1. Introduction The non-central interactions of molecular fluids can be modeled in several ways. Best known among these is probably the multi-centre (interaction site) model El]. A rather different model that can be used is the Kihara... more
1. Introduction The non-central interactions of molecular fluids can be modeled in several ways. Best known among these is probably the multi-centre (interaction site) model El]. A rather different model that can be used is the Kihara potential. In the latter model the ...
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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,... more
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 equilibriu...
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In this work, we studied the effect of Li+, Na+, K+, Mg2+, and Ca2+ chlorides and sulfates on the temperature of maximum density (TMD) of aqueous solutions at room pressure. Experiments at 1 molal salt concentration were carried out to... more
In this work, we studied the effect of Li+, Na+, K+, Mg2+, and Ca2+ chlorides and sulfates on the temperature of maximum density (TMD) of aqueous solutions at room pressure. Experiments at 1 molal salt concentration were carried out to determine the TMD of these solutions. We also performed molecular dynamics simulations to estimate the TMD at 1 and 2 m with the Madrid-2019 force field, which uses the TIP4P/2005 water model and scaled charges for the ions, finding an excellent agreement between experiment and simulation. All the salts studied in this work shift the TMD of the solution to lower temperatures and flatten the density vs temperature curves (when compared to pure water) with increasing salt concentration. The shift in the TMD depends strongly on the nature of the electrolyte. In order to explore this dependence, we have evaluated the contribution of each ion to the shift in the TMD concluding that Na+, Ca2+, and [Formula: see text] seem to induce the largest changes among...
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... C PROGRAM CICLOS * C THIS PROGRAM HAS BEEN WRITTEN BY CABLOS VEGA AND SANTIAGO ... IS THEN C FALSE) * C * C * C ALGESO DETERMINES THE VALUE OF PASO * C * C * * * * * KBOLT i .380 SD 16 ALGESO ( eLi .. ...
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We present a study of homogeneous crystal nucleation from metastable fluids via the seeding technique for four different systems: mW water, Tosi-Fumi NaCl, Lennard-Jones, and Hard Spheres. Combining simulations of spherical crystal seeds... more
We present a study of homogeneous crystal nucleation from metastable fluids via the seeding technique for four different systems: mW water, Tosi-Fumi NaCl, Lennard-Jones, and Hard Spheres. Combining simulations of spherical crystal seeds embedded in the metastable fluid with classical nucleation theory, we are able to successfully describe the nucleation rate for all systems in a wide range of metastability. The crystal-fluid interfacial free energy extrapolated to coexistence conditions is also in good agreement with direct calculations of such parameter. Our results show that seeding is a powerful technique to investigate crystal nucleation.
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We use computer simulations to investigate the effect of salt on homogeneous ice nucleation. The melting point of the employed solution model was obtained both by direct coexistence simulations and by thermodynamic integration from... more
We use computer simulations to investigate the effect of salt on homogeneous ice nucleation. The melting point of the employed solution model was obtained both by direct coexistence simulations and by thermodynamic integration from previous calculations of the water chemical potential. Using a seeding approach, in which we simulate ice seeds embedded in a supercooled aqueous solution, we compute the nucleation rate as a function of temperature for a 1.85 NaCl mol per water kilogram solution at 1 bar. To improve the accuracy and reliability of our calculations, we combine seeding with the direct computation of the ice-solution interfacial free energy at coexistence using the Mold Integration method. We compare the results with previous simulation work on pure water to understand the effect caused by the solute. The model captures the experimental trend that the nucleation rate at a given supercooling decreases when adding salt. Despite the fact that the thermodynamic driving force fo...
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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... more
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 hydrat...
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The so-called thermodynamic anomalies of water form an integral part of the peculiar behaviour of this both important and ubiquitous molecule. In this paper our aim is to establish whether the recently proposed TIP4P/2005 model is capable... more
The so-called thermodynamic anomalies of water form an integral part of the peculiar behaviour of this both important and ubiquitous molecule. In this paper our aim is to establish whether the recently proposed TIP4P/2005 model is capable of reproducing a number of these anomalies. Using molecular dynamics simulations we investigate both the maximum in density and the minimum in the isothermal compressibility along a number of isobars. It is shown that the model correctly describes the decrease in the temperature of the density maximum with increasing pressure. At atmospheric pressure the model exhibits an additional minimum in density at a temperature of about 200K, in good agreement with recent experimental work on super-cooled confined water. The model also presents a minimum in the isothermal compressibility close to 310K. We have also investigated the atmospheric pressure isobar for three other water models; the SPC/E and TIP4P models also present a minimum in the isothermal co...
