Phase Behavior Of Mixtures Research Articles

Microcanonical-ensemble perturbation theory for thermodynamic and diffusion properties of square-well fluids

We present results for thermodynamic and diffusion properties of a square-well fluid (SW) using the Microcanonical-Ensemble perturbation theory (MEPT) [Mol. Phys. 116, 351 (2018)]. The aim of this work is to assess the accuracy of MEPT that is required in order to model real substances properties. We consider the extension of MEPT for binary mixtures of SW fluids, using the Lorentz-Berthelot combining rules. The phase behavior of binary square-well mixtures are studied, exploring in detail the influence of the hard-core size ratios, dispersion energy ratios, and variable attractive range potential. Excellent agreement between the MEPT approach and Gibbs Ensemble Monte Carlo (GEMC) simulation is found, for a wide range of temperatures and compositions. The relevance of perturbation terms of order higher than two is also considered, comparing with SAFT-VR results. This approach is a first step in modeling inhomogeneous systems, such as adsorption and interfacial properties of binary mixtures based on discrete potential systems as reference fluids.

The helical twisting power of chiral dopants in lyotropic chromonic liquid crystals

ABSTRACT Chiral dopants with different levels of lipophilicity were synthesised, and their helical twisting power (HTP) and thermal phase behaviour in mixtures with lyotropic chromonic liquid crystal (LCLC) were investigated. The experimental approach involved measurement of the positions of disclination lines in Θ-cells, where one surface is linearly rubbed, and the other is circularly rubbed. This technique allowed the use of minimal dopant concentrations. Experimental conditions were selected such that nematic temperature range of LCLC remained constant. We found that the twisting power of the used chiral dopants varies over nearly an order of magnitude between the molecules with positive and negative lipophilicity. All chiral molecules of hydrophobic nature investigated showed much higher effectiveness in twisting LCLC than hydrophilic ones, and in addition, the hydrophilic molecules had a much smaller effect on the transition temperatures of LCLC. The dependences were interpreted as a variation of the partition between the concentration of dopant dispersed in the host phase, between the columns and that adhering to the columnar stacks – with hydrophobic molecules better able to adhere, thus creating sources of chirality for the whole system.

Thermodynamic stability and critical points in multicomponent mixtures with structured interactions.

Theoretical work has shed light on the phase behavior of idealized mixtures of many components with random interactions. However, typical mixtures interact through particular physical features, leading to a structured, nonrandom interaction matrix of lower rank. Here, we develop a theoretical framework for such mixtures and derive mean-field conditions for thermodynamic stability and critical behavior. Irrespective of the number of components and features, this framework allows for a generally lower-dimensional representation in the space of features and proposes a principled way to coarse-grain multicomponent mixtures as binary mixtures. Moreover, it suggests a way to systematically characterize different series of critical points and their codimensions in mean-field. Since every pairwise interaction matrix can be expressed in terms of features, our work is applicable to a broad class of mean-field models.

Bridging confined phase behavior of CH[formula omitted]-CO[formula omitted] binary systems across scales

Phase behavior of confined fluids may deviate significantly from that of the bulk fluid due to the fluid–wall interactions being a significant portion of all intermolecular interactions under confinement. Despite recent advancements in understanding confined phase behavior of pure fluids, confined phase behavior of mixtures remains an understudied topic. In this work, we examine the confined phase behavior of a CH4-CO2 binary system by combining Monte Carlo (MC) simulations, a cubic equation of state (EoS), and the lattice Boltzmann method (LBM). First, the effects of confinement on density and phase distribution in nano-size pores are established using Gibbs Ensemble MC calculations, which produce precise results of liquid and vapor confined pressures and account for the modification of the phase change location. By comparing the phase envelopes of bulk and confined mixtures at a fixed temperature, it is observed that the phase envelopes shrink with reductions in pore size. Based on this observation, we extend a modified Peng–Robinson EoS, which was originally developed for pure fluids under confinement, to mixtures via van-der-Waals-type mixing rules and by accounting for shifts in the critical properties of confined CH4-CO2. The resulting phase envelopes are in good agreement with the MC data. In addition, a local density model is used in combination with the confined EoS to calculate adsorption isotherms of CH4-CO2 mixtures and to characterize the behavior of confined matter in nanopores. Finally, we incorporate this EoS in a multicomponent multiphase LBM that uses a pseudopotential model to represent intermolecular forces. This workflow utilizes multiscale simulation techniques to bridge the behavior of multicomponent systems across scales and to shed light on the confined phase behavior of CH4-CO2 binary systems.

