Upper storage-capacity limit and multiple occupancy phenomena in H2-hydroquinone clathrates using Monte Carlo and DFT simulations.

The upper hydrogen-storage capacity limit of the ß-hydroquinone clathrate has been investigated using hybrid Grand-Canonical Monte Carlo/Molecular Dynamics simulations, for temperatures ranging from 77 K to 300 K. The evolution with pressure of the cage occupancies has been monitored in detail, describing the progressive nature of the uptake process. It is found that the storage capacity of the pure ß-HQ + H2 clathrate could reach 0.6 wt% (weight percentage) only for pressures above 1400 bar, at ambient temperature. The enhancement of the storage capacities by the multiple occupancy phenomenom was accordingly shown to be very limited by the need for extreme conditions. Following this observation, an unmodified version of the van der Waals & Platteeuw theory was applied allowing for the prediction of experimentally accessible formation pressures. Density functional theory calculations were addittionnaly performed to comprehensively characterize the hydrogen diffusion process within the clathrate crystalline structure, considering different occupancy scenarios.

Graphene IoNanofluids, Thermal and Structural Characterization.

Graphene is considered a promising substance in applications related to the capture and reduction of the environmental impact of fluorinated gases. However, further research is still required to explore all related possibilities. In this work, the potential use in this context of nanofluids (NFs), obtained by dispersing graphene nanosheets in fluorinated ionic liquids (FILs) is investigated. As a starting step, a thermal and structural characterization for this type of IoNanofluids (IoNFs) is presented. The highly nanostructured nature of FILs has been recently demonstrated. The presence of fluorinated moieties is responsible for enhancing the accommodation of solutes such as small gases. The strong tendency to self-assemble forming continuous and supramolecular structures, and the versatility to rearrange in several conformational features allows the stabilization of nano colloidal systems. It is essential to perform a comprehensive study of their structural features to understand the behavior of this type of heterogeneous systems. Therefore, we present screening on the phase and structural behavior of these novel IoNFs to discover and develop optimized systems where FILs turn out to be advantageous. Thermogravimetric analysis (TGA) was employed to evaluate IoNFs mass losses with temperature, and their solid-fluid phase transitions were located using a differential scanning calorimeter (DSC). Their rheological properties were also determined through oscillatory experiments, obtaining the viscous and loss moduli. In addition, the structural percolation transition was also identified.

A description of hydroquinone clathrates using molecular dynamics: Molecular model and crystalline structures for CH4 and CO2 guests.

The crystalline structure of hydroquinone clathrates has been studied using molecular dynamics. A flexible non-polarizable all-atom molecular model, based on the original Optimized Potentials for Liquid Simulations force field with recalculated point electric charges, has been used to describe the hydroquinone molecule, and the crystalline solid structure of the α native phase has been analyzed. Then, the ß clathrates have been studied, considering CO2 and CH4 as guest molecules, and also the empty clathrate structure. In all cases, the lattice parameters obtained through molecular simulation show excellent agreement with reported experimental values, showing that the molecular model selected is able to reproduce both the native crystalline phase and also the clathrate structures. In addition, the process of clathrate guest molecule release upon heating has been characterized, and the simulations show a good correspondence with the very recent experimental trends observed for both guest molecules analyzed.

Tuning the electrical conductivity of exfoliated graphite nanosheets nanofluids by surface functionalization.

The electrical conductivity of exfoliated graphite in water nanofluids has been experimentally determined, and compared with the same property when the dispersed nanosheets have been oxidized. The effect of oxidation on this property is different if compared with the case of sintered dry nanosheets. In any case, for the sintered raw material the conduction behaves as expected in a metal, while for the nanofluid it shows values and trends typical of a weak electrolyte solution. The effect of oxidation on the electrical conductivity of exfoliated graphite can be explained as being caused by the dissociation in the fluid phase of the moieties resulting from the chemical functionalization process. This opens the possibility of designing a functionalization process to tune the nanofluid electrical conductivity.

