Fernando del Río Haza | Universidad Autónoma Metropolitana-Iztapalapa (original) (raw)

Papers by Fernando del Río Haza

Research paper thumbnail of Las Reuniones De Fisica Estadistica, Forjadoras De Esa Disciplina en Mexico

EL FISICO FERNANDO DEL RIO HAZA HABLA SOBRE LA REALIZACION Y RESULTADOS DE LA XXV REUNION DE INVI... more EL FISICO FERNANDO DEL RIO HAZA HABLA SOBRE LA REALIZACION Y RESULTADOS DE LA XXV REUNION DE INVIERNO DE FISICA ESTADISTICA, CELEBRADA EN CUERNAVACA. ASEVERA QUE EN LOS ULTIMOS 25 ANOS LA CIENCIA EN MEXICO HA AVANZADO CONSIDERABLEMENTE, Y QUE EN LA FISICA, EN PARTICULAR, EXISTE UN CAMBIO CUALITATIVO EN LA COMPOSICION Y EL TAMANO DE LA COMUNIDAD CIENTIFICA. SENALA TAMBIEN QUE GRACIAS A REUNIONES DE ESTE TIPO LA COMUNIDAD CIENTIFICA NACIONAL SE MANTIENE EN CONTACTO CON LA COMUNIDAD INTERNACIONAL, Y SE HA LOGRADO DESARROLLAR EN MEXICO LA FISICA ESTADISTICA, CUYA FUNCION ES CONECTAR LA FISICA MICROSCOPICA CON LA MACROSCOPICA. SE ANOTAN DATOS DE LOS ANTECEDENTES DE LA REUNION INTERNACIONAL DE FISICA ESTADISTICA.

Research paper thumbnail of RESEARCH NOTE On the hard core Yukawa fluid of variable range: Monte Carlo simulations and test of the MSA equation of state

Molecular Physics, 1999

A recently proposed analytical equation of state (EOS) for the hard core Yukawa fluid is tested a... more A recently proposed analytical equation of state (EOS) for the hard core Yukawa fluid is tested against results of Monte Carlo (MC) simulations. The Duh and Mier-y-Teran (DMT) equation of state is based on the mean-spherical approximation (MSA) and gives the free energy of the fluid for arbitrary ranges of the Yukawa interaction. The pressure EOS is compared with results

Research paper thumbnail of Room temperature ionic liquids: A simple model. Effect of chain length and size of intermolecular potential on critical temperature

The Journal of Chemical Physics, 2015

A model of a room temperature ionic liquid can be represented as an ion attached to an aliphatic ... more A model of a room temperature ionic liquid can be represented as an ion attached to an aliphatic chain mixed with a counter ion. The simple model used in this work is based on a short rigid tangent square well chain with an ion, represented by a hard sphere interacting with a Yukawa potential at the head of the chain, mixed with a counter ion represented as well by a hard sphere interacting with a Yukawa potential of the opposite sign. The length of the chain and the depth of the intermolecular forces are investigated in order to understand which of these factors are responsible for the lowering of the critical temperature. It is the large difference between the ionic and the dispersion potentials which explains this lowering of the critical temperature. Calculation of liquid-vapor equilibrium orthobaric curves is used to estimate the critical points of the model. Vapor pressures are used to obtain an estimate of the triple point of the different models in order to calculate the span of temperatures where they remain a liquid. Surface tensions and interfacial thicknesses are also reported.

Research paper thumbnail of Liquid-Vapor Equilibria of Ionic Liquids from a SAFT Equation of State with Explicit Electrostatic Free Energy Contributions

The Journal of Physical Chemistry B, 2015

Statistically associating fluid theory (SAFT) provides closed-form free energies by perturbation ... more Statistically associating fluid theory (SAFT) provides closed-form free energies by perturbation methods. We propose here a SAFT equation of state for ionic liquids that models the contribution from Coulomb forces after that of the Restricted Primitive Model (RPM) in the Mean Spherical Approximation (MSA). The resulting SAFT-MSA equation, fitted to simulated orthobaric curves of imidazolium based ionic liquids, predicts experimental density data with errors ≈1% and the characteristic decrease of all critical coordinates with increasing cation size. The SAFT-MSA equation can be applied to calculate thermodynamic coefficients, the speed of sound and surface tension (among other properties) of pure ionic liquids and can be generalized straightforwardly to mixtures.

