Green function techniques in the treatment of quantum transport at the molecular scale

The theoretical investigation of charge (and spin) transport at nanometer length scales requires the use of advanced and powerful techniques able to deal with the dynamical properties of the relevant physical systems, to explicitly include out-of-equilibrium situations typical for electrical/heat transport as well as to take into account interaction effects in a systematic way. Equilibrium Green function techniques and their extension to non-equilibrium situations via the Keldysh formalism build one of the pillars of current state-of-the-art approaches to quantum transport which have been implemented in both model Hamiltonian formulations and first-principle methodologies. We offer a tutorial overview of the applications of Green functions to deal with some fundamental aspects of charge transport at the nanoscale, mainly focusing on applications to model Hamiltonian formulations.Comment: Tutorial review, LaTeX, 129 pages, 41 figures, 300 references, submitted to Springer series "Lecture Notes in Physics

Elastofibromatous changes and hyperelastosis of the oral mucosa

Three cases of abnormalities of elastic fibers, two of them on the floor of the mouth and one on the lingual alveolar mucosa, close to the floor of the mouth, in a patient with history of homolateral squamous cell carcinoma of the floor of the mouth, are presented. Comparison with elastofibromatous changes and elastofibromas are made and their possible pathogenesis is discussed. It is suggested that increased awareness may facilitate recognition of such lesions as they can be easily overlooked, especially when they do not present as discrete tumors or they are associated with other "more significant" pathologic processes. © 2009 Humana

Modeling of the carbon dioxide solubility in imidazolium-based ionic liquids with the tPC-PSAFT equation of state

In this work, an equation of state (EoS) is developed to predict accurately the phase behavior of ionic liquid + CO2 systems based on the truncated perturbed chain polar statistical associating fluid theory (tPC-PSAFT) EoS. This EoS accounts explicitly for the dipolar interactions between ionic liquid molecules, the quadrupolar interactions between CO2 molecules, and the Lewis acid-base type of association between the ionic liquid and the CO2 molecules. Physically meaningful model pure-component parameters for ionic liquids are estimated based on literature data. All experimental vapor-liquid equilibrium data are correlated with a single linearly temperature-dependent binary interaction parameter. The ability of the model to describe accurately carbon dioxide solubility in various 1-alkyl-3-methylimidazolium-based ionic liquids with different alkyl chain lengths and different anions at pressures from 0 to 100 MPa and carbon dioxide fractions from 0 to 75 mol % is demonstrated. In all cases, good agreement with experimental data is obtained

TPC-PSAFT modeling of gas solubility in imidazolium-based ionic liquids

The truncated perturbed chain-polar statistical associating fluid theory (tPC-PSAFT) is re-parametrized for imidazolium-based ionic liquids (ILs) by fitting IL density data over a wide temperature range and restricting the model to predict very low vapor pressure values, in agreement with recent experimental evidence. The new set of parameters is used for the correlation of carbon dioxide solubility in various ILs using a binary interaction parameter, kij. The correlated kij values are much lower than the values used previously for the same mixtures (Kroon et al., J. Phys. Chem. B 2006, 110, 9262). Furthermore, the solubilities of carbon monoxide, oxygen, and trifluoromethane in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim+][PF6-]) are correlated. In all cases, the agreement between tPC-PSAFT correlation and experimental data for mixtures is very good

Equation of state modeling of the phase equilibria of ionic liquid mixtures at low and high pressure

Accurate design of processes based on ionic liquids (ILs) requires knowledge of the phase behavior of the systems involved. In this work, the truncated perturbed chain polar statistical associating fluid theory (tPC-PSAFT) is used to correlate the phase behavior of binary and ternary IL mixtures. Both non-polar and polar solvents are examined, while methyl imidazolium ILs are used in all cases. tPC-PSAFT accounts explicitly for weak dispersion interactions, highly directive polar interactions between permanent dipolar and quadrupolar molecules and association between hydrogen bonding molecules. For mixtures of non-polar solvents, tPC-PSAFT predicts accurately the binary mixture data. For the case of polar solvents, a binary interaction parameter is fitted to the experimental data and the agreement between experiment and correlation is very good in all cases