Tuning Stoichiometry to Promote Formation of Binary Colloidal Superlattices.

The self-assembly of binary nanoparticle superlattices from colloidal mixtures is a promising method for the fabrication of complex colloidal cocrystal structures. However, binary mixtures often form amorphous or metastable phases instead of the thermodynamically stable phase. Here we show that in binary mixtures of differently sized spherical particles, an excess of the smaller component can promote-and, in some cases, may be necessary for-the self-assembly of a binary cocrystal. Using computer simulations, we identify two mechanisms responsible for this phenomenon. First, excess small particles act like plasticizers and enable systems to reach a greater supersaturation before kinetic arrest occurs. Second, they can disfavor competing structures that may interfere with the growth of the target structure. We find the phase behavior of simulated mixtures of nearly hard spheres closely matches published experimental results. We demonstrate the generality of our findings for mixtures of particles of arbitrary shape by presenting a binary mixture of hard shapes that only self-assembles with an excess of the smaller component.

A Pore-Geometry-Based Thermodynamic Model for the Nanoconfined Phase Behavior in Shale Condensate Reservoirs

Abstract Shale condensate reservoirs, as one of the most important unconventional resources, have been attracting more and more attention recently. Researchers have developed several models for analyzing the phase behavior in nanopores. However, investigation of pore-geometry-based nanopore confinement is still lacking in the oil industry. Therefore, we develop a comprehensive thermodynamic model to satisfy this gas and simulate to the Eagle Ford shale condensate reservoirs. The model is first built with modification of the PR-EOS coupling the pore-geometry-based capillarity and adsorption. The Multicomponent Langmuir (ML) adsorption model is utilized. A good agreement is shown between this proposed model and the experimental data, which is associated with bulk fluid and confined fluid. After that, phase behavior of C1-C10 and C1-C4 mixtures is analyzed. Results indicate that capillarity and adsorption can result in noticeable deviation in the phase envelope. Additionally, pores with square and triangular cross-sections are analyzed, illustrating that pore geometries also have a significant influence on the phase behavior. Finally, a case study of the Eagle Ford shale condensate reservoir is performed. Phase behaviors in different pore geometries are analyzed by the ML adsorption model in detail, which will be more realistic than only circle pores. Moreover, we examine the physical properties of fluid in the Eagle Ford shale condensate reservoir. This work provides useful insights into the phase behavior of nanopores in shale formation, especially in the case of CO2-EOR in the shale condensate reservoirs.

RNA chain length and stoichiometry govern surface tension and stability of protein-RNA condensates

SummaryProteomic studies have shown that cellular condensates are frequently enriched in diverse RNA molecules, which is suggestive of mechanistic links between phase separation and transcriptional activities. Here, we report a systematic experimental and computational study of thermodynamic landscapes and interfacial properties of protein-RNA condensates. We have studied the affinity of protein-RNA condensation as a function of variable RNA sequence length and RNA-protein stoichiometry under different ionic environments and external crowding. We have chosen the PolyU sequences for RNA and arginine/glycine-rich intrinsically disordered peptide (RGG) for proteins as a model system of RNA-protein condensates, which we then investigate through in vitro microscopy measurements and coarse-grained molecular dynamics simulations. We find that crowding and RNA chain length can have a major stabilizing effect on the condensation. We also find that the RNA-protein charge ratio is a crucial variable controlling stability, interfacial properties, and the reentrant phase behavior of RGG-RNA mixtures.

