aspects of the processes of adsorption of phenol from dilute aqueous solutions on different commercial carbons, evaluating how to optimize the removal of this persistent contaminant. Two powdered activated carbons from two different companies were used: Tetrahedron Carbon (Andes Chemistry Lab., Mendoza, Argentina), and Norit (Norit Americas Inc., USA). Both specific surface areas were measured by means of the BET method. The adsorbate was high purity solid phenol (Fluka ® ≥ 99.5%). Experimental isotherms were determined at 293 K, 303 K and 313 K. The Freundlich and Sips theoretical models were used to fit the experimental data. Freundlich isotherm slightly diverges with the experimental results for higher equilibrium concentrations. Thermodynamic parameters were calculated and correlated with the adsorption behaviours. The values of the thermodynamic parameters obtained indicate an exothermic and spontaneous process for both carbons, and mainly for Norit. This is due to the fact that there might be chemically activated regions on the surface of the Norit carbon, which give rise to combined mechanisms of physisorption and chemisorption.

We present the combined results of ab initio and molecular mechanical calculations, computer simulations, and adsorption isotherms investigations of CH(4) adsorbed on HiPco single-walled carbon nanotubes. Isotherms and adsorption energies obtained in our model and simulations are in good agreement with ours and others experimental results. The theoretical analysis conducted for various homogeneous bundles of close-ended and open-ended tubes confirm not only the adsorption in at least two different stages but also the role played by each of the different adsorption sites on the nanotube bundles. The study of different site and nanotube sizes allows us to establish the presence of open tubes in the as-produced HiPco bundles, without regarding the role that adsorption in large interstitial channels may play. Our results also show that predicted scenarios, for the mechanism and the preferential adsorption sites depend on the size of the nanotubes and those of the bundles.

We have studied conductance fluctuations 5g in a mesoscopic quasi-one-dimensional modulationdoped GaAs/AlGaAs heterojunction as a function of Fermi energy EF and applied magnetic field B at 1.3 K. Such a system has no, or negligibly small, spin-orbit scattering. We observe a reduction of the variance ^igiEp)) by a net factor of 4. This factor-of-4 reduction is a novel observation which is absent in a system with strong spin-orbit interaction and is due to the effect of the Zeeman splitting in breaking the spin degeneracy.PACS numbers: 73.40.Lq, 72.15.Gd, 72.20.My A wealth of novel quantum coherence phenomena has recently been observed in disordered conductors of mesoscopic size at low temperatures. * The most striking among these is the observation of "universal" conductance fluctuations (UCF) in such systems. The UCF arise from random quantum interference of electron waves multiply scattered by impurities in a disordered medium. The theoretical study of UCF has then been based on a microscopic perturbative treatment and numerical simulations.^ Recently, it has been pointed out-^ that the transfer matrix T, which determines the conductance g, has statistical properties characteristic of the random-matrix ensembles well known in nuclear physics. This relationship has lately been used to develop a macroscopic random-matrix theory (RMT) of UCF."^"^ In Ref. 7, another RMT approach was developed, based on the relationship between g and the Hamiltonian of the sample. The theoretical result obtained from the RMT approaches gives the variance of g aswhere k is the number of statistically independent eigenvalue sequences of tt^ (t is the transmission matrix through the disordered region), s is the degeneracy of the sequence, and )3 = 1,2,4 for the orthogonal (OE), unitary (UE), and symplectic (SE) ensembles, respectively, possible for the transfer matrix. This fundamental classification is well known in nuclear physics'* and refers to the symmetry of the transformation which diagonalizes the Hamiltonian. The result of Eq. (1) is independent of the average value of g and could be applied to the quantum noise^ or to the fluctuations^ of g as a function of applied magnetic field B or Fermi energy EF-In the absence of applied field (^=0), for a system without spin-orbit scattering the degeneracy is just the spin degeneracy (5=2, A: = I) and its transfer matrix belongs to the orthogonal ensemble ()3 = 1). For a system with a sufficient spin-orbit interaction the spin degeneracy is removed, but time-reversal invariance introduces a twofold Kramers degeneracy (^'=2, /: = 1) and the system belongs to the symplectic ensemble (j3=4). When an applied field is larger than a threshold field 5^i, a transition to the unitary ensemble ()3 = 2) takes place. Bd is the field above which one has more than a flux quantum (h/e) through a phase-coherent area of the sample. For systems with strong spin-orbit scattering the transition (SE-• UE) is accompanied by the suppression of the Kramers degeneracy (s =2--^ s = l), while k remains unchanged....

We present the results of Monte Carlo simulations of the adsorption of single-component ethane and ethylene and of equimolar mixtures of these two gases on bundles of closed, single-walled carbon nanotubes. Two types of nanotube bundles were used in the simulations: homogeneous (i.e., those in which all the nanotubes have identical diameters) and heterogeneous (those in which nanotubes of different diameters are allowed). We found that at the same pressure and temperature more ethane than ethylene adsorbs on the bundles over the entire range of pressures and temperatures explored. The simulation results for the equimolar mixtures show that the pressure at which maximum separation is attained is a very sensitive function of the diameter of the nanotubes present in the bundles. Simulations using heterogeneous bundles yield better agreement with single-component experimental data for isotherms and isosteric heats than those obtained from simulations using homogeneous bundles. Possible applications of nanotubes in gas separation are discussed. We explored the effect of the diameter of the nanotubes on the separation ability of these sorbents, both for the internal and for the external sites. We found that substrate selectivity is a decreasing function of temperature.

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