Molecular modeling of phase behavior and microstructure of acetone-chloroform-methanol binary mixtures

Force fields based on a Lennard-Jones (LJ) 12-6 plus point charge functional form are developed for acetone and chloroform specifically to reproduce the minimum pressure azeotropy found experimentally in this system. Point charges are determined from a CHELPG population analysis performed on an acetone-chloroform dimer. The required electrostatic surface for this dimer is determined from ab initio calculations performed with MP2 theory and the 6-31g++(3df,3pd) basis set. LJ parameters are then optimized such that the liquid-vapor coexistence curve, critical parameters, and vapor pressures are well reproduced by simulation. Histogram-reweighting Monte Carlo simulations in the grand canonical ensemble are used to determine the phase diagrams for the binary mixtures acetone-chloroform, acetone-methanol, and chloroform-methanol. The force fields developed in this work reproduce the minimum pressure azeotrope in the acetone-chloroform mixture found in experiment. The predicted azeotropic composition of x(CHCl3) = 0.77 is in fair agreement with the experimental value of x(CHCl3)expt = 0.64. The new force fields were also found to provide improved predictions of the pressure-composition behavior of acetone-methanol and chloroform-methanol when compared to other force fields commonly used for vapor-liquid equilibria calculations. NPT simulations were conducted at 300 K and 1 bar for equimolar mixtures of acetone-chloroform, acetone-methanol, and methanol-chloroform. Analysis of the microstructure reveals significant hydrogen bonding occurring between acetone and chloroform. Limited interspecies hydrogen bonding was found in the acetone-methanol or chloroform-methanol mixtures.     

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.     

Liquid-liquid-vapor phase equilibria behavior of certain binary carbon dioxide + n-alkylbenzene mixtures

The liquid-liquid-vapor loci for the binary mixtures CO₂ + n-hexylbenzene, n-heptylbenzene, and n-octylbenzene were experimentally studied. The compositions and molar volumes of the liquid phases are reported along with the pressure and temperature. For these three alkylbenzenes, the nature of the liquid-liquid-vapor loci experiences a transition, with the CO₂ + n-heptylbenzene mixture exhibiting two separate liquid-liquid-vapor branches.     

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.     

Structure and properties of thin films of binary rodlike/flexible polyimide mixtures

Binary mixtures of a rodlike poly(p-phenylene pyromellitimide) (PMDA-PDA) and a flexible 6F-BDAF polyimide synthesized from hexafluoroisopropylidene diphthalic anhydride and 2,2-bis(4-aminophenoxy-p-phenylene) hexafluoropropane were prepared by solution-blending of the meta-PMDA-PDA poly(amic ethyl ester) and 6F-BDAF poly(amic acid) precursors, followed by solvent evaporation and thermal imidization. Mixtures containing different molecular weights of 6F-BDAF poly(amic acid) were studied. The size scale of the phase separation, as measured by light scattering, is ca. 1 μm or smaller in most cases. The domain size is primarily set by the demixing of the precursor polymers during solvent evaporation, with no significant coarsening observed during the thermal imidization. The observed variation of the domain size with molecular and process parameters such as composition, molecular weight, and film thickness is discussed in terms of the miscibility of the precursor polymers, rate of solvent evaporation, and solidification. Dynamic mechanical thermal analysis and dielectric relaxation measurements indicate that the glass transition temperature of 6F-BDAF is unaffected in all of the mixtures studied, indicating complete demixing of rodlike and flexible polyimides in agreement with theory. X-ray photoelectron spectroscopy results show a strong surface segregation of 6F-BDAF in mixtures containing as low as 10% by weight of the 6F-BDAF component in the bulk. The mixtures with PMDA-PDA as the major matrix component therefore exhibit excellent mechanical toughness, dimensional stability up to 500°C, low coefficients of thermal expansion (< ca. 10 ppm/°C), and low dielectric constants (<3.0). On the other hand, the surface properties of the mixtures are dominated by the flexible 6F-BDAF, resulting in excellent polymer/polymer self-adhesion (lamination) properties between fully imidized films.     