Research paper thumbnail of Reversible and irreversible heat transfer by radiation

European Journal of Physics, 2015

ABSTRACT The theme of heat transfer by radiation is absent from most textbooks on thermodynamics,... more ABSTRACT The theme of heat transfer by radiation is absent from most textbooks on thermodynamics, and its treatment in the applied literature presents some basic discrepancies concerning the validity of the Clausius relation between the quantity of heat exchanged, δQ, and the accompanying entropy change, dS. We review the reversible and irreversible heat transfers by radiation to clarify the validity of the Clausius relation, and we show that in both cases, the Clausius relation is obeyed, as it should be. We also deal with radiation diluted by the presence of matter, introducing a dilution coefficient, ϕ, and an irreversibility factor, χ ϕ( ). This treatment requires the use of the correct relation between energy and heat fluxes, the spectral fluxes of energy and entropy, and Planck’s equation for the entropy of monochromatic radiation. For the irreversible case of diluted radiation, we recover the ratio between the fluxes of heat and entropy that agree with Clausius’ inequality, including an irreversibility factor, (4 3) ( )χ ϕ . An improved modification for the explicit function χ ϕ( ) is given. As an illustration, the fluxes of energy and entropy from the Sun to the Earth are obtained. We also calculate the fluxes re-emitted by the Earth, taking into account the greenhouse effect. We find the value of1.258 Wm K− −2 1 for the reemitted entropy flux after the radiation has been thermalized, which is much larger than the incident flux, in agreement with other authors.

Research paper thumbnail of Accurate Effective Potentials of Real Substances from Acoustic Virial Coefficients

International Journal of Thermophysics, 1999

Accurate second acoustic virial coefficients have been employed to determine parameters of an eff... more Accurate second acoustic virial coefficients have been employed to determine parameters of an effective intermolecular potential for nine pure substances: argon, nitrogen, carbon dioxide, carbon tetrafluoride, and the first five alkanes (methane, ethane, propane, butane, and pentane). The values used for the second acoustic virial coefficients were taken from data reported in the literature. To obtain the form and parameters

Research paper thumbnail of Collision Diameters, Interaction Potentials, and Virial Coefficients of Small Quasi-Spherical Molecules

The Journal of Physical Chemistry, 1996

... J. Eloy Ramos, and Alejandro Gil-Villegas †. ... The method gives insight into the collision ... more ... J. Eloy Ramos, and Alejandro Gil-Villegas †. ... The method gives insight into the collision diameters and second virial coefficients, B(T), of model molecules, both spherical and nonspherical, including three-parameter potential functions and diatomic Lennard-Jones molecules. ...

Research paper thumbnail of Third Virial Coefficients of Mixtures from a Model of Two- and Three-body Forces

We present a model for the calculation of the third virial coefficient in binary mixtures of non-... more We present a model for the calculation of the third virial coefficient in binary mixtures of non-polar molecules, using as effective potentials approximate non-conformal (ANC) binary interactions and the Axilrod-Teller-Muto (ATM) ternary potential. We propose suitable combination rules to obtain the parameters for the cross-interaction ANC and ATM potentials from the pure-interaction ones, so that the model includes no free

Research paper thumbnail of Nonconformal Interaction Models and Thermodynamics of Polar Fluids

The Journal of Physical Chemistry B, 2005

Research paper thumbnail of Nonconformal Potentials and Second Virial Coefficients in Molecular Fluids. 1. Theory

The Journal of Physical Chemistry B, 1998

... J. Eloy Ramos and ... In this paper we test the applicability of the theory to a variety of n... more ... J. Eloy Ramos and ... In this paper we test the applicability of the theory to a variety of nonspherical models spherocylinders, ellipsoids, Lennard−Jones polyatomics, square-well chains, and Stockmayer molecules and show that their virial coefficients are accounted for accurately ...

Research paper thumbnail of Theoretical Equation of State of Dense Nonconformal Fluids from Effective Potentials. 1. Applications to Model Systems

The Journal of Physical Chemistry B, 2001

Research paper thumbnail of Accurate effective potentials and virial coefficients in real fluids. Part III. Alkanes and perfluoroalkanes

Physical Chemistry Chemical Physics, 2000

ABSTRACT The approximate nonconformal (ANC) theory recently proposed has been applied very succes... more ABSTRACT The approximate nonconformal (ANC) theory recently proposed has been applied very successfully to the determination of interaction pair potentials for the noble gases, homodiatomics and their mixtures. The ANC theory is used here to obtain effective angle-averaged potentials of all those n-alkanes and perfluoroalkanes for which experimental second virial coefficient data are available. The effective parameters conform to theoretical models based on spherocylinders and site–site chain molecules. The critical volumes and temperatures of these gases vary smoothly when scaled with parameters of the ANC potential. Further, the ANC model with only three parameters for each substance is able to reproduce the experimental data Bexp(T) of the second virial coefficient within experimental uncertainty for all 14 gases for which Bexp(T) has been reported.