Computational study of the interplay between intermolecular interactions and CO2 orientations in type I hydrates.

Carbon dioxide (CO2) molecules show a rich orientation landscape when they are enclathrated in type I hydrates. Previous studies have described experimentally their preferential orientations, and some theoretical works have explained, but only partially, these experimental results. In the present paper, we use classical molecular dynamics and electronic density functional theory to advance in the theoretical description of CO2 orientations within type I hydrates. Our results are fully compatible with those previously reported, both theoretical and experimental, the geometric shape of the cavities in hydrate being, and therefore, the steric constraints, responsible for some (but not all) preferential angles. In addition, our calculations also show that guest-guest interactions in neighbouring cages are a key factor to explain the remaining experimental angles. Besides the implication concerning equation of state hydrate modeling approximations, the conclusion is that these guest-guest interactions should not be neglected, contrary to the usual practice.

On interfacial properties of tetrahydrofuran: Atomistic and coarse-grained models from molecular dynamics simulation.

We have determined the interfacial properties of tetrahydrofuran (THF) from direct simulation of the vapor-liquid interface. The molecules are modeled using six different molecular models, three of them based on the united-atom approach and the other three based on a coarse-grained (CG) approach. In the first case, THF is modeled using the transferable parameters potential functions approach proposed by Chandrasekhar and Jorgensen [J. Chem. Phys. 77, 5073 (1982)] and a new parametrization of the TraPPE force fields for cyclic alkanes and ethers [S. J. Keasler et al., J. Phys. Chem. B 115, 11234 (2012)]. In both cases, dispersive and coulombic intermolecular interactions are explicitly taken into account. In the second case, THF is modeled as a single sphere, a diatomic molecule, and a ring formed from three Mie monomers according to the SAFT-γ Mie top-down approach [V. Papaioannou et al., J. Chem. Phys. 140, 054107 (2014)]. Simulations were performed in the molecular dynamics canonical ensemble and the vapor-liquid surface tension is evaluated from the normal and tangential components of the pressure tensor along the simulation box. In addition to the surface tension, we have also obtained density profiles, coexistence densities, critical temperature, density, and pressure, and interfacial thickness as functions of temperature, paying special attention to the comparison between the estimations obtained from different models and literature experimental data. The simulation results obtained from the three CG models as described by the SAFT-γ Mie approach are able to predict accurately the vapor-liquid phase envelope of THF, in excellent agreement with estimations obtained from TraPPE model and experimental data in the whole range of coexistence. However, Chandrasekhar and Jorgensen model presents significant deviations from experimental results. We also compare the predictions for surface tension as obtained from simulation results for all the models with experimental data. The three CG models predict reasonably well (but only qualitatively) the surface tension of THF, as a function of temperature, from the triple point to the critical temperature. On the other hand, only the TraPPE united-atoms models are able to predict accurately the experimental surface tension of the system in the whole temperature range.

Understanding the Phase Behavior of Tetrahydrofuran + Carbon Dioxide, + Methane, and + Water Binary Mixtures from the SAFT-VR Approach.

