Lourdes F. Vega

Thermodynamic behaviour of homonuclear and heteronuclear Lennard-Jones chains with association sites from simulation and theory

Monte Carlo simulation and theoretical results are presented for mixtures of associating and non-associating Lennard-Jones chains. The molecular model accounts explicitly for repulsive, dispersive, chain, and association interactions. A modified statistical associating fluid theory for Lennard-Jones chains is used to compare with the NPT and Gibbs ensemble Monte Carlo simulation results. The equation is extended to describe heteronuclear Lennard-Jones chains, and results are compared with those obtained by Monte Carlo simulations. The influence of several variables, such as chain length, segment size and dispersive energy, polydispersity, etc., on the supercritical properties and the phase equilibria behaviour of these systems is discussed here. The theory seems to predict more accurately the behaviour of long chains rather than of those of intermediate length, due to the approximations made in the radial distribution function of the fluid.

Modeling Complex Associating Mixtures with [Cn-mim][Tf2N] Ionic Liquids: Predictions from the Soft-SAFT Equation

In a previous work (Andreu and Vega, J. Phys. Chem.B2008, 111, 16028), we presented a simple model for the imidazolium-based ionic liquids (ILs) with the bis(trifluorosulfonyl)imide anion [Tf(2)N](-) in the context of the soft-SAFT equation of state. The model was successfully used to predict the solubility of several gases in these ILs. However, the small amount of experimental data made the predictions less accurate when going into more complex mixtures and one or two fitted binary parameters were needed in some cases. In this work, we have reparameterized our previous model and evaluated its reliability to predict the behavior of these ionic liquids in binary mixtures with other associating compounds. Model parameters for the ionic liquids were estimated using new experimental density data at atmospheric pressure in an extended range of temperatures, from 273 until 473 K, consistent within the range of temperatures previously measured by other authors. The new set of molecular parameters has been tested to predict the density of several members of the family at higher pressures up to 60 MPa with the same degree of accuracy than at atmospheric values. In addition to density-temperature data, interfacial tensions and the isothermal compressibility of some compounds were predicted in reasonable good agreement with experimental data. The molecular parameters of the pure compounds were used then, in a predictive manner, to describe the behavior of binary mixtures with other imidazolium ionic liquids, changing either the cation or the anion. Predictions for some mixtures with methanol, ethanol, and water were compared with experimental data, providing an excellent description of the systems, with no fitting to mixture data in almost all the cases. The excellent results obtained in this work reinforce the need to have accurate data, showing that molecular based models can be used to assess the validity of these data. In addition, this work also shows that a simple model in which the physics of the system is kept is good enough to describe the complex behavior of associating mixtures of ionic liquids, without the need of additional parameters that may obscure the real physics of the system.

Thermodynamic properties of Lennard-Jones chain molecules: Renormalization-group corrections to a modified statistical associating fluid theory

A modified version of the statistical associating fluid theory (SAFT), the so-called soft-SAFT equation of state (EOS), has been extended by a crossover treatment to take into account the long density fluctuations encountered when the critical region is approached. The procedure, based on Whites work from the renormalization group theory [Fluid Phase Equilibria 75, 53 (1992); L. W. Salvino and J. A. White, J. Chem. Phys. 96, 4559 (1992)], is implemented in terms of recursion relations where the density fluctuations are successively incorporated. The crossover soft-SAFT equation provides the correct nonclassical critical exponents when approaching the critical point, and reduces to the original soft-SAFT equation far from the critical region. The accuracy of the global equation is tested by direct comparison with molecular simulation results of Lennard-Jones chains, obtaining very good agreement and clear improvements compared to the original soft-SAFT EOS. Excellent agreement with vapor-liquid equilibrium experimental data inside and outside the critical region for the n-alkane series is also obtained. We provide a set of transferable molecular parameters for this family, unique for the whole range of thermodynamic properties.

Capturing the solubility minima of n-alkanes in water by soft-SAFT.