Research paper thumbnail of Thermodynamic properties of non-conformal soft-sphere fluids with effective hard-sphere diameters

Physical Chemistry Chemical Physics, 2012

In this work we study a set of soft-sphere systems characterised by a well-defined variation of t... more In this work we study a set of soft-sphere systems characterised by a well-defined variation of their softness. These systems represent an extension of the repulsive Lennard-Jones potential widely used in statistical mechanics of fluids. This type of soft spheres is of interest because they represent quite accurately the effective intermolecular repulsion in fluid substances and also because they exhibit interesting properties. The thermodynamics of the soft-sphere fluids is obtained via an effective hard-sphere diameter approach that leads to a compact and accurate equation of state. The virial coefficients of soft spheres are shown to follow quite simple relationships that are incorporated into the equation of state. The approach followed exhibits the rescaling of the density that produces a unique equation for all systems and temperatures. The scaling is carried through to the level of the structure of the fluids.

Research paper thumbnail of Mixing rules and effective potentials in gaseous mixtures

Physica A: Statistical Mechanics and its Applications, 2009

ABSTRACT We generate exact mixing rules for dilute gaseous mixtures with nonconformal interaction... more ABSTRACT We generate exact mixing rules for dilute gaseous mixtures with nonconformal interactions by introducing an adequate effective potential. The general mixing rule leads to several approximations of the van der Waals type. The work uses nonconformal model pair potentials that accurately describe the pure fluid properties. The validity of the approximate mixing rules is assessed by comparison with exact results for binary mixtures over wide ranges of temperature and composition. Large differences in energy, size and potential profile are considered. The simplest mixing rules are straightforward generalizations of the van der Waals rules and one version, based on a square-well (SW) approximation, performs with high precision for all systems except for those with extreme differences in molecular size. The SW mixing rule is used to predict excess virial coefficients of mixtures involving Ar, Kr, H2 and CF4.

Research paper thumbnail of Liquid–vapour equilibrium of multipolar square-well fluids.Gibbs ensemble simulations and equation of state

Molecular Physics, 2005

Simulation results for a system comprising a square well plus either a point dipole or a point qu... more Simulation results for a system comprising a square well plus either a point dipole or a point quadrupole are presented. The properties obtained are the vapour-liquid equilibrium densities and the critical properties. Critical densities are not very sensitive to the values of dipole or quadrupole, while critical temperatures increase significantly when the multipole strength rises. A comparison with a perturbation

Research paper thumbnail of The thermodynamics of heteronuclear molecules formed from bonded square-well (BSW) segments using the SAFT-VR approach

Molecular Physics, 1999

We broaden the scope of the statistical associating¯uid theory for potentials of variable attract... more We broaden the scope of the statistical associating¯uid theory for potentials of variable attractive range (SAFT-VR) to treat heteronuclear chain molecules formed from bonded square-well (BSW) segments. The ideas of the bonded hard sphere (BHS) treatment for distributed-site models composed of hard-sphere segments are applied to square-well sites with the SAFT-VR approach. The results of isothermal±isobaric Monte Carlo simulations are reported for heteronuclear square-well diatomics with di erent sets of energy and range parameters. The SAFT-VR approach provides an excellent description of the equation of state of the diatomic systems for a wide range of densities. The goal of the work is to provide a rigorous treatment of distributed-site models of¯uids, and to establish a framework for a group contribution approach with SAFT-VR.