The high-pressure phase diagrams of the tetrahydrofuran(1) + carbon dioxide(2), + methane(2), and + water(2) mixtures are examined using the SAFT-VR approach. Carbon dioxide molecule is modeled as two spherical segments tangentially bonded, water is modeled as a spherical segment with four associating sites to represent the hydrogen bonding, methane is represented as an isolated sphere, and tetrahydrofuran is represented as a chain of m tangentially bonded spherical segments. Dispersive interactions are modeled using the square-well intermolecular potential. In addition, two different molecular model mixtures are developed to take into account the subtle balance between water-tetrahydrofuran hydrogen-bonding interactions. The polar and quadrupolar interactions present in water, tetrahydrofuran, and carbon dioxide are treated in an effective way via square-well potentials of variable range. The optimized intermolecular parameters are taken from the works of Giner et al. (Fluid Phase Equil. 2007, 255, 200), Galindo and Blas (J. Phys. Chem. B 2002, 106, 4503), Patel et al. (Ind. Eng. Chem. Res. 2003, 42, 3809), and Clark et al. (Mol. Phys. 2006, 104, 3561) for tetrahydrofuran, carbon dioxide, methane, and water, respectively. The phase diagrams of the binary mixtures exhibit different types of phase behavior according to the classification of van Konynenburg and Scott, ranging from types I, III, and VI phase behavior for the tetrahydrofuran(1) + carbon dioxide(2), + methane(2), and + water(2) binary mixtures, respectively. This last type is characterized by the presence of a Bancroft point, positive azeotropy, and the so-called closed-loop curves that represent regions of liquid-liquid immiscibility in the phase diagram. The system exhibits lower critical solution temperatures (LCSTs), which denote the lower limit of immiscibility together with upper critical solution temperatures (UCSTs). This behavior is explained in terms of competition between the incompatibility with the alkyl parts of the tetrahydrofuran ring chain and the hydrogen bonding between water and the ether group. A minimum number of unlike interaction parameters are fitted to give the optimal representation of the most representative features of the binary phase diagrams. In the particular case of tetrahydrofuran(1) + water(2), two sets of intermolecular potential model parameters are proposed to describe accurately either the hypercritical point associated with the closed-loop liquid-liquid immiscibility region or the location of the mixture lower- and upper-critical end-points. The theory is not only able to predict the type of phase behavior of each mixture, but also provides a reasonably good description of the global phase behavior whenever experimental data are available.

Molecular dynamics simulation of CO2 hydrates: Prediction of three phase coexistence line.

The three phase equilibrium line (hydrate-liquid water-liquid carbon dioxide) has been estimated for the water + carbon dioxide binary mixture using molecular dynamics simulation and the direct coexistence technique. Both molecules have been represented using rigid nonpolarizable models. TIP4P/2005 and TIP4P/Ice were used for the case of water, while carbon dioxide was considered as a three center linear molecule with the parameterizations of MSM, EPM2, TraPPE, and ZD. The influence of the initial guest occupancy fraction on the hydrate stability has been analyzed first in order to determine the optimal starting configuration for the simulations, paying attention to the influence of the two different cells existing in the sI hydrate structure. The three phase coexistence temperature was then determined for a pressure range from 2 to 500 MPa. The qualitative shape of the equilibrium curve estimated is correct, including the high pressure temperature maximum that determines the hydrate re-entrant behaviour. However, in order to obtain quantitative agreement with experimental results, a positive deviation from the classical Lorentz-Berthelot combining rules must be considered.

Fluid-solid equilibrium of carbon dioxide as obtained from computer simulations of several popular potential models: the role of the quadrupole.

In this work the solid-fluid equilibrium for carbon dioxide (CO2) has been evaluated using Monte Carlo simulations. In particular the melting curve of the solid phase denoted as I, or dry ice, was computed for pressures up to 1000 MPa. Four different models, widely used in computer simulations of CO2 were considered in the calculations. All of them are rigid non-polarizable models consisting of three Lennard-Jones interaction sites located on the positions of the atoms of the molecule, plus three partial charges. It will be shown that although these models predict similar vapor-liquid equilibria their predictions for the fluid-solid equilibria are quite different. Thus the prediction of the entire phase diagram is a severe test for any potential model. It has been found that the Transferable Potentials for Phase Equilibria (TraPPE) model yields the best description of the triple point properties and melting curve of carbon dioxide. It is shown that the ability of a certain model to predict the melting curve of carbon dioxide is related to the value of the quadrupole moment of the model. Models with low quadrupole moment tend to yield melting temperatures too low, whereas the model with the highest quadrupole moment yields the best predictions. That reinforces the idea that not only is the quadrupole needed to provide a reasonable description of the properties in the fluid phase, but also it is absolutely necessary to describe the properties of the solid phase.

