On the fluid phase behaviour of fluid binary mixtures using the Yukawa fluid molecular model

By expanding Ginoza’s mean spherical approximation (MSA) results in an inverse-temperature expansion, Henderson et al. obtained explicit results for the thermodynamic functions of a pure Yukawa fluid. We have recently published explicit results for the coefficients in an inverse-temperature expansion of the thermodynamic functions for the MSA for mixtures of Yukawa fluids. Attention is drawn to the fact that the MSA in the Ginoza formulation, does not always yield a convergent solution. The expansion used in this paper will always yield a result. In this work we present our investigations of the fluid phase diagram of Yukawa binary mixtures by considering an expansion of the MSA Helmholtz free energy up to the fifth order of the inverse-temperature expansion. The calculated fluid phase diagrams for Yukawa binary mixtures are similar to those of real mixtures.     

Molecular simulation of the high-pressure phase equilibria of binary atomic fluid mixtures using the exponential-6 intermolecular potential

Molecular simulation results using the exponential-6 intermolecular potential are reported for the phase behaviour of the atomic binary mixtures of neon+xenon, helium+neon, helium+argon and helium+xenon. These binary mixtures exhibit both vapour–liquid and liquid–liquid phase equilibria up to very high pressures. Comparison with experiment indicates good overall agreement. The results indicate that the exponential-6 intermolecular potential is a useful generic potential for molecular simulation.     

An apparent critical point in binary mixtures: experimental and simulation study

We report the experimental and simulation studies for the system of nitrobenzene-cyclododecane, showing an apparent critical point, which lies in their metastable, experimentally inaccessible state, below their melting point, affecting physical and chemical properties of this system in the stable liquid phase. The nonlinear dielectric effect (NDE) was measured in the mixture of nitrobenzene with cyclododecane. The mixture has been found to show an apparent critical point which lies below the melting point, manifested as anomalous NDE behavior in the vicinity of the critical concentrations in the stable liquid phase. The melting temperature of this system was estimated using the differential scanning calorimetry method. For such a system, we also performed Monte Carlo (MC) simulations that aimed to analyze the kinds of phase transitions observed and the conditions of their occurrence in Lennard-Jones mixture. The enthalpy, configurational energy, and radial distribution function have been estimated by the MC simulation method in the N-P-T system. Immiscibility conditions according to the approach by Schoen and Hoheisel [Mol. Phys. 57, 65 (1986)] are also discussed.     

Electric permittivity near the critical point of binary mixtures

Electric permittivity for binary solutions of nitrobenzene in 2,2,4-trimethyl pentane, hexane, cyclohexane and cycloheptane was measured versus temperature for critical concentrations. The critical temperatures were determined. It was observed that the derivative d?/dT decreased with decreasing temperature when the system passed from a single phase to a two-phase state.     

Nonlinear dielectric effect near the critical point of binary mixtures

The nonlinear dielectric effect (NDE) was measured in two binary systems near the critical point. The effect of independent of specific dipole-dipole interactions in contrast with previous conceptions. Its singular part is proportional to (T ? T_c)^{?$\text{1}\text{2}$} in agreement with Snider's theory as applied to NDE.     

Static and dynamic critical behavior of a symmetrical binary fluid: a computer simulation

A symmetrical binary, A+B Lennard-Jones mixture is studied by a combination of semi-grand-canonical Monte Carlo (SGMC) and molecular dynamics (MD) methods near a liquid-liquid critical temperature T(c). Choosing equal chemical potentials for the two species, the SGMC switches identities (A-->B-->A) to generate well-equilibrated configurations of the system on the coexistence curve for TT(c). A finite-size scaling analysis of the concentration susceptibility above T(c) and of the order parameter below T(c) is performed, varying the number of particles from N=400 to 12 800. The data are fully compatible with the expected critical exponents of the three-dimensional Ising universality class. The equilibrium configurations from the SGMC runs are used as initial states for microcanonical MD runs, from which transport coefficients are extracted. Self-diffusion coefficients are obtained from the Einstein relation, while the interdiffusion coefficient and the shear viscosity are estimated from Green-Kubo expressions. As expected, the self-diffusion constant does not display a detectable critical anomaly. With appropriate finite-size scaling analysis, we show that the simulation data for the shear viscosity and the mutual diffusion constant are quite consistent both with the theoretically predicted behavior, including the critical exponents and amplitudes, and with the most accurate experimental evidence.     

Soret motion in non-ionic binary molecular mixtures

We study the Soret coefficient of binary molecular mixtures with dispersion forces. Relying on standard transport theory for liquids, we derive explicit expressions for the thermophoretic mobility and the Soret coefficient. Their sign depends on composition, the size ratio of the two species, and the ratio of Hamaker constants. Our results account for several features observed in experiment, such as a linear variation with the composition; they confirm the general rule that small molecules migrate to the warm, and large ones to the cold.     