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Ice nucleation is a phenomenon that, despite the relevant implications for life, atmospheric sciences, and technological applications, is far from being completely understood, especially under extreme thermodynamic conditions. In this... more
Ice nucleation is a phenomenon that, despite the relevant implications for life, atmospheric sciences, and technological applications, is far from being completely understood, especially under extreme thermodynamic conditions. In this work we present a computational investigation of the homogeneous ice nucleation at negative pressures. By means of the seeding technique we estimate the size of the ice critical nucleus N_{c} for the TIP4P/Ice water model. This is done along the isotherms 230, 240, and 250 K, from positive to negative pressures until reaching the liquid-gas kinetic stability limit (where cavitation cannot be avoided). We find that N_{c} is nonmonotonic upon depressurization, reaching a minimum at negative pressures in the doubly metastable region of water. According to classical nucleation theory we establish the nucleation rate J and the surface tension γ, revealing a retracing behavior of both when the liquid-gas kinetic stability limit is approached. We also predict...
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Molecular Physics Publication details, including instructions for authors and subscription information:
... Carlos Vega, Santiago Lago,* and Paz Padilla Departamento de Quimica Fisica, Facultad de Ciencias Quimicas, Uniuersidad Complutense, 28040 Madrid, Spain (Received ... where {pij) stands for the set of shortest distances between the... more
... Carlos Vega, Santiago Lago,* and Paz Padilla Departamento de Quimica Fisica, Facultad de Ciencias Quimicas, Uniuersidad Complutense, 28040 Madrid, Spain (Received ... where {pij) stands for the set of shortest distances between the rod labeled as i of molecule 1 and rod ...
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The vapor-liquid equilibria of 20 substances, most of them widely used as organic solvents, were obtained by means of a Gibbs ensemble Monte Carlo method. All these substances can be represented by linear or angular models with only two... more
The vapor-liquid equilibria of 20 substances, most of them widely used as organic solvents, were obtained by means of a Gibbs ensemble Monte Carlo method. All these substances can be represented by linear or angular models with only two bonds. The intermolecular interaction was described by a Kihara potential and, where appropriate, an additional multipolar potential using meaningful microscopic parameters.
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The role of cubic ice, ice Ic, in the nucleation of ice from supercooled water has been widely debated in the past decade. Computer simulations can provide insightful information about the mechanism of ice nucleation at a molecular scale.... more
The role of cubic ice, ice Ic, in the nucleation of ice from supercooled water has been widely debated in the past decade. Computer simulations can provide insightful information about the mechanism of ice nucleation at a molecular scale. In this work, we use molecular dynamics to study the competition between ice Ic and hexagonal ice, ice Ih, in the process of ice nucleation. Using a seeding approach, in which classical nucleation theory is combined with simulations of ice clusters embedded in supercooled water, we estimate the nucleation rate of ice for a pathway in which the critical nucleus has an Ic structure. Comparing our results with those previously obtained for ice Ih [Sanz et al., J. Am. Chem. Soc. 135, 15008 (2013)], we conclude that within the accuracy of our calculations both nucleation pathways have the same rate for the studied water models (TIP4P/Ice and TIP4P/2005). We examine in detail the factors that contribute to the nucleation rate and find that the chemical potential difference with the fluid, the attachment rate of particles to the cluster, and the ice-water interfacial free energy are the same within the estimated margin of error for both ice polymorphs. Furthermore, we study the morphology of the ice clusters and conclude that they have a spherical shape.
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ABSTRACT The sedimentation of a polydisperse suspension of small spherical particles dispersed in a viscous fluid, where particles belong to N species differing in size, can be described by a strongly coupled system of N scalar, nonlinear... more
ABSTRACT The sedimentation of a polydisperse suspension of small spherical particles dispersed in a viscous fluid, where particles belong to N species differing in size, can be described by a strongly coupled system of N scalar, nonlinear first‐order conservation laws for the evolution of the volume fractions. The hyperbolicity of this system is a property of theoretical importance because it limits the range of validity of the model and is of practical interest for the implementation of numerical methods. The present work, which extends the results of R. Bürger, R. Donat, P. Mulet, and C.A. Vega (SIAM Journal on Applied Mathematics 2010; 70:2186–2213), is focused on the fluxes corresponding to the models by Batchelor and Wen, Höfler and Schwarzer, and Davis and Gecol, for which the Jacobian of the flux is a rank‐3 or rank‐4 perturbation of a diagonal matrix. Explicit estimates of the regions of hyperbolicity of these models are derived via the approach of the so‐called secular equation (J. Anderson. Linear Algebra and Applications 1996; 246:49–70), which identifies the eigenvalues of the Jacobian with the poles of a particular rational function. Hyperbolicity of the system is guaranteed if the coefficients of this function have the same sign. Sufficient conditions for this condition to be satisfied are established for each of the models considered. Some numerical examples are presented. Copyright © 2012 John Wiley & Sons, Ltd.