Research paper thumbnail of A model for viscosity coefficients of gases with potentials differing in form

Molecular Physics, 2003

A recent theory of non-conformal interactions has been very succesful in providing effective sphe... more A recent theory of non-conformal interactions has been very succesful in providing effective spherical potentials for the pressure of more than 40 real gases and many of their binary mixtures. Here, this theory is applied to deal with low-density viscosity coefficients. In its simplest form, the approximate non-conformal (ANC) theory introduces, besides the usual corresponding states parameters - energy ε and distance rm - a softness parameter s to account for the form of a particular potential function. We investigate the effects of the potential form on the temperature behaviour of the viscosisty coefficient eta. It is shown that the softer potentials, with wider attractive wells, have larger viscosities and an explicit expression for eta (T, ε, rm, s) is obtained. The ANC potentials are tested in their capacity to reproduce the temperature dependence of eta for the heavier noble gases (Ar, Kr and Xe), diatomics (H2, N2, O2 and Cl2) and a dozen small polyatomics. It is found that the ANC model eta (T, ε, rm, s), with only three substance-dependent parameters, reproduces experimental eta data within their estimated error.

Research paper thumbnail of Effective potential for three-body forces in fluids

Molecular Physics, 2011

... Guzmán a , Fernando del Río a * & J. Eloy Ramos b pages 955-967. ... In a pre... more ... Guzmán a , Fernando del Río a * & J. Eloy Ramos b pages 955-967. ... In a previous paper we presented a calculation of C(T) adequate for treating a variety of small non-polar molecules including the effect of three-body forces 66. Guzmán, O and del Río, F. 2007. J. Phys. B: At. ...

Research paper thumbnail of Systematic prediction of critical point coordinates from molecular parameters of equations of state and interaction potentials

Molecular Physics, 2012

We compare the accuracy of three types of models for prediction of experimental values of the cri... more We compare the accuracy of three types of models for prediction of experimental values of the critical volume VC, temperature TC and pressure PC of 15 fluids. These models (van der Waals equations of state, two-body Lennard-Jones and Approximate Non-Conformal potentials, and effective potentials including three-body-force effects) can be regarded as systematic improvements on the description of molecular interactions. We

Research paper thumbnail of Third virial coefficient of nonpolar gases from accurate binary potentials and ternary forces

Journal of Physics B: Atomic, Molecular and Optical Physics, 2007

... Third virial coefficient of nonpolar gases from accurate binary potentials and ternary forces... more ... Third virial coefficient of nonpolar gases from accurate binary potentials and ternary forces Orlando Guzmán and Fernando del Rıo Departamento de Fısica, Universidad Autónoma Metropolitana, Iztapalapa, Apdo 55 534, México DF, 09340, Mexico E-mail: fdr@xanum.uam.mx ...

Research paper thumbnail of Las Reuniones De Fisica Estadistica, Forjadoras De Esa Disciplina en Mexico

EL FISICO FERNANDO DEL RIO HAZA HABLA SOBRE LA REALIZACION Y RESULTADOS DE LA XXV REUNION DE INVI... more EL FISICO FERNANDO DEL RIO HAZA HABLA SOBRE LA REALIZACION Y RESULTADOS DE LA XXV REUNION DE INVIERNO DE FISICA ESTADISTICA, CELEBRADA EN CUERNAVACA. ASEVERA QUE EN LOS ULTIMOS 25 ANOS LA CIENCIA EN MEXICO HA AVANZADO CONSIDERABLEMENTE, Y QUE EN LA FISICA, EN PARTICULAR, EXISTE UN CAMBIO CUALITATIVO EN LA COMPOSICION Y EL TAMANO DE LA COMUNIDAD CIENTIFICA. SENALA TAMBIEN QUE GRACIAS A REUNIONES DE ESTE TIPO LA COMUNIDAD CIENTIFICA NACIONAL SE MANTIENE EN CONTACTO CON LA COMUNIDAD INTERNACIONAL, Y SE HA LOGRADO DESARROLLAR EN MEXICO LA FISICA ESTADISTICA, CUYA FUNCION ES CONECTAR LA FISICA MICROSCOPICA CON LA MACROSCOPICA. SE ANOTAN DATOS DE LOS ANTECEDENTES DE LA REUNION INTERNACIONAL DE FISICA ESTADISTICA.

