Excess properties of mixtures of <i>n</i>-alkanes from perturbation theory

Vega, Carlos; MacDowell, Luis G.; López-Rodríguez, A.

doi:10.1063/1.479599

Cited by 10 publications

(17 citation statements)

References 67 publications

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“…However, the results given in Table unfortunately suggest that the theory is unable to predict positive values for the excess heat at low temperatures. This point is consistent with the results from Vega and co-workers 1mixture T (K) (cm 3 /mol) (cm 3 /mol) (J/mol) (J/mol) c 6 /c 10 293.15 −0.20 87 −0.29 16.7 87 −25.7 c 6 /c 16 293.15 −0.49 88 −0.76 129 89-91 −59.4 c 8 /c 16 293.15 −0.19 88 −0.31 88.8 87 −21.9 c 8 /c 24 379.15 −1.33 80 −1.56 −25.1 81 −177 a c 6 : n -hexane, c 8 : n -octane, c 10 : n -decane, c 16 : n -hexadecane, and c 24 : n -tetracosane.…”

Section: Model and Theorysupporting

confidence: 92%

“…The theory presented in the previous sections, along with other versions of SAFT, neglects the effect the conformational changes in molecules have on the thermodynamic properties, including excess functions. Small conformational changes exist when two different alkanes are mixed. , On the basis of the work of Vega et al, we assume that positive values for the excess enthalpies of n -alkanes at low temperatures are due to conformational changes in the mixture. According to Vega and collaborators, the free energy due to the intramolecular interactions of a pure fluid is given by in which n c is the number of carbon atoms forming the molecules of the system; x t , x g + , and x g − are the fractions of torsional degrees of freedom in the trans (t), gauche + (g + ), and gauche - (g - ) states, respectively; and E gt is the energy difference between the gauche and trans states.…”

Section: Model and Theorymentioning

confidence: 99%

“…Although results indicated that the theory could be used to predict the excess properties of real n -alkanes, we only concentrated on model systems in that previous work. Independently, Vega and co-workers have also examined the excess properties of n -alkane binary mixtures using a perturbation theory based on the work of Wertheim. This work is particularly interesting because they show how the conformational population of a mixture of n -alkanes plays a determinant role in the behavior of the excess heats of n -alkanes.…”

Section: Introductionmentioning

confidence: 99%

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Examination of the Excess Thermodynamic Properties of n-Alkane Binary Mixtures: A Molecular Approach

Ramos

Blas

2005

J. Phys. Chem. B

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A modification of the statistical associating fluid theory, the so-called Soft-SAFT equation of state, is proposed to predict the excess thermodynamic properties of binary mixtures of n-alkanes. n-Alkane molecules are modeled as fully flexible Lennard-Jones chains. This molecular model accounts for the most important microscopic features of real chainlike molecules: attractive and repulsive interactions between different chemical groups and the connectivity of the segments that form the molecules. In this work we consider an additional microscopic effect that can profoundly affect certain thermodynamic properties, namely, the conformational changes when two different n-alkane molecules are mixed. We propose, following the work of Vega and co-workers [J. Chem. Phys. 1999, 111, 3192], a simple model to account for the conformational changes in molecules. The resulting free energy is combined with the SAFT free energy to describe the excess thermodynamic properties of binary mixtures of n-alkanes. Predictions from the theory are compared with experimental data taken from the literature. The agreement between the experiments and the theoretical predictions is excellent in all cases. This work shows that although minor microscopic effects, such as the conformational changes in the molecules that form the mixtures, have only a very small effect on the usual thermodynamic properties, such as pressure, chemical potential, phase equilibria, and excess volumes, they can contribute significantly to other thermodynamic properties. In fact, one of the main conclusions of this work is that it is essential that conformational effects be taken into account in molecular-based theories if an accurate description of certain excess properties (excess enthalpy for instance) is desired.

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Section: Model and Theorysupporting

confidence: 92%

Section: Model and Theorymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Examination of the Excess Thermodynamic Properties of n-Alkane Binary Mixtures: A Molecular Approach

Ramos

Blas

2005

J. Phys. Chem. B

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“…It is also worth noting that an extrapolation of the high temperature regime of the H m E curves for both systems may lead to small and positive values of H m E at the lowest temperatures, which are much closer to the experimental estimations. Vega 34 and Blas 22,35 have shown that for mixtures of n-alkanes, the experimentally observed positive values of H m E at low temperatures were due to conformational changes in the molecules when they are mixed. Although conformational changes are included in the TraPPE model used in the present simulations, it is nevertheless a united atom model.…”

Section: Resultsmentioning

confidence: 99%

Excess Thermodynamic Properties of Mixtures Involving Xenon and Light Alkanes: A Study of Their Temperature Dependence by Computer Simulation

et al. 2011

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As a natural extension of a previous work, excess molar enthalpies and excess molar volumes as a function of composition in a wide range of temperatures have been obtained for binary mixtures of xenon with ethane, propane, and n-butane by Monte Carlo computer simulation. Xenon was modeled by a simple spherical Lennard-Jones potential, and the TraPPE-UA force field was used to describe the n-alkanes. One of the main goals of this study is to investigate the temperature dependence of the excess properties for mixtures of xenon and n-alkanes and, if possible, to supplement the lack of experimental data. For all three systems, the simulation results predicted excess volumes in good agreement with the experimental data. As for the excess enthalpies, in the case of (xenon + ethane), the simulation results confirm the negative experimental result and the weak temperature dependence. In the case of (xenon + propane) and (xenon + n-butane), however, the simulation predicts negative excess enthalpies, but those estimated from experimental data are positive. Both excess volumes and enthalpies display a complex dependence on temperature that in some aspects resembles that found for mixtures of n-alkanes.The structure of the liquid mixtures was also investigated by calculating radial distribution functions [g αβ(r)] between each pair of interaction groups for all the binary systems at all temperatures. It is found that the mean distance between xenon and CH2 groups is systematically higher than the distance between xenon and CH3. In addition, the number of groups around xenon in the first coordination sphere was calculated and seems to be proportionally more populated by methyl groups than by methylene groups. The results seem to reflect a preferential and stronger interaction between xenon and CH3, in agreement with previous findings.

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“…Vega and collaborators 47,48 have probably proposed the first model in the literature able to provide a nearly quantitative description of V E (but considering also H E ) of mixtures of n-alkanes. In the first paper the authors developed a perturbation theory, based on Wertheim ideas, and in the second one they refined the model to make it possible to predict the V E and H E of n-alkanes.…”

Section: Molecular Modelsmentioning

confidence: 99%

CHAPTER 16. SAFT and Molecular Simulation Techniques: Application to Determination of Volumetric Excess Properties

Blas¹,

Piñeiro²

2014

Volume Properties

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Excess properties of mixtures of n-alkanes from perturbation theory

Cited by 10 publications

References 67 publications

Examination of the Excess Thermodynamic Properties of n-Alkane Binary Mixtures: A Molecular Approach

Examination of the Excess Thermodynamic Properties of n-Alkane Binary Mixtures: A Molecular Approach

Excess Thermodynamic Properties of Mixtures Involving Xenon and Light Alkanes: A Study of Their Temperature Dependence by Computer Simulation

CHAPTER 16. SAFT and Molecular Simulation Techniques: Application to Determination of Volumetric Excess Properties

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