Simple models of adsorption in nanotubes

We present two very simple models of adsorption in cylindrical pores. It is assumed that a layer-by-layer mechanism occurs similarly to that in the BET theory. The major assumption is that in the pores having an adsorption space with cylindrical geometry, the surface area of the upper surface (in comparison with the bottom surface) should be diminished in proportion to the radii of a cylinder. Two cases are considered: the adsorbate-adsorbate interactions are neglected or they are taken into account according to the lattice model developed by Fowler and Guggenheim. It is shown that the data simulated by Ohba and Kaneko for adsorption of nitrogen in the internal space of carbon nanotubes are successfully described by our models. On the basis of the fitted data we show that the relation between the monolayer capacity in cylindrical pores and on flat surfaces is in excellent agreement with the equation developed recently by Salmas and Androutsopoulos. Moreover, our models are verified for two sets of experimental data reported by Kaneko et al. We obtain excellent agreement between the values of the pore diameters calculated by us and suggested by these authors (from HRTEM, the GCMC simulations, and the IDBdB model). It is concluded that proposed simple and fast models can be applied as a first approximation to the estimation of the internal nanotube diameters if they do not exceed ca. 5 nm and are slightly dispersed.     

CO2 sorption on substituted carbon materials: Computational chemistry studies

Theoretical study of sorption of CO₂ on the 4-ring graphene (“unmodified” or N-, O-, and OH-substituted) structures possessing one completely unsaturated edge zigzag site is reported using the DFT (B3LYP/6-31G(d,p)) method. Lactone and heterocyclic complexes (due to thermodynamic favourability) are taken into account. The analysis of theoretical results shows that the enthalpy of reaction strongly depends on the chemical nature, i.e. the position of the doping of atom(s) is crucial. All substitutions do not change or decrease the enthalpy in comparison with the “unmodified” graphene sheet. The well-known theoretical reactivity indices (ionization potential, electron affinity, global softness, and HOMO-LUMO gaps) are calculated for the studied adsorbents in order to explain the above-mentioned tendencies. Finally, the effect of the presence of heteroatoms on the enthalpy of reaction (ΔH²⁹⁸) for all CO₂-heteroatom-doping adsorbent complexes is shown. Thus, carbon dioxide molecules adsorb on the edge plane surface of N-, O-, OH-containing carbon surfaces similarly or much less favourably in comparison with the “unmodified” adsorbents. This confirms some experimental observations.     

Homogeneous and heterogeneous micropore structures in carbonaceous adsorbents--twenty years later

The major aim of this study is to show that, developed by Dubinin and Stoeckli, the relation between the parameters of DA (Dubinin-Astakhov) and DRS (Dubinin-Radushkevich-Stoeckli) equations is an approximation. It is shown that the parameter B(0) of DRS isotherm should be taken into account in such a relationship; strictly speaking, the parameter n(DA) of DA adsorption isotherm equation is not only the function of the dispersion of the heterogeneity of carbon, it also depends on the location of the maximum of the distribution of this heterogeneity. Moreover, taking into account that the DR isotherm rather does not describe the adsorption in a homogeneous micropore system, the analogous relationship, however, for DA adsorption isotherm as a local equation in GAI (General Adsorption Isotherm) is proposed.     

Simulating the effect of carbon nanotube curvature on adsorption of polycyclic aromatic hydrocarbons

The results of Molecular Dynamics simulation of polycyclic aromatic hydrocarbons adsorption on single-walled (13,9) carbon nanotube are reported. We discuss the angular orientation and plausible adsorbed states of molecules. It is shown, that suggested by Gotovac et al. orientation of adsorbed molecules is correct.     

Metal-insulator crossover in high T c cuprates: A gauge field approach

A metal-insulator crossover appears in the experimental data for in-plane resistivity of underdoped cuprates and a range of superconducting cuprates in the presence of a strong magnetic field suppressing superconductivity. We propose an explanation for this phenomenon based on a gauge field theory approach to the t-J model. In this approach, based on a formal spin-charge separation, the low energy effective action describes gapful spinons (with a theoretically derived doping dependence of the gap m _s ² ∼ δ| ln δ|) and holons with finite Fermi surface (ɛF ∼ tδ) interacting via a gauge field whose basic effect on the spinons is to bind them into overdamped spin waves, shifting their gap by a damping term linear in T, which causes the metal-insulator crossover. The presence of a magnetic field perpendicular to the plane acts by increasing the damping, in turn producing a big positive transverse in-plane magnetoresistance at low T, as experimentally observed.     