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Chromosome arrangements of twenty-eight cultivars of common wheat, Triticum aestivum L., from or introduced into Spain are compared with that of 'Chinese Spring' taken as a pattern. All the cultivars analyzed differ from... more
Chromosome arrangements of twenty-eight cultivars of common wheat, Triticum aestivum L., from or introduced into Spain are compared with that of 'Chinese Spring' taken as a pattern. All the cultivars analyzed differ from 'Chinese Spring' by one or two reciprocal translocations. When 12 out of 28 cultivars were compared it was concluded that a minimum number of thirteen interchanges are present, involving at least ten different chromosomes of the complement. The interest of a reappraisal of the rôle of interchanges in the evolution of Gramineae is pointed out.
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Abstract. Polydisperse suspensions consist of small particles which are dispersed in a viscous fluid, and which belong to a finite number N of species that differ in size or density. Spatially one-dimensional kinematic models for the... more
Abstract. Polydisperse suspensions consist of small particles which are dispersed in a viscous fluid, and which belong to a finite number N of species that differ in size or density. Spatially one-dimensional kinematic models for the sedimentation of such mixtures are given by systems of N non-linear first-order conservation laws for the vector Φ of the N local solids volume
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Two independent reciprocal translocations in the Spanish cultivar Canaleja of common wheat, Triticum aestivum L., are identified as 5B–7B and 3A-3D when compared with ‘Chinese Spring’. The translocated segments of the chromosomes 5B and... more
Two independent reciprocal translocations in the Spanish cultivar Canaleja of common wheat, Triticum aestivum L., are identified as 5B–7B and 3A-3D when compared with ‘Chinese Spring’. The translocated segments of the chromosomes 5B and 7B are much longer than those of the chromosomes 4A and 3D. The origin of the 5B–7B interchange ‘Canaleja’ and several other cultivars analyzed by different
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Page 1. Carlos Alberto Vega THE OLD SPANISH SANALEJO: HORIZON OF EXPECTATIONS AND REWORKINGS As Karl Uitti has observed, there is at least as much virtue in examining the individual textual diversities within a hagiographie tradition as... more
Page 1. Carlos Alberto Vega THE OLD SPANISH SANALEJO: HORIZON OF EXPECTATIONS AND REWORKINGS As Karl Uitti has observed, there is at least as much virtue in examining the individual textual diversities within a hagiographie tradition as in attempts to ...
Wertheim's theory is used to determine the critical properties of chains formed by m tangent spheres interacting through the pair potential u…r†. It is shown that within Wertheim's theory the critical temperature and... more
Wertheim's theory is used to determine the critical properties of chains formed by m tangent spheres interacting through the pair potential u…r†. It is shown that within Wertheim's theory the critical temperature and compressibility factor reach a ® nite non-zero value for in® nitely long chains, whereas the critical density and pressure vanish as m¡1:5. Analysing the zero density limit
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Liquid-Vapor Equilibria of Llnear Kihara Molecules ... Carlos Vega,* Santiago Lago, Departamento de Quimica Fisica, Facultad de Ciencias Quimicas. Universidad Complutense, 28040 Madrid, Spain ... Enrique de Miguel, and Luis F. Rull... more
Liquid-Vapor Equilibria of Llnear Kihara Molecules ... Carlos Vega,* Santiago Lago, Departamento de Quimica Fisica, Facultad de Ciencias Quimicas. Universidad Complutense, 28040 Madrid, Spain ... Enrique de Miguel, and Luis F. Rull Departamento de Fisica Atomica ...
ABSTRACT
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... It is seen that the metric tensor term favours conformers with gauche bonds. This explains the large differences found in the population of the ttt conformer for the RM and the FM, presented in table 2. For butane, Almarza et al. ...