Three-phase equilibria of hydrates from computer simulation. I. Finite-size effects in the methane hydrate.

Clathrate hydrates are vital in energy research and environmental applications. Understanding their stability is crucial for harnessing their potential. In this work, we employ direct coexistence simulations to study finite-size effects in the determination of the three-phase equilibrium temperature (T3) for methane hydrates. Two popular water models, TIP4P/Ice and TIP4P/2005, are employed, exploring various system sizes by varying the number of molecules in the hydrate, liquid, and gas phases. The results reveal that finite-size effects play a crucial role in determining T3. The study includes nine configurations with varying system sizes, demonstrating that smaller systems, particularly those leading to stoichiometric conditions and bubble formation, may yield inaccurate T3 values. The emergence of methane bubbles within the liquid phase, observed in smaller configurations, significantly influences the behavior of the system and can lead to erroneous temperature estimations. Our findings reveal finite-size effects on the calculation of T3 by direct coexistence simulations and clarify the system size convergence for both models, shedding light on discrepancies found in the literature. The results contribute to a deeper understanding of the phase equilibrium of gas hydrates and offer valuable information for future research in this field.

Three-phase equilibria of hydrates from computer simulation. II. Finite-size effects in the carbon dioxide hydrate.

In this work, the effects of finite size on the determination of the three-phase coexistence temperature (T3) of the carbon dioxide (CO2) hydrate have been studied by molecular dynamic simulations and using the direct coexistence technique. According to this technique, the three phases involved (hydrate-aqueous solution-liquid CO2) are placed together in the same simulation box. By varying the number of molecules of each phase, it is possible to analyze the effect of simulation size and stoichiometry on the T3 determination. In this work, we have determined the T3 value at 8 different pressures (from 100 to 6000 bar) and using 6 different simulation boxes with different numbers of molecules and sizes. In two of these configurations, the ratio of the number of water and CO2 molecules in the aqueous solution and the liquid CO2 phase is the same as in the hydrate (stoichiometric configuration). In both stoichiometric configurations, the formation of a liquid drop of CO2 in the aqueous phase is observed. This drop, which has a cylindrical geometry, increases the amount of CO2 available in the aqueous solution and can in some cases lead to the crystallization of the hydrate at temperatures above T3, overestimating the T3 value obtained from direct coexistence simulations. The simulation results obtained for the CO2 hydrate confirm the sensitivity of T3 depending on the size and composition of the system, explaining the discrepancies observed in the original work by Míguez et al. [J. Chem Phys. 142, 124505 (2015)]. Non-stoichiometric configurations with larger unit cells show a convergence of T3 values, suggesting that finite-size effects for these system sizes, regardless of drop formation, can be safely neglected. The results obtained in this work highlight that the choice of a correct initial configuration is essential to accurately estimate the three-phase coexistence temperature of hydrates by direct coexistence simulations.

Three-phase equilibria of hydrates from computer simulation. III. Effect of dispersive interactions in the methane and carbon dioxide hydrates.

In this work, the effect of the range of dispersive interactions in determining the three-phase coexistence line of the CO2 and CH4 hydrates has been studied. In particular, the temperature (T3) at which solid hydrate, water, and liquid CO2/gas CH4 coexist has been determined through molecular dynamics simulations using different cutoff values (from 0.9 to 1.6 nm) for dispersive interactions. The T3 of both hydrates has been determined using the direct coexistence simulation technique. Following this method, the three phases in equilibrium are put together in the same simulation box, the pressure is fixed, and simulations are performed at different temperatures T. If the hydrate melts, then T > T3. Conversely, if the hydrate grows, then T < T3. The effect of the cutoff distance on the dissociation temperature has been analyzed at three different pressures for CO2 hydrate: 100, 400, and 1000 bar. Then, we have changed the guest and studied the effect of the cutoff distance on the dissociation temperature of the CH4 hydrate at 400 bar. Moreover, the effect of long-range corrections for dispersive interactions has been analyzed by running simulations with homo- and inhomogeneous corrections and a cutoff value of 0.9 nm. The results obtained in this work highlight that the cutoff distance for the dispersive interactions affects the stability conditions of these hydrates. This effect is enhanced when the pressure is decreased, displacing the T3 about 2-4 K depending on the system and the pressure.

Homogeneous nucleation rate of methane hydrate formation under experimental conditions from seeding simulations.

