Phase equilibria molecular simulations of hydrogen hydrates via the direct phase coexistence approach

We report the three-phase (hydrate-liquid water-vapor) equilibrium conditions of the hydrogen-water binary system calculated with molecular dynamics simulations via the direct phase coexistence approach. A significant improvement of ∼10.5 K is obtained in the current study, over earlier simulation attempts, by using a combination of modifications related to the hydrogen model that include (i) hydrogen Lennard-Jones parameters that are a function of temperature and (ii) the water-guest energy interaction parameters optimized further by using the Lorentz-Berthelot combining rules, based on an improved description of the solubility of hydrogen in water.

Synthesis and characterization of TiFe(0.7-x)Mn(0.3)V(x) (x = 0.05, and 0.1) and Ti(1-y)Ta(y)Fe(0.7)Mn(0.3) (y = 0.2, and 0.4) nanostructured metal hydrides for low temperature applications

Metal hydrides (MH) are often preferred to absorb and desorb hydrogen at ambient temperature and pressure with a high volumetric density. These hydrogen storage alloys create promising prospects for hydrogen storage and can solve the energetic and environmental issues. In the present research work, the goal of our studies is to find the influence of partial substitution of small amounts of vanadium and tantalum on the hydrogenation properties of TiFe(0.7-x)Mn(0.3)V(x) (x = 0.05, and 0.1) and Ti(1-y)Ta(y)Fe(0.7)Mn(0.3) (y = 0.2, and 0.4) alloys, respectively. The nominal compositions of these materials are TiFe(0.6)Mn(0.3)V(0.05), TiFe(0.6)Mn(0.3)V(0.1), Ti(0.8)Ta(0.2)Fe(0.7)Mn(0.3), and Ti(0.6)Ta(0.4)Fe(0.7)Mn(0.3). All samples were synthesized by arc-melting high purity elements under argon atmosphere. The structural and microstructural properties of the samples were studied by using XRD and SEM, respectively, while the corresponding microchemistry was determined by obtaining EDS measurements at specific regions of the samples. Mapping was obtained in order to investigate atomic distribution in microstructure. Moreover, to ensure the associations between the properties and structure, all samples were examined by an optical microscope for accessional characterization. From all these microscopic examinations a variety of photomicrographs were taken with different magnifications. The hydrogenation properties were obtained by using a Magnetic Suspension Balance (Rubotherm). In this equipment, the hydrogen desorption and re-absorption, can be investigated at constant hydrogen pressures in the range of 1 to 20 MPa (flow-through mode). At least 3.43 wt.% of absorbed hydrogen amount was measured while the effect of substitutions was investigated at the same temperature.

Structural, microchemistry, and hydrogenation properties of TiMn0.4Fe0.2V0.4, TiMn0.1Fe0.2V0.7 and Ti0.4Zr0.6Mn0.4Fe0.2V0.4 metal hydrides

In this work, TiFe-based alloys have been developed according to the stoichiometry Ti1-xAx Fe1-yBy (A [triple bond] Zr; B [triple bond] Mn, V). The hydrogen solubility properties have been investigated to develop dynamic hydrides of Ti-based alloys for hydrogen storage applications. The hydrogenation behavior of these alloys has been studied, and their hydrogen storage capacities and kinetics have been evaluated. Several activation modes, including activation at high temperatures under hydrogen pressure, have been attempted for the as-milled powders. In order to clarify the structural/microstructural characteristics, and chemical composition before and after hydrogenation, X-Ray Diffraction (XRD), EDAX-Mapping Analysis and Scanning Electron Microscopy (SEM), have been carried out for the samples. Modeling of the isotherms has been performed by using MATLAB programming. The maximum gravimetric density of 4.3 wt%, has been obtained on the sample with the BCC main phase. The calculated enthalpy of reaction (deltaH) is found to be about 4 kJ/mol.

Graphene fluoride: a stable stoichiometric graphene derivative and its chemical conversion to graphene

Stoichoimetric graphene fluoride monolayers are obtained in a single step by the liquid-phase exfoliation of graphite fluoride with sulfolane. Comparative quantum-mechanical calculations reveal that graphene fluoride is the most thermodynamically stable of five studied hypothetical graphene derivatives; graphane, graphene fluoride, bromide, chloride, and iodide. The graphene fluoride is transformed into graphene via graphene iodide, a spontaneously decomposing intermediate. The calculated bandgaps of graphene halides vary from zero for graphene bromide to 3.1 eV for graphene fluoride. It is possible to design the electronic properties of such two-dimensional crystals.