Research paper thumbnail of RESEARCH NOTE On the hard core Yukawa fluid of variable range: Monte Carlo simulations and test of the MSA equation of state

Molecular Physics, 1999

A recently proposed analytical equation of state (EOS) for the hard core Yukawa fluid is tested a... more A recently proposed analytical equation of state (EOS) for the hard core Yukawa fluid is tested against results of Monte Carlo (MC) simulations. The Duh and Mier-y-Teran (DMT) equation of state is based on the mean-spherical approximation (MSA) and gives the free energy of the fluid for arbitrary ranges of the Yukawa interaction. The pressure EOS is compared with results

Research paper thumbnail of Room temperature ionic liquids: A simple model. Effect of chain length and size of intermolecular potential on critical temperature

The Journal of Chemical Physics, 2015

A model of a room temperature ionic liquid can be represented as an ion attached to an aliphatic ... more A model of a room temperature ionic liquid can be represented as an ion attached to an aliphatic chain mixed with a counter ion. The simple model used in this work is based on a short rigid tangent square well chain with an ion, represented by a hard sphere interacting with a Yukawa potential at the head of the chain, mixed with a counter ion represented as well by a hard sphere interacting with a Yukawa potential of the opposite sign. The length of the chain and the depth of the intermolecular forces are investigated in order to understand which of these factors are responsible for the lowering of the critical temperature. It is the large difference between the ionic and the dispersion potentials which explains this lowering of the critical temperature. Calculation of liquid-vapor equilibrium orthobaric curves is used to estimate the critical points of the model. Vapor pressures are used to obtain an estimate of the triple point of the different models in order to calculate the span of temperatures where they remain a liquid. Surface tensions and interfacial thicknesses are also reported.

Research paper thumbnail of Liquid-Vapor Equilibria of Ionic Liquids from a SAFT Equation of State with Explicit Electrostatic Free Energy Contributions

The Journal of Physical Chemistry B, 2015

Statistically associating fluid theory (SAFT) provides closed-form free energies by perturbation ... more Statistically associating fluid theory (SAFT) provides closed-form free energies by perturbation methods. We propose here a SAFT equation of state for ionic liquids that models the contribution from Coulomb forces after that of the Restricted Primitive Model (RPM) in the Mean Spherical Approximation (MSA). The resulting SAFT-MSA equation, fitted to simulated orthobaric curves of imidazolium based ionic liquids, predicts experimental density data with errors ≈1% and the characteristic decrease of all critical coordinates with increasing cation size. The SAFT-MSA equation can be applied to calculate thermodynamic coefficients, the speed of sound and surface tension (among other properties) of pure ionic liquids and can be generalized straightforwardly to mixtures.

Research paper thumbnail of Reversible and irreversible heat transfer by radiation

European Journal of Physics, 2015

ABSTRACT The theme of heat transfer by radiation is absent from most textbooks on thermodynamics,... more ABSTRACT The theme of heat transfer by radiation is absent from most textbooks on thermodynamics, and its treatment in the applied literature presents some basic discrepancies concerning the validity of the Clausius relation between the quantity of heat exchanged, δQ, and the accompanying entropy change, dS. We review the reversible and irreversible heat transfers by radiation to clarify the validity of the Clausius relation, and we show that in both cases, the Clausius relation is obeyed, as it should be. We also deal with radiation diluted by the presence of matter, introducing a dilution coefficient, ϕ, and an irreversibility factor, χ ϕ( ). This treatment requires the use of the correct relation between energy and heat fluxes, the spectral fluxes of energy and entropy, and Planck’s equation for the entropy of monochromatic radiation. For the irreversible case of diluted radiation, we recover the ratio between the fluxes of heat and entropy that agree with Clausius’ inequality, including an irreversibility factor, (4 3) ( )χ ϕ . An improved modification for the explicit function χ ϕ( ) is given. As an illustration, the fluxes of energy and entropy from the Sun to the Earth are obtained. We also calculate the fluxes re-emitted by the Earth, taking into account the greenhouse effect. We find the value of1.258 Wm K− −2 1 for the reemitted entropy flux after the radiation has been thermalized, which is much larger than the incident flux, in agreement with other authors.

Research paper thumbnail of Accurate Effective Potentials of Real Substances from Acoustic Virial Coefficients

International Journal of Thermophysics, 1999

Accurate second acoustic virial coefficients have been employed to determine parameters of an eff... more Accurate second acoustic virial coefficients have been employed to determine parameters of an effective intermolecular potential for nine pure substances: argon, nitrogen, carbon dioxide, carbon tetrafluoride, and the first five alkanes (methane, ethane, propane, butane, and pentane). The values used for the second acoustic virial coefficients were taken from data reported in the literature. To obtain the form and parameters

Research paper thumbnail of Collision Diameters, Interaction Potentials, and Virial Coefficients of Small Quasi-Spherical Molecules

The Journal of Physical Chemistry, 1996

... J. Eloy Ramos, and Alejandro Gil-Villegas †. ... The method gives insight into the collision ... more ... J. Eloy Ramos, and Alejandro Gil-Villegas †. ... The method gives insight into the collision diameters and second virial coefficients, B(T), of model molecules, both spherical and nonspherical, including three-parameter potential functions and diatomic Lennard-Jones molecules. ...