Quantum fluctuations increase the self-diffusive motion of para-hydrogen in narrow carbon nanotubes

Quantum fluctuations significantly increase the self-diffusive motion of para-hydrogen adsorbed in narrow carbon nanotubes at 30 K comparing to its classical counterpart. Rigorous Feynman's path integral calculations reveal that self-diffusive motion of para-hydrogen in a narrow (6,6) carbon nanotube at 30 K and pore densities below ～29 mmol cm(-3) is one order of magnitude faster than the classical counterpart. We find that the zero-point energy and tunneling significantly smoothed out the free energy landscape of para-hydrogen molecules adsorbed in a narrow (6,6) carbon nanotube. This promotes a delocalization of the confined para-hydrogen at 30 K (i.e., population of unclassical paths due to quantum effects). Contrary the self-diffusive motion of classical para-hydrogen molecules in a narrow (6,6) carbon nanotube at 30 K is very slow. This is because classical para-hydrogen molecules undergo highly correlated movement when their collision diameter approached the carbon nanotube size (i.e., anomalous diffusion in quasi-one dimensional pores). On the basis of current results we predict that narrow single-walled carbon nanotubes are promising nanoporous molecular sieves being able to separate para-hydrogen molecules from mixtures of classical particles at cryogenic temperatures.     

Molecular dynamics of zigzag single walled carbon nanotube immersion in water

The results of enthalpy of immersion in water for finite single-walled carbon nanotubes are reported. Using molecular dynamics simulation, we discuss the relation between the value of this enthalpy and tube diameters showing that the obtained plot can be divided into three regions. The structure of water inside tubes in all three regions is discussed and it is shown that the existence of the strong maximum of enthalpy observed for tube diameter ca. 1.17 nm is due to freezing of water under confinement. The calculations of hydrogen bond statistics and water density profiles inside tubes are additionally reported to confirm the obtained results. Next, we show the results of calculation for the same tubes but containing surface carbonyl oxygen groups at pore entrances. A remarkable rise in the value of enthalpy of immersion in comparison to the initial tubes is observed. We also discuss the influence of charge distribution between oxygen and carbon atom forming surface carbonyls on the structure of confined water. It is concluded for the first time that the presence of surface oxygen atoms at the pore entrances remarkably influences the structure and stability of ice created inside nanotubes, and surface carbonyls appear to be chaotropic (i.e. structure breaking) for confined water. This effect is explained by the pore blocking leading to a decrease (compared to initial structure) in the number of confined water molecules after introduction of surface oxygen groups at pore entrances.     

泥沙输运模拟综述——现状及其发展趋势

随着流体力学中数值方法的飞速发展,计算模型已经成为研究流体运动,泥沙输运和不同环境(诸如河流,湖泊及沿海地区)中相应污染物归趋过程等方面非常有吸引力的工具,在过去的30多年里,发展了许多计算水动力学/泥沙输运模型.文章追溯当前具有代表性的(一维、二维、三维)模型的发展历程,描述他们各自的特点,优势及局限,力图作为对模型方面感兴趣读者的第一指南,同时也为大家讨论模型的局限性,未来的发展趋势和研究需求方面搭建一个平台.给出了模型的表达,时空特性,水动力学和沉积物的耦合方式,处理非恒定流,推移质和悬移质,泥沙交换过程,泥沙类型(粘性或非粘性)及非均匀泥沙输运的能力.总结了不同模型的应用实例,读者可以运用这些例子作为研究模型设置,模型率定及模型验证的参考.给出了选择泥沙输运模型应遵循的原则,模型输入及率定方面存在的问题及改进的途径.探讨了现有水动力学/泥沙输运模型在处理复杂湍流,泥沙携带,流动与输沙耦合,非均匀泥沙,离散和扩散系数,河岸来沙处理等方面的局限性及改进的方向.最后,对基于多相流思想的泥沙输运模型及其它一些交叉性问题作了评述与展望.