QSPR modeling of critical properties of organic binary mixtures

QSPR models for the critical temperatures, critical volumes, and critical pressures of binary organic mixtures are given. The binary organic mixtures have been described in terms of the mixture modification of simplex representation of molecular structure. The accuracy of the obtained models is comparable to the recommended one, the mean error ranging from 6.8 to 14.6%. The models imply that electronic polarizability is the most important factor for the critical volume and that the critical temperature and critical pressure are determined primarily by van der Waals and electrostatic interactions.     

Structure,thermodynamics and diffusion in asymmetric binary mixtures: a molecular dynamics simulation study

Structural and thermodynamic properties as well as diffusion coefficients of binary fluid mixtures with asymmetry in mass, size, charge and their combinations have been studied using classical molecular dynamics simulations. The fluid mixture is modelled as spherical particles interacting via the Weeks–Chandler–Andersen and Coulomb potential. The diameter, charge and mass of the fluid particles are in the range 6–60 Å, 1–10e and 1—500 amu, respectively. Systematic variations in pair-correlation functions, thermodynamic properties as well as the self-diffusion coefficient are found with the size, charge and mass ratio of the particles. The self-diffusion coefficient for systems having more than one type of asymmetry is calculated and expressed in terms of diffusion coefficients of systems with only one type of asymmetry.     

Structural features of binary mixtures of supercritical CO2 with polar entrainers by molecular dynamics simulation

Computer simulations of supercritical carbon dioxide and its mixtures with polar cosolvents: water, methanol, and ethanol (concentration, 0.125 mole fractions) at T = 318 K and ρ = 0.7 g/cm³ are performed. Atom-atom radial distribution functions are calculated by classical molecular dynamics, while the probability distributions of relative orientation of CO₂ molecules in the first and second coordination spheres describing the geometry of the nearest environment of CO₂ molecules and the trajectories of cosolvent molecules are found using Car-Parrinello molecular dynamics. Based on the latter, the conclusions regarding structure and interactions of polar entrainers in their mixtures with supercritical CO₂ are made. It is shown that the microstructure of carbon dioxide varies only slightly upon the introduction of cosolvents.     

Global fluid phase equilibria and critical phenomena of selected mixtures using the crossover soft-SAFT equation

We present here the extension of the crossover soft-statistical associating fluid theory (soft-SAFT) equation of state to mixtures, as well as some illustrative applications of the methodology to mixtures of particular scientific and technological interest. The procedure is based on White's work (White, J. A. Fluid Phase Equilib. 1992, 75, 53) from the renormalization group theory, as for the pure fluids, with the isomorphism assumption applied to the mixtures. The equation is applied to three groups of mixtures: selected mixtures of n-alkanes, the CO2/n-alkane homologous series, and the CO2/1-alkanol homologous series. The crossover equation is first applied to the pure components of the mixtures, CO2 and the 1-alkanol family, while an available correlation is used for the molecular parameters of the n-alkane series (Llovell et al. J. Chem. Phys 2004, 121, 10715). A set of transferable molecular parameters is provided for the 1-alkanols series; these are accurate for the whole range of thermodynamic conditions. The crossover soft-SAFT equation is able to accurately describe these compounds near to and far from the critical point. The theory is then used to represent the phase behavior and the critical phenomena of the selected mixtures. We use binary interaction parameters xi and eta for dissimilar mixtures. These parameters are fitted at some particular conditions (one subcritical temperature or binary critical data) and used to predict the behavior of the mixture at different conditions (other subcritical conditions and/or critical conditions). The equation is able to capture the continuous change in the critical behavior of the CO2/n-alkane and the CO2/1-alkanol homologous series as the chain length of the second compound increases. Excellent agreement with experimental data is obtained, even in the most nonideal cases. The new equation is proved to be a powerful tool to study the global phase behavior of complex systems, as well as other thermodynamic properties of very challenging mixtures.     

Critical point estimation of the Lennard-Jones pure fluid and binary mixtures

The apparent critical point of the pure fluid and binary mixtures interacting with the Lennard-Jones potential has been calculated using Monte Carlo histogram reweighting techniques combined with either a fourth order cumulant calculation (Binder parameter) or a mixed-field study. By extrapolating these finite system size results through a finite size scaling analysis we estimate the infinite system size critical point. Excellent agreement is found between all methodologies as well as previous works, both for the pure fluid and the binary mixture studied. The combination of the proposed cumulant method with the use of finite size scaling is found to present advantages with respect to the mixed-field analysis since no matching to the Ising universal distribution is required while maintaining the same statistical efficiency. In addition, the accurate estimation of the finite critical point becomes straightforward while the scaling of density and composition is also possible and allows for the estimation of the line of critical points for a Lennard-Jones mixture.     