Research paper thumbnail of Third Virial Coefficients of Mixtures from a Model of Two- and Three-body Forces

We present a model for the calculation of the third virial coefficient in binary mixtures of non-... more We present a model for the calculation of the third virial coefficient in binary mixtures of non-polar molecules, using as effective potentials approximate non-conformal (ANC) binary interactions and the Axilrod-Teller-Muto (ATM) ternary potential. We propose suitable combination rules to obtain the parameters for the cross-interaction ANC and ATM potentials from the pure-interaction ones, so that the model includes no free

Research paper thumbnail of Nonconformal Interaction Models and Thermodynamics of Polar Fluids

The Journal of Physical Chemistry B, 2005

Research paper thumbnail of Nonconformal Potentials and Second Virial Coefficients in Molecular Fluids. 1. Theory

The Journal of Physical Chemistry B, 1998

... J. Eloy Ramos and ... In this paper we test the applicability of the theory to a variety of n... more ... J. Eloy Ramos and ... In this paper we test the applicability of the theory to a variety of nonspherical models spherocylinders, ellipsoids, Lennard−Jones polyatomics, square-well chains, and Stockmayer molecules and show that their virial coefficients are accounted for accurately ...

Research paper thumbnail of Theoretical Equation of State of Dense Nonconformal Fluids from Effective Potentials. 1. Applications to Model Systems

The Journal of Physical Chemistry B, 2001

Research paper thumbnail of Accurate effective potentials and virial coefficients in real fluids. Part III. Alkanes and perfluoroalkanes

Physical Chemistry Chemical Physics, 2000

ABSTRACT The approximate nonconformal (ANC) theory recently proposed has been applied very succes... more ABSTRACT The approximate nonconformal (ANC) theory recently proposed has been applied very successfully to the determination of interaction pair potentials for the noble gases, homodiatomics and their mixtures. The ANC theory is used here to obtain effective angle-averaged potentials of all those n-alkanes and perfluoroalkanes for which experimental second virial coefficient data are available. The effective parameters conform to theoretical models based on spherocylinders and site–site chain molecules. The critical volumes and temperatures of these gases vary smoothly when scaled with parameters of the ANC potential. Further, the ANC model with only three parameters for each substance is able to reproduce the experimental data Bexp(T) of the second virial coefficient within experimental uncertainty for all 14 gases for which Bexp(T) has been reported.

Research paper thumbnail of Thermodynamic properties of non-conformal soft-sphere fluids with effective hard-sphere diameters

Physical Chemistry Chemical Physics, 2012

In this work we study a set of soft-sphere systems characterised by a well-defined variation of t... more In this work we study a set of soft-sphere systems characterised by a well-defined variation of their softness. These systems represent an extension of the repulsive Lennard-Jones potential widely used in statistical mechanics of fluids. This type of soft spheres is of interest because they represent quite accurately the effective intermolecular repulsion in fluid substances and also because they exhibit interesting properties. The thermodynamics of the soft-sphere fluids is obtained via an effective hard-sphere diameter approach that leads to a compact and accurate equation of state. The virial coefficients of soft spheres are shown to follow quite simple relationships that are incorporated into the equation of state. The approach followed exhibits the rescaling of the density that produces a unique equation for all systems and temperatures. The scaling is carried through to the level of the structure of the fluids.

Research paper thumbnail of Mixing rules and effective potentials in gaseous mixtures

Physica A: Statistical Mechanics and its Applications, 2009

ABSTRACT We generate exact mixing rules for dilute gaseous mixtures with nonconformal interaction... more ABSTRACT We generate exact mixing rules for dilute gaseous mixtures with nonconformal interactions by introducing an adequate effective potential. The general mixing rule leads to several approximations of the van der Waals type. The work uses nonconformal model pair potentials that accurately describe the pure fluid properties. The validity of the approximate mixing rules is assessed by comparison with exact results for binary mixtures over wide ranges of temperature and composition. Large differences in energy, size and potential profile are considered. The simplest mixing rules are straightforward generalizations of the van der Waals rules and one version, based on a square-well (SW) approximation, performs with high precision for all systems except for those with extreme differences in molecular size. The SW mixing rule is used to predict excess virial coefficients of mixtures involving Ar, Kr, H2 and CF4.