In this work, we shall estimate via computer simulations the homogeneous nucleation rate for the methane hydrate at 400 bars for a supercooling of about 35 K. The TIP4P/ICE model and a Lennard-Jones center were used for water and methane, respectively. To estimate the nucleation rate, the seeding technique was employed. Clusters of the methane hydrate of different sizes were inserted into the aqueous phase of a two-phase gas-liquid equilibrium system at 260 K and 400 bars. Using these systems, we determined the size at which the cluster of the hydrate is critical (i.e., it has 50% probability of either growing or melting). Since nucleation rates estimated from the seeding technique are sensitive to the choice of the order parameter used to determine the size of the cluster of the solid, we considered several possibilities. We performed brute force simulations of an aqueous solution of methane in water in which the concentration of methane was several times higher than the equilibrium concentration (i.e., the solution was supersaturated). From brute force runs, we infer the value of the nucleation rate for this system rigorously. Subsequently, seeding runs were carried out for this system, and it was found that only two of the considered order parameters were able to reproduce the value of the nucleation rate obtained from brute force simulations. By using these two order parameters, we estimated the nucleation rate under experimental conditions (400 bars and 260 K) to be of the order of log10 (J/(m3 s)) = -7(5).

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.

Interfacial properties of binary mixtures of square-well molecules from Monte Carlo simulation.

We determine the interfacial properties of mixtures of spherical square-well molecules from direct simulation of the vapor-liquid interface. We consider mixtures with the same molecular size and intermolecular potential range but different dispersive energy parameter values. We perform Monte Carlo simulations in the canonical ensemble to obtain the interfacial properties of mixtures of square-well molecules. In particular, we determine the pressuretensor using the mechanical (virial) route and the vapor-liquid interfacial tension evaluated using the Irving-Kirkwood method. In addition to the pressuretensor and the surface tension, we also obtain density profiles, coexistence densities, and interfacial thickness as functions of pressure, at a given temperature. This work can be considered as the extension of our previous work [F. J. Martínez-Ruiz and F. J. Blas, Mol. Phys. 113, 1217 (2015)] to deal with mixtures of spherical molecules that interact through a discontinuous intermolecular potential. According to our results, the main effect of increasing the ratio between the dispersive energy parameters of the mixture, ϵ22/ϵ11, is to sharpen the vapor-liquid interface and to increase the width of the biphasic coexistence region. Particularly interesting is the presence of a relative maximum in the density profiles of the more volatile component at the interface. This maximum is related with adsorption or accumulation of these molecules at the interface, since there are stronger attractive interactions between these molecules in comparison with the rest of intermolecular interactions. Also, the interfacial thickness decreases and the surface tension increases as ϵ22/ϵ11 is larger, a direct consequence of the increasing of the cohesive energy of the system.

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.

Liquid-liquid interfacial properties of a symmetrical Lennard-Jones binary mixture.

We determine the interfacial properties of a symmetrical binary mixture of equal-sized spherical Lennard-Jones molecules, σ11 = σ22, with the same dispersive energy between like species, ϵ11 = ϵ22, but different dispersive energies between unlike species low enough to induce phase separation. We use the extensions of the improved version of the inhomogeneous long-range corrections of Janec̆ek [J. Phys. Chem. B 110, 6264 (2006)], presented recently by MacDowell and Blas [J. Chem. Phys. 131, 074705 (2009)] and Martínez-Ruiz et al. [J. Chem. Phys. 141, 184701 (2014)], to deal with the interaction energy and microscopic components of the pressure tensor. We perform Monte Carlo simulations in the canonical ensemble to obtain the interfacial properties of the symmetrical mixture with different cut-off distances rc and in combination with the inhomogeneous long-range corrections. The pressure tensor is obtained using the mechanical (virial) and thermodynamic route. The liquid-liquid interfacial tension is also evaluated using three different procedures, the Irving-Kirkwood method, the difference between the macroscopic components of the pressure tensor, and the test-area methodology. This allows to check the validity of the recent extensions presented to deal with the contributions due to long-range corrections for intermolecular energy and pressure tensor in the case of binary mixtures that exhibit liquid-liquid immiscibility. In addition to the pressure tensor and the surface tension, we also obtain density profiles and coexistence densities and compositions as functions of pressure, at a given temperature. According to our results, the main effect of increasing the cut-off distance rc is to sharpen the liquid-liquid interface and to increase the width of the biphasic coexistence region. Particularly interesting is the presence of a relative minimum in the total density profiles of the symmetrical mixture. This minimum is related with a desorption of the molecules at the interface, a direct consequence of a combination of the weak dispersive interactions between unlike species of the symmetrical binary mixture, and the presence of an interfacial region separating the two immiscible liquid phases in coexistence.