Lattice Boltzmann method for Lennard-Jones fluids based on the gradient theory of interfaces

In the present study we propose a lattice Boltzmann equation (LBE) model derived from density gradient expansions of the discrete BBGKY evolution equations. The model is based on the mechanical approach of the gradient theory of interfaces. The basic input is the radial distribution function, which is related exclusively to the molecular interaction potential, rather than semiempirical equations of state used in previous LBE models. This function can be provided from independent molecular simulations or from approximate theories. Evidently the accuracy of the interaction potential, and thus the radial distribution function, reflects on the accuracy of the thermodynamic properties and consistency of the derived LBE model. We have applied the proposed model to obtain equilibrium bulk and interfacial properties of a Lennard-Jones fluid at different temperatures, T, close to critical, T(c). The results demonstrate that the LBE model is in excellent agreement with gradient theory as well as with independent literature results based on different molecular simulation approaches. Hence the proposed LBE model can recover accurately bulk and interfacial thermodynamics for a Lennard Jones fluid at T/T(c)>0.9.

Organic functionalisation of graphenes

Graphene sheets derived from dispersion of graphite in pyridine were functionalised by the 1,3 dipolar cycloaddition of azomethine ylide. The organically modified graphene sheets are easily dispersible in polar organic solvents and water, and they are extensively characterised using several spectroscopic and microscopy techniques.

Thermodynamic consistency of liquid-gas lattice Boltzmann methods: interfacial property issues

In the present study we examine the thermodynamic consistency of lattice Boltzmann equation (LBE) models that are based on the forcing method by comparing different numerical treatments of the LBE for van der Waals fluids. The different models are applied for the calculation of bulk and interfacial thermodynamic properties at various temperatures. The effect of the interface density gradient parameter, kappa , that controls surface tension, is related explicitly with the fluid characteristics, including temperature, molecular diameter, and lattice spacing, through the employment of a proper intermolecular interaction potential. A comprehensive analysis of the interfacial properties reveals some important shortcomings of the LBE methods when central finite difference schemes are employed in the directional derivative calculations and proposes a proper treatment that ensures thermodynamically consistent interfacial properties in accord with the van der Waals theory. The results are found to be in excellent quantitative agreement with exact results of the van der Waals theory preserving all the major features of the interfacial characteristics of vapor-liquid systems of different shapes and sizes.

Effect of liquid films on the isothermal drying of porous media

We study the effects of liquid films on the isothermal drying of porous media. They are important for the transport of liquid to an evaporation interface, far from the receding liquid clusters. Through a transformation, the drying problem is mapped to the Laplace equation around the percolation liquid clusters. From its solution, the properties of drying are obtained in terms of the capillary number. Consistent with experimental evidence, film flow is shown to accelerate drying significantly.

Low Peclet mass transport in assemblages of spherical particles for two different adsorption mechanisms

The problem of flow and mass transport within an assemblage of spherical solid absorbers is investigated. We present and compare results from the numerical solution of the convection-diffusion equation in the sphere-in-cell geometry and in stochastically constructed 3-D spherical particle assemblages. In the first case, we make use of an analytical solution of the creeping flow field in the sphere-in-cell model while in the second we employ a full numerical solution of the flow field in the realistic geometry of sphere assemblages. Low to moderate Peclet numbers (Pe<10(2)) are considered where the validity of the sphere-in-cell model is uncertain. On the other hand, the selected porosities range from values close to unity, where the sphere-in-cell approximation is expected to hold, to intermediate values, where its applicability becomes again uncertain. In all cases, instantaneous and Langmuir adsorption is studied. It is found that the simplified sphere-in-cell approach performs adequately provided that proper account of the actual porous media properties (porosity and internal surface area) is taken. A simple match of porosity is not sufficient for a reliable estimation of adsorption efficiencies.

Numerical and experimental investigation of the diffusional release of a dispersed solute from polymeric multilaminate matrices

In the present work the release behavior of special, multilaminate matrix-type polymer systems, is studied both theoretically and experimentally. Two different mathematical models have been employed to describe the release of a dispersed solute from both single- and multilayer matrices. A parameter sensitivity study shows that the incorporation of supersaturated matrices in the formation of multilaminate devices, with a nonuniform initial solute loading, can provide a delivery system with optimized performance compared to monolithic ones. Finally, the findings of this theoretical analysis show good agreement with measurements of the release rates of a model disperse dye from both single- and multilayer matrices.

Structural and Transport Properties of Alumina Porous Membranes from Process-Based and Statistical Reconstruction Techniques

We study the structural and transport properties of two model porous membranes made by compaction of spherical monosize gamma-alumina particles. A ballistic deposition process of spherical particles has been employed as a process-based representation method for accurately simulating the pore structure of the membranes. Comparison between the computed and experimental permeability values obtained in the Knudsen regime shows very good agreement for both membranes and indicates that sufficient representation of the original pore structure is achieved with the random sphere packs. In a further step, a medium with the same porosity and autocorrelation function as the sphere pack has been stochastically reconstructed. Comparison between the structural properties of the random sphere pack system (process-based model) and the stochastically reconstructed medium (statistical model) shows nearly identical correlation functions and pore chord length distributions but widely different mass chord length distributions. This is reflected to a significant difference in the prediction of a dynamic property like the Knudsen permeability by a factor of about 4. The results suggest that matching of the porosity and the two-point correlation function alone is not always adequate when pursuing an accurate representation of the structure of a porous material. In such cases, higher order statistical properties of the material contained in the chord length distribution of both pore and solid phase should be satisfied as well. It is also found that proper account of the formation process in the reconstruction of a porous material (process-based model) leads to representations of its structure more accurate than those of statistical reconstruction models. Copyright 2000 Academic Press.