Research paper thumbnail of Liquid–vapour equilibrium of multipolar square-well fluids.Gibbs ensemble simulations and equation of state

Molecular Physics, 2005

Simulation results for a system comprising a square well plus either a point dipole or a point qu... more Simulation results for a system comprising a square well plus either a point dipole or a point quadrupole are presented. The properties obtained are the vapour-liquid equilibrium densities and the critical properties. Critical densities are not very sensitive to the values of dipole or quadrupole, while critical temperatures increase significantly when the multipole strength rises. A comparison with a perturbation

Research paper thumbnail of The thermodynamics of heteronuclear molecules formed from bonded square-well (BSW) segments using the SAFT-VR approach

Molecular Physics, 1999

We broaden the scope of the statistical associating¯uid theory for potentials of variable attract... more We broaden the scope of the statistical associating¯uid theory for potentials of variable attractive range (SAFT-VR) to treat heteronuclear chain molecules formed from bonded square-well (BSW) segments. The ideas of the bonded hard sphere (BHS) treatment for distributed-site models composed of hard-sphere segments are applied to square-well sites with the SAFT-VR approach. The results of isothermal±isobaric Monte Carlo simulations are reported for heteronuclear square-well diatomics with di erent sets of energy and range parameters. The SAFT-VR approach provides an excellent description of the equation of state of the diatomic systems for a wide range of densities. The goal of the work is to provide a rigorous treatment of distributed-site models of¯uids, and to establish a framework for a group contribution approach with SAFT-VR.

Research paper thumbnail of A model for viscosity coefficients of gases with potentials differing in form

Molecular Physics, 2003

A recent theory of non-conformal interactions has been very succesful in providing effective sphe... more A recent theory of non-conformal interactions has been very succesful in providing effective spherical potentials for the pressure of more than 40 real gases and many of their binary mixtures. Here, this theory is applied to deal with low-density viscosity coefficients. In its simplest form, the approximate non-conformal (ANC) theory introduces, besides the usual corresponding states parameters - energy ε and distance rm - a softness parameter s to account for the form of a particular potential function. We investigate the effects of the potential form on the temperature behaviour of the viscosisty coefficient eta. It is shown that the softer potentials, with wider attractive wells, have larger viscosities and an explicit expression for eta (T, ε, rm, s) is obtained. The ANC potentials are tested in their capacity to reproduce the temperature dependence of eta for the heavier noble gases (Ar, Kr and Xe), diatomics (H2, N2, O2 and Cl2) and a dozen small polyatomics. It is found that the ANC model eta (T, ε, rm, s), with only three substance-dependent parameters, reproduces experimental eta data within their estimated error.

Research paper thumbnail of Effective potential for three-body forces in fluids

Molecular Physics, 2011

... Guzmán a , Fernando del Río a * & J. Eloy Ramos b pages 955-967. ... In a pre... more ... Guzmán a , Fernando del Río a * & J. Eloy Ramos b pages 955-967. ... In a previous paper we presented a calculation of C(T) adequate for treating a variety of small non-polar molecules including the effect of three-body forces 66. Guzmán, O and del Río, F. 2007. J. Phys. B: At. ...

Research paper thumbnail of Systematic prediction of critical point coordinates from molecular parameters of equations of state and interaction potentials

Molecular Physics, 2012

We compare the accuracy of three types of models for prediction of experimental values of the cri... more We compare the accuracy of three types of models for prediction of experimental values of the critical volume VC, temperature TC and pressure PC of 15 fluids. These models (van der Waals equations of state, two-body Lennard-Jones and Approximate Non-Conformal potentials, and effective potentials including three-body-force effects) can be regarded as systematic improvements on the description of molecular interactions. We

Research paper thumbnail of Third virial coefficient of nonpolar gases from accurate binary potentials and ternary forces

Journal of Physics B: Atomic, Molecular and Optical Physics, 2007

... Third virial coefficient of nonpolar gases from accurate binary potentials and ternary forces... more ... Third virial coefficient of nonpolar gases from accurate binary potentials and ternary forces Orlando Guzmán and Fernando del Rıo Departamento de Fısica, Universidad Autónoma Metropolitana, Iztapalapa, Apdo 55 534, México DF, 09340, Mexico E-mail: fdr@xanum.uam.mx ...