A modified SAFT equation of state is developed by applying the perturbation theory of Barker and Henderson to a hard-chain reference fluid. With conventional one-fluid mixing rules, the equation of state is applicable to mixtures of small spherical molecules such as gases, nonspherical solvents, and chainlike polymers. The three pure-component parameters required for nonassociating molecules were identified for 78 substances by correlating vapor pressures and liquid volumes. The equation of state gives good fits to these properties and agrees well with caloric properties. When applied to vapor−liquid equilibria of mixtures, the equation of state shows substantial predictive capabilities and good precision for correlating mixtures. Comparisons to the SAFT version of Huang and Radosz reveal a clear improvement of the proposed model. A brief comparison with the Peng−Robinson model is also given for vapor−liquid equilibria of binary systems, confirming the good performance of the suggested equation of state. ...

Perturbed-Chain SAFT: An Equation of State Based on a Perturbation Theory for Chain Molecules

羟氨苄青霉素引起大疱性类天疱疮样疹

We discuss existing and new computational analysis techniques to classify local atomic arrangements in large-scale atomistic computer simulations of crystalline solids. This article includes a performance comparison of typical analysis algorithms such as common neighbor analysis (CNA), centrosymmetry analysis, bond angle analysis, bond order analysis and Voronoi analysis. In addition we propose a simple extension to the CNA method that makes it suitable for multi-phase systems. Finally, we introduce a new structure identification algorithm, the neighbor distance analysis, which is designed to identify atomic structure units in grain boundaries.

https://iopscience.iop.org/article/10.1088/0965-0393/20/4/045021/pdf

Structure identification methods for atomistic simulations of crystalline materials

We discuss existing and new computational analysis techniques to classify local atomic arrangements in large-scale atomistic computer simulations of crystalline solids. This article includes a performance comparison of typical analysis algorithms such as Common Neighbor Analysis, Centrosymmetry Analysis, Bond Angle Analysis, Bond Order Analysis, and Voronoi Analysis. In addition we propose a simple extension to the Common Neighbor Analysis method that makes it suitable for multi-phase systems. Finally, we introduce a new structure identification algorithm, the Neighbor Distance Analysis, that is designed to identify atomic structure units in grain boundaries.

We present a review of recent advances in the statistical associating fluid theory (SAFT). In contrast to the “chemical theory”, in which nonideality is explained in terms of chemical reactions between the species, SAFT and similar approaches relate nonideality to the intermolecular forces involved. Such physical theories can be tested against molecular simulations, and improvements to the theory can be made where needed. We describe the original SAFT approach and more recent modifications to it. Emphasis is placed on pointing out that SAFT is a general method and not a unique equation of state. Applications to a wide variety of fluids and mixtures are reviewed, including aqueous mixtures and electrolytes, liquid−liquid immiscible systems, amphiphilic systems, liquid crystals, polymers, petroleum fluids, and high-pressure phase equilibria.

Molecular-Based Equations of State for Associating Fluids: A Review of SAFT and Related Approaches

The phase equilibria of the n-alkanes and the 1-alkanols have been calculated with the Lennard-Jones−SAFT (statistical association fluid theory) equation of state of Muller and Gubbins. This equation includes contributions from a dipole−dipole term, a modified association term, a chain term, and a Lennard-Jones term to account for monomer dispersion and overlap interactions. The influence of electrostatic forces due to the dipole moment has been investigated, and a simple treatment of the polarizability has been tested. It is shown by comparison with some sample calculations based on the renormalized perturbation theory that this approach is reasonable. The calculated phase equilibria are in good agreement with experimental data. The deviation between calculated and experimental data is significantly lower than for the original SAFT equation of state and a recently published chemical theory.

