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 availabl

Systems involving hydrogenated and fluorinated chains: Volumetric properties of Perfluoroalkanes and Perfluoroalkylalkane Surfactants

As part of a combined experimental and theoretical study of the thermodynamic properties of perfluoroalkylalkanes (PFAAs), the liquid density of perfluorobutylpentane (F4H5), perfluorobutylhexane (F4H6), and perfluorobutyloctane (F4H8) was measured as a function of temperature from 278.15 to 353.15 K and from atmospheric pressure to 70 MPa. The liquid densities of n-perfluoropentane, n-perfluorohexane, n-perfluorooctane, and n-perfluorononane were also measured at room pressure over the same temperature range. The PVT behavior of the PFAAs was also studied using the SAFT-VR equation of state. The PFAA molecules were modeled as heterosegmented diblock chains, using different parameters for the alkyl and perfluoroalkyl segments, that were developed in earlier work. Through this simple approach, we are able to predict the thermodynamic behavior of the perfluoroalkylalkanes, without fitting to any experimental data for the systems being studied. Molecular dynamics simulations have also been performed and used to calculate the densities of the perfluoroalkylalkanes studied

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

Test-area simulation method for the direct determination of the interfacial tension of systems with continuous or discontinuous potentials

A novel test-area TA technique for the direct simulation of the interfacial tension of systems interacting through arbitrary intermolecular potentials is presented in this paper. The most commonly used method invokes the mechanical relation for the interfacial tension in terms of the tangential and normal components of the pressure tensor relative to the interface the relation of Kirkwood and Buff J. Chem. Phys. 17, 338 1949 . For particles interacting through discontinuous intermolecular potentials e.g., hard-core fluids this involves the determination of functions which are impractical to evaluate, particularly in the case of nonspherical molecules. By contrast we employ a thermodynamic route to determine the surface tension from a free-energy perturbation due to a test change in the surface area. There are important distinctions between our test-area approach and the computation of a free-energy difference of two or more systems with different interfacial areas the method of Bennett J. Comput. Phys. 22, 245 1976 , which can also be used to determine the surface tension. In order to demonstrate the adequacy of the method, the surface tension computed from test-area Monte Carlo TAMC simulations are compared with the data obtained with other techniques e.g., mechanical and free-energy differences for the vapor-liquid interface of Lennard-Jones and square-well fluids; the latter corresponds to a discontinuous potential which is difficult to treat with standard methods. Our thermodynamic test-area approach offers advantages over existing techniques of computational efficiency, ease of implementation, and generality. The TA method can easily be implemented within either Monte Carlo TAMC or molecular-dynamics TAMD algorithms for different types of interfaces vapor-liquid, liquid-liquid, fluid-solid, etc. of pure systems and mixtures consisting of complex polyatomic molecules

Uso de composta, minerales primarios amorfos y microorganismos para la producción y calidad de tomate

This research was carried out with the aim to determine the influence of the bocashi compost, amorphous primary minerals and microorganisms in post-harvest yield and quality of saladette tomato (Lycopersicum esculentum Mill.) hibrid “Moctezuma”. The crop was carried in soil under greenhouse conditions during the agricultural cycle 2011-2012 autumn-winter. A population density of 2.5 plants m2 managed to a single stem and a separation of 1.6 m between rows was used. The bocashi compost and amorphous primary minerals were incorporated into the soil 30 days before the start of cultivation. Total and extra-large, large, medium and small tomatoes yields of eight clusters per plant and their postharvest quality were evaluated. An experimental design of randomized complete blocks with nine treatments was applied for yield variables, while a completely randomized design was used for postharvest quality variables. The highest yield (113.97 Mg ha-1) was obtained with the treatment consisting of 25 and 6 Mg ha-1 of compost and amorphous primary minerals, respectively, which exceeded 9.3% those achieved with the Steiner solution based treatments, although no significant statistically difference were found between the two averages. pH, titratable acidity and total soluble solids of fruits were not significantly affected by treatments.La presente investigación se realizó con el objetivo de determinar el efecto de la composta bocashi, minerales primarios amorfos y microorganismos en el rendimiento y calidad postcosecha de tomate tipo saladette (Lycopersicum esculentum Mill.), híbrido “Moctezuma”. El cultivo se realizó en suelo bajo condiciones de invernadero, durante el ciclo agrícola otoño-invierno 2011-2012. Se utilizó una densidad de población de 2,5 plantas m2, manejadas a un tallo y con una separación de 1,6 m entre hileras. La composta bocashi, los minerales primarios amorfos y los microorganismos fueron incorporados al suelo 30 días antes del inicio del cultivo, y estos últimos también se aplicaron después del trasplante. Se evaluó el rendimiento total y de tomates extra grandes, grandes, medianos y chicos de ocho racimos por planta, así como su calidad postcosecha. Para las variables de rendimiento se consideró un diseño experimental de bloques completos al azar conformado por nueve tratamientos, mientras que para el estudio de calidad postcosecha se utilizó un diseño completamente al azar con los mismos tratamientos. El mayor rendimiento (113,97 Mg ha-1) se obtuvo con el tratamiento compuesto por 25 y 6 Mg ha-1 de composta y minerales primarios amorfos, respectivamente, el cual superó en 9,3 % a los rendimientos logrados solo con la solución Steiner, aunque sin diferencia estadísticamente significativa entre ambos promedios. Los tratamientos no tuvieron ningún efecto en el pH, la acidez titulable y los sólidos solubles totales