Influence of the long-range corrections on the interfacial properties of molecular models using Monte Carlo simulation.

We analyze the influence of the long-range corrections, due to the dispersive term of the intermolecular potential energy, on the surface tension using direct simulation of the vapour-liquid interface of different molecular models. Although several calculation methods have been proposed recently to compute the fluid-fluid interfacial properties, the truncation of the intermolecular potential or the use of the tail corrections represents a contribution relevant from a quantitative perspective. In this work, a simplified model for methane, namely a spherical Lennard-Jones intermolecular potential, has been considered first, and afterwards other models including rigid non polarizable structures with both Lennard-Jones sites and point electric charges, representing some of the most popular models to describe water (namely the original TIP4P model, and the TIP4P/Ew and TIP4P/2005 versions), and carbon dioxide (MSM, EPM2, TraPPE, and ZD models) have been studied. Our results show that for all cases tested, including those in which the electrostatic interactions may be predominant, an incomplete account of the long-range corrections produces a systematic underestimation of the computed interfacial tension.

Vapor-liquid interfacial properties of rigid-linear Lennard-Jones chains.

We have obtained the interfacial properties of short rigid-linear chains formed from tangentially bonded Lennard-Jones monomeric units from direct simulation of the vapour-liquid interface. The full long-range tails of the potential are accounted for by means of an improved version of the inhomogeneous long-range corrections of Janecek [J. Phys. Chem. B 110, 6264-6269 (2006)] proposed recently by MacDowell and Blas [J. Chem. Phys. 131, 074705 (2009)] valid for spherical as well as for rigid and flexible molecular systems. Three different model systems comprising of 3, 4, and 5 monomers per molecule are considered. The simulations are performed in the canonical ensemble, and the vapor-liquid interfacial tension is evaluated using the test-area method. In addition to the surface tension, we also obtain density profiles, coexistence densities, critical temperature and density, and interfacial thickness as functions of temperature, paying particular attention to the effect of the chain length and rigidity on these properties. According to our results, the main effect of increasing the chain length (at fixed temperature) is to sharpen the vapor-liquid interface and to increase the width of the biphasic coexistence region. As a result, the interfacial thickness decreases and the surface tension increases as the molecular chains get longer. The surface tension has been scaled by critical properties and represented as a function of the difference between coexistence densities relative to the critical density.

On interfacial tension calculation from the test-area methodology in the grand canonical ensemble.

We propose the extension of the test-area methodology, originally proposed to evaluate the surface tension of planar fluid-fluid interfaces along a computer simulation in the canonical ensemble, to deal with the solid-fluid interfacial tension of systems adsorbed on slitlike pores using the grand canonical ensemble. In order to check the adequacy of the proposed extension, we apply the method for determining the density profiles and interfacial tension of spherical molecules adsorbed in slitlike pore with different pore sizes and solid-fluid dispersive energy parameters along the same simulation. We also calculate the solid-fluid interfacial tension using the original test-area method in the canonical ensemble. Agreement between the results obtained from both methods indicate that both methods are fully equivalent. The advantage of the new methodology is that allows to calculate simultaneously the density profiles and the amount of molecules adsorbed onto a slitlike pore, as well as the solid-fluid interfacial tension. This ensures that the chemical potential at which all properties are evaluated during the simulation is exactly the same since simulations can be performed in the grand canonical ensemble, mimicking the conditions at which the adsorption experiments are most usually carried out in the laboratory.

An examination of the ternary methane + carbon dioxide + water phase diagram using the SAFT-VR approach.