Molecular simulation of the phase behavior of fluids and fluid mixtures using the synthetic method

A new molecular simulation procedure is reported for determining the phase behavior of fluids and fluid mixtures, which closely follows the experimental synthetic method. The simulation procedure can be implemented using Monte Calro or molecular dynamics in either the microcanonical or canonical statistical ensembles. Microcanonical molecular dynamics simulations are reported for the phase behavior of both the pure Lennard-Jones fluid and a Lennard-Jones mixture. The vapor pressures for the pure fluid are in good agreement with Monte Carlo Gibbs ensemble and Gibbs-Duhem calculations. The Lennard-Jones mixture is composed of equal size particles, with dissimilar energy parameters (?(2)∕?(1) = 1∕2, ?(12)∕?(1) = 1∕2). The binary Lennard-Jones mixture exhibits liquid-liquid equilibria at high pressures and the simulation procedure allows us to estimate the coordinates of the high-pressure branch of the critical curve.     

Concentration dependences of the critical temperatures of binary mixtures of nonaqueous components

The critical (liquid-vapor) temperatures were determined by the ampule method for 10 binary mixtures containing aliphatic alcohols, n-octane, n-decane, methyl ethyl ketone, benzene, and toluene in various combinations. The predictive potential of several calculation methods was considered. The methods using binary interaction parameters as functions of the ratio between the critical volumes of compounds were found to correctly describe experimental data.     

Prediction of the critical locus of binary mixtures of fluorocompounds with hydrocarbons

The critical temperatures and pressures for 14 binary mixtures of fluorocompounds with hydrocarbons were correlated using an iteration procedure based on a cubic equation of state. The predicted values were compared with previously measured values, the agreement being very good.     

Prediction of microscopic structure and physical properties of complex fluid mixtures based on molecular simulation

Molecular simulation calculations are presented for two types of complex fluid mixtures, namely elastomer polymer mixtures and water–1-octanol binary and ternary mixtures. Elastomer polymers are used widely as membrane materials for gas separation. In this respect, the solubility and diffusion coefficient of gases need to be known accurately. Predictions for both properties are presented here. Water–1-octanol mixture is a widely used prototype system used to assess the partitioning of various chemical compounds with applications to chemical industry, biotechnology, etc. The microscopic structure of the water–1-octanol mixture is examined and the Gibbs free energy of solvation of four organic solutes is calculated. In all cases, detailed atomistic force fields are used to account for inter- and intra-molecular interactions. Simulation results are shown to be in excellent agreement with literature experimental data.     

A method for calculating the molecular refraction of binary solvent mixtures

A method for calculating the molecular refraction of binary mixtures of different solvents is developed using an original equation of substance polarization theory.     

Effect of three-body interactions on the vapor-liquid phase equilibria of binary fluid mixtures

Gibbs-Duhem Monte Carlo simulations are reported for the vapor-liquid phase coexistence of binary argon+krypton mixtures at different temperatures. The calculations employ accurate two-body potentials in addition to contributions from three-body dispersion interactions resulting from third-order triple-dipole interactions. A comparison is made with experiment that illustrates the role of three-body interactions on the phase envelope. In all cases the simulations represent genuine predictions with input parameters obtained independently from sources other than phase equilibria data. Two-body interactions alone are insufficient to adequately describe vapor-liquid coexistence. In contrast, the addition of three-body interactions results in very good agreement with experiment. In addition to the exact calculation of three-body interactions, calculations are reported with an approximate formula for three-body interactions, which also yields good results.     

Scaling function of critical binary mixtures: Nitrobenzene-n-hexane data revisited

Ultrasonic attenuation spectra of the nitrobenzene-n-hexane mixture of critical composition have been analysed. Data between 50 kHz and 1 GHz from different sources have been included to show that at a given temperature the spectra, in addition to the critical contribution, reveal a non-critical relaxation term. Taking this additional term into account inconsistencies in the scaling function reported in the literature are avoided. In the final analysis the scaling function of the nitrobenzene-n-hexane system follows the predictions of the Bhattacharjee-Ferrell theory with critical amplitude and relaxation rate of concentration fluctuations in nice agreement with determinations from independent methods. The low-frequency attenuation data are briefly discussed with a view to a bulk viscosity approach which yields a slightly different proportionality constant in the linear regime of the scaling function than the Bhattacharjee-Ferrell theory. Evidence in favour of the latter is obtained.