The purpose of this work is twofold: (1) to provide an accurate molecular model for water within the soft-SAFT equation of state [Blas, F.J.; Vega, L.F. Mol. Phys. 1997, 92, 135; Llovell, F., et al. J. Chem. Phys. 2004, 121, 10715] and (2) to check the capability of this molecular-based equation of state for capturing the solubility minima of n-alkanes in water experimentally found at room temperature for these mixtures. Water was modeled as a Lennard-Jones sphere with four associating sites, with parameters obtained by fitting to experimental vapor-liquid equilibrium data. Special care was taken to the value of these parameters depending on the range of applicability of the equation, which turned out to be essential for accurate predictions for mixtures. A correlation available in the literature was used for the molecular parameters of the n-alkane series. The crossover soft-SAFT equation was able to accurately describe the phase behavior of water near to and far from the critical point, up to 350 K. If instead of obtaining an overall good agreement one is interested in a more precise description of the near-ambient conditions, a more refined fitting of the parameters is needed. The model was used to describe the water+methane up to water+n-decane binary mixtures. The equation was able to predict the mutual solubilities in almost quantitative agreement with experimental data, including the presence of the solubility minima at ambient temperature, with a single transferable energy binary parameter, independent of temperature and chain length. Predictions obtained from the soft-SAFT approach are clearly superior than those obtained from the Huang and Radosz version of the SAFT equation [Economou, I. G.; Tsonopoulos, C. Chem. Eng. Sci. 1997, 52, 511], due to the more refined reference term and the more accurate radial distribution function used in the chain and association terms. This is the first time a SAFT approach is able to describe this minima.

Interfacial properties of Lennard-Jones chains by direct simulation and density gradient theory

We perform a series of molecular dynamics simulations of Lennard-Jones chains systems, up to tetramers, in order to investigate the influence of temperature and chain length on their phase separation and interfacial properties. Simulation results serve as a test to check the accuracy of a statistical associated fluid theory (soft-SAFT) coupled with the density gradient theory. We focus on surface tension and density profiles. The simulations allow us to discuss the success and limitations of the theory and how to estimate the only adjustable parameter, the influence parameter. This parameter is obtained by fitting the surface tension, and then used to obtain the density profiles in a predictive manner. A good agreement is found if the temperature dependence of this parameter is neglected.(c) 2004 American Institute of Physics.

Some issues on the calculation of interfacial properties by molecular simulation

Some of the pitfalls that may befall molecular simulations of interfaces are discussed. They are all related to the calculation of the pressure tensor profiles, which are needed in order to compute surface tensions. We focus on three controversial points: (1) the calculation of the pressure tensor profiles for polyatomic systems, in particular, when the SHAKE algorithm is employed, (2) the addition of long-range corrections to compensate the truncation of the potential, and (3) the importance of including adequate error intervals with the results. Most of the conclusions are general, but some specifically apply to multiple site molecular-dynamics simulations.

Phase and interface behaviors in type-I and type-V Lennard-Jones mixtures: theory and simulations.

Density gradient theory (DGT) and molecular-dynamics (MD) simulations have been used to predict subcritical phase and interface behaviors in type-I and type-V equal-size Lennard-Jones mixtures. Type-I mixtures exhibit a continuum critical line connecting their pure critical components, which implies that their subcritical phase equilibria are gas liquid. Type-V mixtures are characterized by two critical lines and a heteroazeotropic line. One of the two critical lines begins at the more volatile pure component critical point up to an upper critical end point and the other one comes from the less volatile pure component critical point ending at a lower critical end point. The heteroazeotropic line connects both critical end points and is characterized by gas-liquid-liquid equilibria. Therefore, subcritical states of this type exhibit gas-liquid and gas-liquid-liquid equilibria. In order to obtain a correct characterization of the phase and interface behaviors of these types of mixtures and to directly compare DGT and MD results, the global phase diagram of equal-size Lennard-Jones mixtures has been used to define the molecular parameters of these mixtures. According to our results, DGT and MD are two complementary methodologies able to obtain a complete and simultaneous prediction of phase equilibria and their interfacial properties. For the type of mixtures analyzed here, both approaches have shown excellent agreement in their phase equilibrium and interface properties in the full concentration range.