In this work, the molecular based Variable Range Statistical Associating Fluid Theory (SAFT-VR) has been used to estimate the global phase equilibria diagram of the ternary mixture water + carbon dioxide + methane, over a wide pressure and temperature range. An accurate determination of the phase equilibria of this mixture is relevant in Petrophysics, as, for instance, in enhanced natural gas recovery from low permeability reservoirs (the so-called tight gas reservoirs), or in geology, as it is the basic composition of many geological fluids. A previous study on the phase behavior of the binary mixtures involved is presented, using in a transferable manner the characteristic molecular parameters for the three molecules involved. The ternary mixture presents a very rich and complex phase behavior, with a wide region of the thermodynamic space of phases (at higher pressures) presenting a large gap of ternary liquid-liquid equilibria, that upon descending pressures leads to the transition to a three-phase liquid-liquid-vapor equilibria region, and both regions are separated by a continuous critical end point line. The ability of the theory to describe this complex multicomponent mixture phase transition with a reduced and physically sound set of characteristic parameters must be underlined.

Analysis of the orientational order effect on n-alkanes: Evidences on experimental response functions and description using Monte Carlo molecular simulation.

Short-range correlations of the molecular orientations in liquid n-alkanes have been extensively studied from depolarized Rayleigh scattering and thermodynamic measurements. These correlations between segments induce structural anisotropy in the fluid bulk. This phenomenon, which is characteristic of linear chain molecules when the constituting segments are nor freely jointed, but interact through a given angular potential, is then present in the linear n-Cn series, increasing its magnitude with chain length, and it is therefore less relevant or even completely absent in branched alkanes. This intermolecular effect is clearly revealed in second-order excess magnitudes such as heat capacities when the linear molecule is mixed with one whose structure approaches sphericity. The mixing process of different aspect ratio chain molecules is thought to modify the original pure fluid structure, by producing a diminution of the orientational order previously existing between pure n-alkane chains. However, second-order thermodynamics quantities of pure liquids C(P), ( partial differentialv/ partial differentialT)(P), and ( partial differentialv/ partial differentialP)(P) are known to be very sensitive to the specific interactions occurring at the microscopic level. In other words, the behavior of these derived properties versus temperature and pressure can be regarded as response functions of the complexity of the microscopic interactions. Thus, the purpose of the present work is to rationalize the orientational order evolution with both temperature and molecular chain length from the analysis of pure fluid properties. To this aim, we focused on two linear alkanes, n-octane (n-C(8)) and n-hexadecane (n-C(16)), and two of their branched isomers, i.e., 2,2,4-trimethylpentane (br-C(8)) and 2,2,4,4,6,8,8-heptamethylnonane (br-C(16)). For each compound, we propose a combined study from direct experimental determination of second-order derivative properties and Monte Carlo simulations. We performed density rho, speed of sound c, and isobaric heat capacity C(P) measurements in broad ranges of pressure and temperature allowing a complete thermodynamic characterization of these compounds. Monte Carlo simulations provide a link between the molecular scale model and the experimental thermodynamic properties. Additional information about the microscopic structure of the simulated fluid model was derived, through the calculation of the radius of gyration and average end-to-end distances. Orientational order is clearly revealed by the experimental residual heat capacity trend of pure linear alkanes. The close agreement observed between this experimental macroscopic property and the calculated theoretical structural parameters support the conclusion that the orientational order between segments of linear molecules should be regarded as a conformational effect due to the flexibility of the chain.

A combined pressure-controlled scanning calorimetry and Monte Carlo determination of the Joule-Thomson inversion curve. Application to methane.

A combination of a pressure-controlled scanning calorimetry (PCSC) and Monte Carlo simulations (MCS) is presented for an unequivocal determination of the Joule-Thomson inversion curve (JTIC) with high accuracy over wide ranges of pressure and temperature. The MCS performed with the fluctuation method are fast and easy to operate, but the results can vary significantly depend on the set of primary molecular data needed for the calculations. The PCSC is an experimental and more laborious technique, but supplies data of high quality. Thus, it can be used to check the MCS data and to verify the molecular parameters used for the calculations. Such a combined procedure was used in the present study for determination of the JTIC for methane, for which a correlation equation was established valid from 302.9 to 586.5 K. A combination of a direct experimental technique with molecular simulations permits also to better understand the complex behavior of the Joule-Thomson inversion phenomenon over wide ranges of pressure and temperature.