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.

Effect of dispersive long-range corrections to the pressure tensor: the vapour-liquid interfacial properties of the Lennard-Jones system revisited.

We propose an extension of the improved version of the inhomogeneous long-range corrections of Janecek [J. Phys. Chem. B 110, 6264-6269 (2006)], presented recently by MacDowell and Blas [J. Chem. Phys. 131, 074705 (2009)] to account for the intermolecular potential energy of spherical, rigid, and flexible molecular systems, to deal with the contributions to the microscopic components of the pressure tensor due to the dispersive long-range corrections. We have performed Monte Carlo simulations in the canonical ensemble to obtain the interfacial properties of spherical Lennard-Jones molecules with different cutoff distances, r(c) = 2.5, 3, 4, and 5σ. In addition, we have also considered cutoff distances r(c) = 2.5 and 3σ in combination with the inhomogeneous long-range corrections proposed in this work. The normal and tangential microscopic components of the pressure tensor are obtained using the mechanical or virial route in combination with the recipe of Irving and Kirkwood, while the macroscopic components are calculated using the Volume Perturbation thermodynamic route proposed by de Miguel and Jackson [J. Chem. Phys. 125, 164109 (2006)]. The vapour-liquid interfacial tension is evaluated using three different procedures, the Irving-Kirkwood method, the difference between the macroscopic components of the pressure tensor, and the Test-Area methodology. In addition to the pressure tensor and the surface tension, we also obtain density profiles, coexistence densities, vapour pressure, critical temperature and density, and interfacial thickness as functions of temperature, paying particular attention to the effect of the cutoff distance and the long-range corrections on these properties. According to our results, the main effect of increasing the cutoff distance (at fixed temperature) is to sharpen the vapour-liquid interface, to decrease the vapour pressure, and to increase the width of the biphasic coexistence region. As a result, the interfacial thickness decreases, the width of the tangential microscopic component of the pressure tensor profile increases, and the surface tension increases as the cutoff distance is larger. We have also checked the effect of the impulsive contribution to the pressure due to the discontinuity of the intermolecular interaction potential when it is cut. If this contribution is not accounted for in the calculation of the microscopic components of the pressure tensor, incorrect values of both components as well as a wrong structure along the vapour-liquid interface are obtained.

Effect of molecular flexibility of Lennard-Jones chains on vapor-liquid interfacial properties.

We have determined the interfacial properties of short fully flexible chains formed from tangentially bonded Lennard-Jones monomeric units from direct simulation of the vapor-liquid interface. The results obtained are compared with those corresponding to rigid-linear chains formed from the same chain length, previously determined in the literature [F. J. Blas, A. I. M.-V. Bravo, J. M. Míguez, M. M. Piñeiro, and L. G. MacDowell, J. Chem. Phys. 137, 084706 (2012)]. 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 129, 6264 (2006)] proposed recently by MacDowell and Blas [J. Chem. Phys. 131, 074705 (2008)] valid for spherical as well as for rigid and flexible molecular systems. Three different model systems comprising of 3, 5, and 6 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 obtained 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. Comparison between predictions for fully flexible and rigid-linear chains, formed by the same number of monomeric units, indicates that the main effects of increasing the flexibility, i.e., passing from a rigid-linear to a fully flexible chain, are: (a) to decrease the difference between the liquid and vapor densities; (b) to decrease the critical temperature and to increase the critical density; (c) to smooth the density profiles along the interfacial region; (d) to increase the interfacial thickness; and (e) to decrease the vapor-liquid surface 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.

Universal scaling behaviour of surface tension of molecular chains.

We use and extend the universal relationship recently proposed by Galliero [G. Galliero, J. Chem. Phys. 133, 074705 (2010)], based on a combination of the corresponding-states principle of Guggenheim [E. A. Guggenheim, J. Chem. Phys. 13, 253 (1945)] and the parachor approach of Macleod [J. Macleod, Trans. Faraday Soc. 19, 38 (1923)], to predict the vapour-liquid surface tension of fully flexible chainlike Lennard-Jones molecules. In the original study of Galliero, the reduced surface tension of short-chain molecules formed by up to five monomers is expressed as a unique function of the difference between the liquid and vapour coexistence densities. In this work, we extend the applicability of the recipe and demonstrate that it is also valid for predicting the surface tension of two different chainlike molecular models, namely, linear tangent chains that interact through the Lennard-Jones intermolecular potential and fully flexible chains formed by spherical segments interacting through the square-well potential. Computer simulation data for vapour-liquid surface tension of fully flexible and rigid linear Lennard-Jones, and fluid flexible square-well chains is taken from our previous works. Our results indicate that the universal scaling relationship is able to correlate short- and long-chain molecules with different degrees of flexibility and interacting through different intermolecular potentials.