A study on structural and diffusion properties of porcine stratum corneum based on very small angle neutron scattering data

PURPOSE

Generation of valuable information about the biphasic geometrical configuration of porcine stratum corneum from Very Small Angle Neutron Scattering (VSANS) data and investigation of its effect on the corresponding effective diffusivity.

METHODS

Spectra of porcine stratum corneum are mathematically transformed in order to obtain the corresponding auto-correlation function (ACF). Model stratum corneum structures, matching this experimentally determined ACF, are then produced based on the "brick-and-mortar" configuration. The effective diffusivity through these model domains is calculated using an appropriate numerical method.

RESULTS

The most appropriate geometry of porcine stratum corneum's lipid and protein phases in a "brick-and-mortar" configuration is quantitatively determined and correlated with the barrier properties (diffusivity) of the stratum corneum model structures.

CONCLUSIONS

The ACF analysis indicates the most appropriate values for the dimensions of the corneocyte thickness and the surrounding lipid gap, while the corneocyte length is estimated from the diffusion study.

The Structure of Adsorbed CO(2) in Slitlike Micropores at Low and High Temperature and the Resulting Micropore Size Distribution Based on GCMC Simulations

The Monte Carlo method is used in its grand ensemble variant in combination with CO(2) experimental isotherm data at low (195.5 K) and high temperatures (at 298 and 308 K, i.e., slightly below and above the CO(2) critical temperature, respectively) to characterize microporous carbons and obtain the corresponding pore size distribution (PSD). Specifically, the CO(2) density inside a single, slit-shaped, graphitic pore of given width is found on the basis of grand canonical Monte Carlo (GCMC) simulations for a predefined temperature and different relative pressures. The simulation results provide useful insights concerning the densification process in the micropores and the structure of the CO(2) molecules packing in the individual pores as the temperature or pressure changes from 195.5 K to ambient or from very low to 70 bar, respectively. Effects of temperature, pore size, quadrupole interactions, and molecule elongation on the local density profile within the pore are examined and discussed. In an additional step, we determine the optimal PSD for which the best match is obtained between computed and measured CO(2) isotherms. Comparisons are made between the PSDs found for the same carbon sample at low and high temperatures and conclusions are drawn concerning the applicability of the method and the reliability of the resulting micropore size distributions. Copyright 2000 Academic Press.

Adsorption-Desorption Flow of Condensable Vapors through Mesoporous Media: Network Modeling and Percolation Theory

Flow of condensable vapors in mesoporous media is investigated theoretically and experimentally during adsorption and desorption processes. A typical permeability curve of a condensable vapor is strongly enhanced in the capillary condensation region. This is because additional capillary pressure gradients are imposed on the capillary-condensed pores, which act as "good" conductors compared to the noncondensed pores, which are considered "poor" conductors. The percolation scaling properties that hold for a system of "good" and "poor" conductors are confirmed for the cases examined. As the ratio of gas flow/capillary-enhanced flow decreases, the rise of permeability with pressure becomes sharper. The network connectivity has a strong impact on the maximum permeability value and on the width of the scaling law regions. The contribution of surface flow does not affect the permeability in the peak region, but results in a shrinkage of the scaling law regions. During desorption, a marked hysteresis in the permeability curves is found and it is attributed only to thermodynamic hysteresis. The maximum permeability values in this case are higher and shifted to lower relative pressures. Copyright 2000 Academic Press.

A two-phase model for controlled drug release from biphasic polymer hydrogels

A comprehensive two phase model is developed to describe the sustained release of a solute or drug from a biphasic hydrogel substrate. Such a material consists of a continuous hydrophilic phase (polymer backbone in water) and a dispersion of spherical microdomains made of the hydrophobic side chains of the polymer organised in a micelle like fashion. The solute or drug is assumed to be encapsulated within the dispersed microdomains, and to diffuse from the interior to the surface of the microdomain where it exchanges following a Langmuir isotherm. Mass transfer to the bulk phase occurs by desorption of the drug from the surface through a driving force that is proportional to the difference of surface and bulk concentration. Accordingly the drug is released to the surroundings by diffusion through the bulk. Depending on the values of the Langmuir constant and assuming well stirred behaviour in the interior of the microdomain, the present model results in either of the two asymptotic models developed in previous studies. The results of a parametric study show that the desired steady state flux of a specific drug to the surroundings may be obtained given appropriate values of structural properties of the material. This conclusion is further supported when using this model to simulate earlier experimental results. The polymer structural properties can be manipulated easily during the fabrication of dispersed-phase networks, as indicated by preliminary experiments.