Adsorption of N2, CH4, CO and CO2 gases in single walled carbon nanotubes: A combined experimental and Monte Carlo molecular simulation study

In this study, the adsorption capacity of single-wall carbon nanotubes (SWCNTs) bundles with regard to the pure CH4, N2, CO and CO2 gases at 298 K and pressure range from 0.01 up to 2.0 MPa has been investigated experimentally and computationally. Experimental work refers to gravimetric surface excess adsorption measurements of each gas studied in this nanomaterial. Commercial samples of pristine SWCNTs, systematically prepared and characterized at first, were used for the evaluation of their adsorption capacity. A Langmuir type equation was adopted to estimate the total adsorption isotherm based on the experimental surface excess adsorption data for each system studied. Computational work refers to Monte Carlo (MC) simulation of each adsorbed gas on a SWCNTs model of the type (9, 9) in the grand canonical (GC) ensemble at the same conditions with experiment using Scienomics' MAPS platform software simulation packages such as Towhee. The GCMC simulation technique was employed to obtain the uptake wt% of each adsorbed gas by considering a SWCNTs model of arrays with parallel tubes exhibiting open-ended cylindrical structures as in experiment. Both experimental and simulation adsorption data concerning these gases within the examined carbon material are presented and discussed in terms of the adsorbate fluid molecular characteristics and corresponding interactions among adsorbate species and adsorbent material. The adsorption isotherms obtained exhibited type I (Langmuir) behavior, providing enhanced gas-substrate interactions. We found that both the experimental as well as the simulated adsorption uptake of the examined SWCNTs at these conditions with regard to the aforementioned fluids and in comparison with adsorbate H2 on the same material increase similarly and in the following order: H2 ≪ N2 ≈ CH4 < CO ≪ CO2. Furthermore, for each adsorbate fluid the calculations exhibit somewhat greater gas uptake with pressure compared to the corresponding experiment. The difference in the absolute uptake values between experiment and simulation has been discussed and ascribed to the following implicit factors: (i) to the employed model calculations, (ii) to the remained carbonaceous impurities in the sample, and (iii) to a proportion of close ended tubes, contained in the experimental sample even after preparation. © 2010 Elsevier B.V. All rights reserved

Thermophysical properties of imidazolium tricyanomethanide ionic liquids: experiments and molecular simulation

The low-viscous tricyanomethanide ([TCM]-)-based ionic liquids (ILs) are gaining increasing interest as attractive fluids for a variety of industrial applications. The thermophysical properties (density, viscosity, surface tension, electrical conductivity and self-diffusion coefficient) of the 1-alkyl-3-methylimidazolium tricyanomethanide [Cnmim][TCM] (n = 2, 4 and 6-8) IL series were experimentally measured over the temperature range from 288 to 363 K. Moreover, a classical force field optimized for the imidazolium-based [TCM]- ILs was used to calculate their thermodynamic, structural and transport properties (density, surface tension, self-diffusion coefficients, viscosity) in the temperature range from 300 to 366 K. The predictions were directly compared against the experimental measurements. The effects of anion and alkyl chain length on the structure and thermophysical properties have been evaluated. In cyano-based ILs, the density decreases with increasing molar mass, in contrast to the behavior of the fluorinated anions, being in agreement with the literature. The contribution per -CH2- group to the increase of the viscosity presents the following sequence: [PF6]- > [BF4]- > [Tf2N]- > [DCA]- > [TCB]- > [TCM]-. [TCM]--based ILs show lower viscosity than dicyanamide ([DCA]-)- and tetracyanoborate ([TCB]-)-based ILs, while the latter two exhibit a crossover which depends both on temperature and the alkyl chain length of the cation. The surface tension of the investigated ILs decreases with increasing alkyl chain length. [C2mim][TCM] shows an outlier behavior compared to other members of the homologous series. The surface enthalpies and surface entropies for all the studied systems have been calculated based on the experimentally determined surface tensions. The relationship between molar conductivity and viscosity was analyzed using the Walden rule. The experimentally determined self-diffusion coefficients of the cations are in good agreement with the molecular simulation predictions, in which a decrease of the self-diffusion of the cations with increasing alkyl chain length is observed with a simultaneous increase in viscosity and for the longer alkyl lengths the anion becomes more mobile than the cation