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.

Transferable SAFT-VR models for the calculation of the fluid phase equilibria in reactive mixtures of carbon dioxide, water, and n-alkylamines in the context of carbon capture.

The amine functional groups are fundamental building blocks of many molecules that are central to life, such as the amino acids, and to industrial processes, such as the alkanolamines, which are used extensively for gas absorption. The modeling of amines and of mixtures of amines with water (H(2)O) and carbon dioxide (CO(2)) is thus relevant to a number of applications. In this contribution, we use the statistical associating fluid theory for potentials of variable range (SAFT-VR) to describe the fluid phase behavior of ammonia + H(2)O + CO(2) and n-alkyl-1-amine + H(2)O + CO(2) mixtures. Models are developed for ammonia (NH(3)) and n-alkyl-1-amines up to n-hexyl-1-amine (CH(3)NH(2) to C(6)H(13)NH(2)). The amines are modeled as homonuclear chain molecules formed from spherical segments with additional association sites incorporated to mediate the effect of hydrogen-bonding interactions. The SAFT-VR approach provides a representation of the pure component fluid phase equilibria, on average, to within 1.48% of the experimental data in relative terms for the saturated liquid densities and vapor pressures. A simple empirical correlation is derived for the SAFT-VR parameters of the n -alkylamine series as a function of molecular weight. Aqueous mixtures of the amines are modeled using a model of water taken from previous work. The models developed for the mixtures are of high fidelity and can be used to calculate the binary fluid phase equilibrium of these systems to within 2.28% in relative terms for the temperature or pressure and 0.027 in absolute terms for the mole fraction. Regions of both vapor-liquid and liquid-liquid equilibria are considered. We also consider the reactive mixtures of amines and CO(2) in aqueous solution. To model the reaction of CO(2) with the amine, an additional site is included on the otherwise nonassociating CO(2) model. The unlike interaction parameters for the NH(3) + H(2)O + CO(2) ternary mixture are obtained by comparison to the experimental data available for this system. The resulting model is found to correlate and predict the liquid-phase loading (moles of CO(2) per mole of amine) to within 0.091 of experimental data in absolute terms. The parameters describing the NH(3)-CO(2) interaction are then transferred to other n-alkyl-1-amines, and sample predictions of the fluid phase equilibria for the n-propyl-1-amine + H(2)O + CO(2), n-butyl-1-amine + H(2)O + CO(2), and n-hexyl-1-amine + H(2)O + CO(2) mixtures are presented. The latter mixture is found to exhibit regions of liquid-liquid immiscibility.

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.

Computer simulation study of the global phase behavior of linear rigid Lennard-Jones chain molecules: comparison with flexible models.

The global phase behavior (i.e., vapor-liquid and fluid-solid equilibria) of rigid linear Lennard-Jones (LJ) chain molecules is studied. The phase diagrams for three-center and five-center rigid model molecules are obtained by computer simulation. The segment-segment bond lengths are L = sigma, so that models of tangent monomers are considered in this study. The vapor-liquid equilibrium conditions are obtained using the Gibbs ensemble Monte Carlo method and by performing isobaric-isothermal NPT calculations at zero pressure. The phase envelopes and critical conditions are compared with those of flexible LJ molecules of tangent segments. An increase in the critical temperature of linear rigid chains with respect to their flexible counterparts is observed. In the limit of infinitely long chains the critical temperature of linear rigid LJ chains of tangent segments seems to be higher than that of flexible LJ chains. The solid-fluid equilibrium is obtained by Gibbs-Duhem integration, and by performing NPT simulations at zero pressure. A stabilization of the solid phase, an increase in the triple-point temperature, and a widening of the transition region are observed for linear rigid chains when compared to flexible chains with the same number of segments. The triple-point temperature of linear rigid LJ chains increases dramatically with chain length. The results of this work suggest that the fluid-vapor transition could be metastable with respect to the fluid-solid transition for chains with more than six LJ monomer units.