Solubility of Methane in Water: Some Useful Results for Hydrate Nucleation

In this paper, the solubility of methane in water along the 400 bar isobar is determined by computer simulations using the TIP4P/Ice force field for water and a simple LJ model for methane. In particular, the solubility of methane in water when in contact with the gas phase and the solubility of methane in water when in contact with the hydrate has been determined. The solubility of methane in a gas–liquid system decreases as temperature increases. The solubility of methane in a hydrate–liquid system increases with temperature. The two curves intersect at a certain temperature that determines the triple point T3 at a certain pressure. We also determined T3 by the three-phase direct coexistence method. The results of both methods agree, and we suggest 295(2) K as the value of T3 for this system. We also analyzed the impact of curvature on the solubility of methane in water. We found that the presence of curvature increases the solubility in both the gas–liquid and hydrate–liquid systems. The change in chemical potential for the formation of hydrate is evaluated along the isobar using two different thermodynamic routes, obtaining good agreement between them. It is shown that the driving force for hydrate nucleation under experimental conditions is higher than that for the formation of pure ice when compared at the same supercooling. We also show that supersaturation (i.e., concentrations above those of the planar interface) increases the driving force for nucleation dramatically. The effect of bubbles can be equivalent to that of an additional supercooling of about 20 K. Having highly supersaturated homogeneous solutions makes possible the spontaneous formation of the hydrate at temperatures as high as 285 K (i.e., 10K below T3). The crucial role of the concentration of methane for hydrate formation is clearly revealed. Nucleation of the hydrate can be either impossible or easy and fast depending on the concentration of methane which seems to play the leading role in the understanding of the kinetics of hydrate formation

Classical density functional theory for the prediction of the surface tension and interfacial properties of fluids mixtures of chain molecules based on the statistical associating fluid theory for potentials of variable range

The statistical associating fluid theory for attractive potentials of variable range (SAFT-VR) density functional theory (DFT) developed by [G. J. Gloor et al., J. Chem. Phys. 121, 12740 (2004)] is revisited and generalized to treat mixtures. The Helmholtz free-energy functional, which is based on the SAFT-VR approach for homogeneous fluids, is constructed by partitioning the free-energy density into a reference term (which incorporates all of the short-range interactions and is treated locally) and an attractive perturbation (which incorporates the long-range dispersion interactions). In this work, two different functionals are compared. In the first, one uses a mean-field version of the theory to treat the long-range dispersive interaction, incorporating an approximate treatment of the effect of the correlations on the attractive energy between the segments by introducing a short-range attractive contribution in the reference term. In the second, one approximates the correlation function of the molecular segments in the inhomogeneous system with that of a homogeneous system for an average density of the two positions, following the ideas proposed by Toxvaerd [S. Toxvaerd, J. Chem. Phys. 64, 2863 (1976)]. The SAFT-VR DFT formalism is then used to study interfacial properties and adsorption phenomena at the interface. A detailed analysis of the influence of the molecular parameters on the surface tension and density/composition profiles of the mixtures is undertaken for binary mixtures of molecules of different chain length, segment diameter, dispersive energy, and attractive range. The effect of the asymmetry of the molecular species on the adsorption phenomena is examined in some depth. The adequacy of the approach is demonstrated by comparing the theoretical predictions with the interfacial properties of some real mixtures. The relative merits of the two approximate free-energy functionals are assessed by examining the vapor-liquid interfacial tension of selected mixtures of n-alkanes. The theory generally provides an excellent description of the interfacial properties of the mixtures without the need for further adjustment of intermolecular parameters obtained from an examination of the bulk fluid-phase behavior alone.F.J.B. acknowledges the financial support from the Spanish Dirección General de Investigación (Project No. FIS2007-66079-C02-02) and the Proyecto de Excelencia from Junta de Andalucía (Grant No. P07-FQM02884). Support from Universidad de Huelva and Junta de Andalucía is also acknowledged. F.L. thanks Shell for funding a research fellowship. Additional funding from the Engineering and Physical Sciences Research Council of the U.K. (EPSRC Grant No. EP/E016340) is also gratefully acknowledged by the Molecular Systems Engineering Group, as is the award of a refurbishment grant for the Royal Society-Wolfson Foundation

Quid: observatorio de medios

El informe está dividido en cuatro apartados: “Derecho a la información y transparencia”, “La televisión mexicana”, “Empresas y prácticas periodísticas” y “Los que se fueron”. En el primero de ellos se presenta un texto que ayuda a entender cuál es el momento en el que se encuentran las propuestas legislativas para regular a los medios y las telecomunicaciones en México, y una evaluación de los primeros cinco años del Instituto de Transparencia e Información Pública de Jalisco. El segundo apartado del informe es ecléctico, pues se compone de artículos que trabajan distintas temáticas de la televisión:la estructura y oferta de la televisión en nuestro país (en particular en la ciudad de Guadalajara), la televisión por cable (enfatizando el caso de Megacable), un recuento de cómo se gestó el Canal 44 y de sus prospectivas en 2011, y los mundiales de futbol. La tercera parte del informe documenta algunas de las situaciones más importantes que se viven en el periodismo local: estos trabajos presentan sistemas en crisis (alta vulnerabilidad de los periodistas mexicanos ante un clima de violencia que lejos de disminuir va en aumento, y la participación, por acción u omisión, del Estado mexicano en la sistemática violación de los derechos de quienes dedican su vida al trabajo periodístico. Los siguientes artículos tratan sobre las transformaciones de las empresas periodísticas, particularmente las del sector de la prensa escrita: la rápida e inexorable desaparición de los suplementos culturales, y una radiografía sobre las formas de producción de algunas secciones internacionales de los periódicos tapatíos. Al final se presentan las semblanzas de José Galindo, Raúl Mora Lomelí, S.J., Tomás Eloy Martínez y Juan Pablo Rosell.ITESO, A.C