Unusual phase behavior of decane-dodecane mixtures confined in SBA-15:Size effect on binary phase diagram

Phase behavior of normal decane-dodecane(n-C₁₀H₂₂-C₁₂H₂₆,C₁₀-C₁₂) system confined in SBA-15（Santa Barbara Amorphous,pore diameters 3.8,7.8,and 17.2 nm） has been studied by using differential scanning calorimetry.It has been found solid-liquid phase diagram of the C₁₀-C₁₂/SBA-15 system is composed of a straight line（3.8 nm）,a curve（7.8 nm） and a loop line （17.2 nm）.The growth of the phase diagram clearly shows the size effect on phase behavior of binary alkanes.Phase behavior has been compared among the systems C₁₀H₂₂-C₁₂H₂₆/SBA-15,C₁₂H₂₆-C₁₄H₃₀/SBA-15 and C₁₄H₃₀-C₁₆H₃₄/SBA-15.     

Pattern formation in phase separating binary mixtures

We experimentally investigate the interplay of thermodynamics with hydrodynamics during phase separation of (quasi-) binary mixtures. Well defined patterns emerge while slowly crossing the cloud point curve. Depending on the material parameters of the experimental system, two distinct scenarios are observed. In quasi-binary mixtures of methanol-hexane patterns appear before macroscopic phase separation sets in. In course of time the patterns turn faint while the overall turbidity of the sample increases until the mixtures become completely turbid. We attribute this pattern formation to a latent heat induced instability resembling a Rayleigh-Bénard instability. This is confirmed by calorimetric data and an estimate of its Rayleigh number. Mixtures of C(4)E(1)-water doped with decane phase separate under heating. After passing the cloud point curve these mixtures first become homogenously turbid. While clearing up, pattern formation is observed. We attribute this type of pattern formation to an interfacial tension induced Bénard-Marangoni instability. The occurrence of the two scenarios is supported by the relevant dimensionless numbers.     

Structure of binary mixtures of ethylene-vinylacetate copolymers

The method of differential scanning calorimetry was used to analyze the structure of binary mixtures of ethylene and vinylacetate copolymers with different contents of reagent monomers. The anomalous deviation of the degree of crystallinity from additivity in the system is found and a possible interpretation of this experimental fact is suggested.     

Bulk and interfacial properties of binary polymer mixtures

A microscopic density functional theory is used to investigate a binary mixture of polymers, built of freely jointed tangent hard spheres. The difference in the chain length and in the segment diameter of polymers gives rise to a demixing transition. We evaluate the bulk fluid phase equilibria (binodal) and the limit of stability of a mixed state (spinodal) for selected systems, and analyze the decay of the critical packing fraction, critical mole fraction, and critical pressure with an increase of the chain length. The bulk results are subsequently used in the calculations of the density profiles across the fluid-fluid interface. The obtained profiles are smooth and do not exhibit any oscillations on the length scale of the segment diameter. Upon approaching the critical point the interfacial tension vanishes as (Deltarho)3, where Deltarho is the difference between bulk densities of one component in bulk phases rich and poor in that species. This indicates that the microscopic density functional theory applied here is of a mean-field type.     

A monolayer study on phase behavior and morphology of binary mixtures of sulfatides with DPPC and DPPE

Sulfatides are important constituents of brain myelin membranes and it is thought to be involved in lateral domain formation in biological membranes. In this work, the interaction of mixed systems of sulfatide with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1, 2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE), two of the major components in biological membranes, was investigated using the monolayer technique at the air–water interface. Based on the regular solution theory, the miscibility of the two binary systems in the mixed monolayer was evaluated in terms of mean surface area per molecule (A_m), excess molecular area (ΔA_(ex)), surface excess Gibbs energy (ΔG_(ex)), interaction parameter (ω) as well as activity coefficients (f₁ and f₂) of formed films. Thermodynamic analysis indicates in the two binary systems with negative deviations from the ideal behavior. Accordingly, the values of the Gibbs energy of mixing, sulfatide-DPPC form stable mixtures at X_sul = 0.4 (X_sul is molar ratio of sulfatide in binary mixture) for all the selected pressures. As for sulfatide/DPPE system, at π = 5 and 30 mN m⁻¹, the minimum for the Gibbs energy of mixing was found at X_sul = 0.6 and 0.2 respectively. But the minimum appeared at X_sul = 0.4 for other surface pressures. The activity coefficients (f₁ and f₂) of mixed monolayers were evaluated which show a marked dependence on the mole faction of sulfatide X_sul. AFM images could support the above findings as well as interpretation.     