Phase Behavior of Internal Mixtures of Hydrocarbon-like Primary Organic Aerosol and Secondary Aerosol Based on Their Differences in Oxygen-to-Carbon Ratios.

The phase behavior, the number and type of phases, in atmospheric particles containing mixtures of hydrocarbon-like organic aerosol (HOA) and secondary organic aerosol (SOA) is important for predicting their impacts on air pollution, human health, and climate. Using a solvatochromic dye and fluorescence microscopy, we determined the phase behavior of 11 HOA proxies (O/C = 0–0.29) each mixed with 7 different SOA materials generated in environmental chambers (O/C 0.4–1.08), where O/C represents the average oxygen-to-carbon atomic ratio. Out of the 77 different HOA + SOA mixtures studied, we observed two phases in 88% of the cases. The phase behavior was independent of relative humidity over the range between 90% and <5%. A clear trend was observed between the number of phases and the difference between the average O/C ratios of the HOA and SOA components (ΔO/C). Using a threshold ΔO/C of 0.265, we were able to predict the phase behavior of 92% of the HOA + SOA mixtures studied here, with one-phase particles predicted for ΔO/C < 0.265 and two-phase particles predicted for ΔO/C ≥ 0.265. The threshold ΔO/C value provides a relatively simple and computationally inexpensive framework for predicting the number of phases in internal SOA and HOA mixtures in atmospheric models.

Unusual Phase Behavior of Hydrocarbon Mixtures

Unusual Phase Behavior of Hydrocarbon Mixtures

Phase behavior of multicomponent mixtures with structured interactions

Multicomponent mixtures are ubiquitous in biology and there is experimental evidence that their phase behavior plays a functional role for signaling and control of biochemical reactions. Theoretical work has shed light on the phase behavior of idealized mixtures with random pairwise interactions. But many biological systems like lipid membranes and protein solutions contain a large number of components that interact through a smaller number of features, leading to a structured interaction matrix.

Transient mixing behavior of a supercritical fluid injected into supercritical and subcritical environments

Transient, near-field mixing of a supercritical fluid injected into supercritical and subcritical environments is investigated experimentally. Fuel (propane) from a 100 μm diameter Bosch diesel injector is injected into a high-pressure test rig with a steady flow of heated, low-velocity nitrogen to control the ambient pressure and temperature. The fuel supply pressure is approximately 190 bar, allowing short injections lasting 3–4 ms to be performed in quick succession to acquire statistically significant datasets to represent the unsteady fuel–air mixing. Two test cases with the same ambient density and injector inlet pressure and temperature were chosen in an attempt to isolate effects of mixture phase behavior. Microscopic rainbow Schlieren deflectometry (RSD) is used for visualization and quantitative data acquisition. In the case of the supercritical ambient, the injector inlet and exit and ambient conditions are all supercritical. The RSD analysis reveals a mixing process similar to the single phase fluid mixing, though condensation is also observed locally, and can be explained by the adiabatic mixing theory presented in the paper. For the subcritical ambient case, the injector inlet is supercritical, but the injector exit and ambient conditions are both subcritical. The results show a rapid expansion in the near-field promoting mixing between propane and nitrogen. At downstream locations, significant condensation is also observed as predicted by the adiabatic mixing theory. The supercritical ambient case exhibited longer and narrower jet penetration compared to the subcritical ambient case.

Bulk phase behavior and interfacial properties of binary mixtures of Lennard-Jones chains and solvent: a density functional theory study.

A Density Functional Theory study is performed to analyze both bulk and interfacial properties of solvent-polymer binary mixtures. The effects of increasing polymer chain length on the bulk phase diagram morphology and interfacial tension are presented and compared to the prior simulation results. Good agreement between simulation and Density Functional Theory is found, including its ability to reproduce the density inversion phenomenon for highly asymmetric solvent-polymer binary mixtures. The data on the interfacial tensions is used to compute contact angles of the mixture at a planar wall, with particular focus on the wetting transition. The dependence of the wetting temperature on the polymer chain length and the mixture composition is analyzed in detail.