Phase equilibria calculations with a modified SAFT equation of state. 1. Pure alkanes, alkanols, and water

The Lennard-Jones−statistical association fluid theory (LJ−SAFT) is applied to binary mixtures containing one self-associating and one nonassociating substance. The binary systems studied here are n-alkane/n-alkane, 1-alkanol/n-alkane, and water/n-alkane mixtures. For cases where the dipole−dipole term is needed, the influence of induction is also investigated. The results with LJ−SAFT exhibit better agreement with experimental data than SAFT. This improvement is due to the exchange of the hard-sphere reference system by the LJ reference system.

Phase equilibria calculations with a modified SAFT equation of state. 2. Binary mixtures of n-alkanes, 1-alkanols, and water

Phase diagrams of binary fluid mixtures have been calculated from the Carnahan–Starling–Redlich–Kwong equation of state in connection with standard quadratic mixing rules. The phase diagrams were classified according to the system of van Konynenburg and Scott and then used to construct global phase diagrams showing the extent of the various phase diagram classes in the space of the parameters of the equation of state. For molecules of equal size, the global phase diagram is rather similar to that of the Redlich–Kwong or the van der Waals equation. For molecules of different sizes, however, a new tricritical line appears. Such a behavior is observed for cubic equations of state only if nonadditive covolumes are assumed. Along this new tricritical line, some unusual phase diagrams involving four‐phase states and high‐density instabilities can be found. The influence of molecular size ratios on the global phase diagrams and the relationship of the equation of state of this work to the ternary symmetric lattice gas and the van der Waals lattice gas are discussed.

Systematic investigation of the phase behavior in binary fluid mixtures. II. Calculations based on the Carnahan-Starling-Redlich-Kwong equation of state

The formation of iron particles from the supersaturated gas phase is investigated by molecular dynamics simulation. The atomic interaction is modelled with a recent parameterization of the embedded atom method which is able to describe the bcc phase of bulk iron. The influence of the state conditions such as temperature and density on the growth mechanisms of the iron particles is analysed. With the common neighbour analysis method the structural changes of the particles over the course of the growth process is traced. Furthermore, the surface fraction is investigated as an order parameter for the morphology of the particles. It appears that in the early state of the growth process the atomic structure is dominated by icosahedral structures which are, over the course of the growth process, transformed into other structures such as fcc or hcp. Iron atoms in the bcc structure were not found for small particles. The structural reorganization depends mainly on the temperature of the system and on the magnitude of the heat removal from the system. The results are in agreement with experimental and other theoretical investigations on the structure of iron nanoparticles.

https://iopscience.iop.org/article/10.1088/0957-4484/15/5/021/pdf

Investigation of the formation of iron nanoparticles from the gas phase by molecular dynamics simulation

Empirical thermal cohesion functions, α(Tr), are frequently used in conventional equations of state (EOS) for fitting the vapor pressures of pure fluids Accurate vapor pressure predictions are required for correlating and/or predicting the phase equilibrium and interfacial tension of multicomponent mixtures This is the case for the Redlich−Kwong−Soave and Peng−Robinson models, two well-established models for engineering applications In this work, we demonstrate that, in the case of pure fluids, the α(Tr) function can potentially predict multiple mechanically stable critical points, thus affecting the global topology of phase equilibrium predictions A detailed analysis, based on the consistency of the prediction of the Joule−Thomson inversion curve, reveals that these predictions are not reliable from a physical point of view In fact, conventional cubic EOS are able to predict multiple Joule−Thomson inversion curves, a behavior symptomatic of the prediction of multiple stable critical points for pure 

Thomas Kraska

Papers

Phase equilibria calculations with a modified SAFT equation of state. 1. Pure alkanes, alkanols, and water

Phase equilibria calculations with a modified SAFT equation of state. 2. Binary mixtures of n-alkanes, 1-alkanols, and water

Systematic investigation of the phase behavior in binary fluid mixtures. II. Calculations based on the Carnahan-Starling-Redlich-Kwong equation of state

Investigation of the formation of iron nanoparticles from the gas phase by molecular dynamics simulation

Unnoticed Pitfalls of Soave-Type Alpha Functions in Cubic Equations of State