Free-Volume Theory Coupled with Soft-SAFT for Viscosity Calculations: Comparison with Molecular Simulation and Experimental Data

The evaluation of phase equilibria and solubility properties through theoretical approaches is a well-known field, where a significant amount of models are able to describe them with a good degree of accuracy. However, the simultaneous calculation of transport properties together with thermodynamic phase properties still remains a challenge, due to the difficulties in describing the behavior of properties like the viscosity of fluids with the same approach. In this work, the free-volume theory (FVT) has been coupled with the soft-SAFT equation for the first time to extend the capabilities of the equation to the calculation of transport properties. The theory has been first tested using simulation data of the viscosity of the Lennard-Jones (LJ) fluid and LJ chains over a wide range of temperature and pressure. Good agreement has been found at all chain lengths, except for some deviations at near-zero density values. Several trends of the viscosity parameters with the length of the chain are identified, allowing the prediction of other chain fluids. Finally, the new equation has been applied to the n-alkanes family, where viscosity is a key property, and results are compared with experimental data. The three viscosity parameters were fitted to viscosity data of the pure fluid at several isotherms or isobars, whereas the density and pressure (or temperature) were taken from the soft-SAFT output. Again, the effect of these parameters on the viscosity has been investigated and compared with results obtained for the LJ chains and with previous work of other authors. The new equation performs very well in all cases, with a global average absolute deviation of 2.12% and shows predictive capabilities for heavier compounds. This empowers soft-SAFT with new capabilities, allowing the equation to calculate phase, interfacial, and transport properties with the same model and degree of accuracy.

Surface Tension of Binary Mixtures of 1-Alkyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquids: Experimental Measurements and Soft-SAFT Modeling

Ionic liquids have attracted a large amount of interest in the past few years. One approach to better understand their peculiar nature and characteristics is through the analysis of their surface properties. Some research has provided novel information on the organization of pure ionic liquids at the vapor-liquid interface; yet, a systematic study on the surface properties of mixtures of ionic liquids and their organization at the surface has not previously been carried out in the literature. This work reports, for the first time, a comprehensive analysis of the surface organization of mixtures of ionic liquids constituted by 1-alkyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ionic liquids, [C(n)mim][NTf(2)]. The surface tension of mixtures composed of [C(4)mim][NTf(2)] + [C(n)mim][NTf(2)] (n = 1, 2, 5, 6, 8, and 10) was experimentally determined, at 298.2 K and atmospheric pressure, in the whole composition range. From the experimental data, the surface tension deviations and the relative Gibbs adsorption isotherms were estimated showing how the surface composition of an ionic liquid mixture differs from that of the liquid bulk and that the surface is enriched by the ionic liquid with the longest alkyl chain length. Finally, the soft-SAFT equation of state coupled with the density gradient theory (DGT) was used, for the first time, to successfully reproduce the surface tension experimental data of binary mixtures of ionic liquids using a molecular-based approach. In addition, the DGT was used to compute the density profiles of the two components across the interface, confirming the experimental results for the components distribution at the bulk and at the vapor-liquid interface.

Critical behavior and partial miscibility phenomena in binary mixtures of hydrocarbons by the statistical associating fluid theory

Predictions of critical lines and partial miscibility of binary mixtures of hydrocarbons have been made by using a modified version of the statistical associating fluid theory (SAFT). The so-called soft-SAFT equation of state uses the Lennard-Jones potential for the reference fluid, instead of the hard-sphere potential of the original SAFT, accounting explicitly for the repulsive and dispersive forces in the reference term. The mixture behavior is predicted once an adequate set of molecular parameters (segment size, dispersive energy, and chain length) of the pure fluid is available. We use two sets of such parameters. The first set is obtained by fitting to the experimental saturated liquid density and by equating the chemical potential in the liquid and vapor phases for a range of temperatures and pressures. The second set is obtained from the previous one, by rescaling the segment size and dispersive energy to the experimental critical temperature and pressure. Results obtained from the theory with these parameters are compared to experimental results of hydrocarbon binary mixtures. The first set gives only qualitative agreement with experimental critical lines, although the general trend is correctly predicted. The agreement is excellent, however, when soft-SAFT is used with the rescaled molecular parameters, showing the ability of SAFT to quantitatively predict the behavior of mixtures. The equation is also able to predict transitions from complete to partial miscibility in binary mixtures containing methane.