Bulk and adsorbed monolayer phase behavior of binary mixtures of undecanoic acid and undecylamine: catanionic monolayers

Differential scanning calorimetry (DSC) and X-ray powder diffraction (PXRD) have been used to determine the phase behavior of the binary mixtures of undecanoic acid (A) and undecylamine (B) in the bulk. In addition, we report DSC data that indicates very similar behavior for the solid monolayers of these materials adsorbed on the surface of graphite. The two species are found to form a series of stoichiometric complexes of the type AB, A(2)B, and A(3)B on the acid rich side of the phase diagram. Interestingly, no similar series of complexes is evident on the amine rich side. As a result of this complexation, the solid monolayers of the binary mixtures exhibit a very pronounced enhancement in stability relative to the pure adsorbates.     

Nematic-isotropic phase transition of binary liquid crystal mixtures

We experimentally studied the nematic-isotropic phase transition of (a) binary mixtures consisting of nematic and racemic liquid crystals and (b) binary mixtures consisting of positive and negative dielectric liquid crystals. We observed that the phase transition temperature is very sensitive to the chemical structures of the constituent components. We also used Maier-Saupe theory to calculate the transition temperature of binary mixtures. By fitting the experimental data, we obtained the interaction coupling constant between the constituent components.     

Dielectric properties of binary mixtures of n-methylbenzenesulfonamide

The dielectric permittivities of binary mixtures of N-methylbenzenesul-fonamide (N-MBS) with benzyl alcohol, 1,2-dichloroethane, 1,4-dioxane and hexamethylphosphortriamide were measured as a function of mole fraction over the whole composition range at 30 and 50°C. The excess dielectric permittivities and the excess molar polarizations were also calculated. The excess dielectric permittivities ^E and excess molar polarizations P_E were found to be negative for N-methylbenzene-sulfonamide mixtures with benzyl alcohol and 1,4-dioxane, ^E values were positive and P^E values negative for mixtures with 1,2-dichloroethane, and ^E and P^E values were clearly positive for mixtures with hexamethylphosphortriamide. The results are discussed in terms of the strength of the dipolar and hydrogen-bonding interactions between the molecules in various binary mixtures.     

Thermodynamic properties of binary mixtures containing hydrofluoroether

Excess molar enthalpy and excess molar volume at T =  298.15 K are reported for binary mixtures of (nonafluorobutylmethylether  +  butylmethylether, or nonane, or heptane, or pentane, or 1-propanol, or 2-propoxyethanol). Excess molar enthalpies of the mixture of (nonafluorobutylmethylether  +  1-pentanol) also are reported at T =  298.15 K. The results of excess molar enthalpy are endothermic and the results of excess molar volume are positive in the whole concentration for all the mixtures. The phase separation is found in the range of 0.15  < x <  0.92 for the 1-pentanol system. The results are explained by means of the destruction of the dipolar interactions and hydrogen bonds in the component liquids, the difference of the dispersion interaction, and the formation of the intermolecular hydrogen bonds between unlike molecules.     

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.     

Liquid-crystal phase diagrams of binary mixtures of hard spherocylinders

We have built the liquid crystal phase diagram of several binary mixtures of freely rotating hard spherocylinders employing a second-order virial density functional theory with Parsons scaling, suitably generalized to deal with mixtures and smectic phases. The components have the same diameter and aspect ratio of moderate value, typical of many mesogens. Attention has been paid to smectic-smectic demixing and the types of arrangement that rods can adopt in layered phases. Results are shown to depend on the aspect ratio of the individual components and on the ratio of their lengths. Smectic phases are seen not to easily mix together at sufficiently high pressures. Layered phases where the longer rods are the majority component have a smectic-A structure. In the opposite case, a smectic-A(2) phase is obtained where the shorter particles populate the layers and the longer ones prefer to stay parallel to the latter in the interlayer region.     

Structural relaxation and glass transition behavior of binary hard-ellipse mixtures

Structural relaxation and glass transition in binary hard-spherical particle mixtures have been reported to exhibit unusual features depending on the size disparity and composition. However, the mechanism by which the mixing effects lead to these features and whether these features are universal for particles with anisotropic geometries remains unclear. Here, we employ event-driven molecular dynamics simulation to investigate the dynamical and structural properties of binary two-dimensional hard-ellipse mixtures. We find that the relaxation dynamics for translational degrees of freedom exhibit equivalent trends as those observed in binary hard-spherical mixtures. However, the glass transition densities for translational and rotational degrees of freedom present different dependencies on size disparity and composition. Furthermore, we propose a mechanism based on structural properties that explain the observed mixing effects and decoupling behavior between translational and rotational motions in binary hard-ellipse systems.