On crystallization of water confined in liposomes and cryoprotective action of DMSO.

In this work, the phase behavior of cryoprotective mixtures based on dimethyl sulfoxide (DMSO) mixed with a lipid bilayer consisting of dimyristoyl phosphatidylcholine (DMPC) was studied. This system represented a model of a biological cell and its membrane. The aim of the work was to clarify the origin of the cryoprotective action of low-concentrated mixtures (1–10 vol%) DMSO in water, representing mixtures used in cryopreservation in cell therapy. The combination of experimental techniques of differential scanning calorimetry (DSC) and positron annihilation lifetime spectroscopy (PALS) allowed a study of crystallization behavior of water confined in liposomes imitating the intracellular environment. The ability of liposomes to show the fundamental aspects of water phase behavior seen during freezing of biological cells was proved. The presence of an amorphous freeze-concentrated phase of DMSO in the frozen state was confirmed and its possible crystallization into the DMSO trihydrate and ice during thawing was demonstrated. Correlation between the critical temperature range for the loss of cell viability during slow thawing and the temperatures of freeze-concentrated phase crystallization was found. Based on this finding, possible mechanisms of DMSO cryoprotection are discussed with support brought by results for the studied model system. Quantification of the ice phase fraction in the frozen mixtures revealed that even low concentrations of DMSO can induce a considerable decrease in the amount of ice present.

Temperature effects on alcohol aggregation phenomena and phase behavior in n-butanol aqueous solution

Liquid-liquid phase separation is an important phenomenon that prevails from simple to complex molecular systems where temperature plays a crucial role in the fundamental understanding of phase behavior in a theoretical and industrial perspective. The n-butanol aqueous solutions have two separate liquid phases of alcohol-rich and water-rich formed at room temperature. The change in phase behavior occurs from two phases to one with an increase of temperature, exhibiting an upper critical solution temperature (UCST) behavior. We carried out molecular dynamics simulations and graph theoretical analysis to understand how temperature regulates n-butanol aggregation and the network properties of water at various concentrations.The theoretical studies suggest that the n-butanol molecules form two distinct alcohol aggregates, water-compatible or water-incompatible, determined by the interactions between alcohol and solvent molecules. Moreover, the proposed bifurcating aggregation pathway is sensitive to the temperature and composition of the n-butanol aqueous solution. The current study shows how temperature is significant in modulating the alcohol aggregation behavior and affects water structure which determines the phase behavior of aqueous solution systems. The effect of temperature on the alcohol aggregation pattern and water structure in n-butanol/water mixtures provides a critical clue in the fundamental understanding and determining the phase behavior of binary water mixtures. The current study also provides a molecular framework to examine and govern the UCST behavior of aqueous solution systems.

Dynamics and phase behavior of two-dimensional size-asymmetric binary mixtures of core-softened colloids.

The self-assembly of binary colloidal mixtures provides a bottom-up approach to create novel functional materials. To elucidate the effect of composition, temperature, and pressure on the self-assembly behavior of size-asymmetric mixtures, we performed extensive dynamics simulations of a simple model of polymer-grafted colloids. We have used a core-softened interaction potential and extended it to represent attractive interactions between unlike colloids and repulsions between like colloids. Our study focused on size-asymmetric mixtures where the ratio between the sizes of the colloidal cores was fixed at σBσA=0.5. We have performed extensive simulations in the isothermal-isobaric and canonical (NVT) ensembles to elucidate the phase behavior and dynamics of mixtures with different stoichiometric ratios. Our simulation results uncovered a rich phase behavior, including the formation of hierarchical structures with many potential applications. For compositions where small colloids are the majority, sublattice melting occurs for a wide range of densities. Under these conditions, large colloids form a well-defined lattice, whereas small colloids can diffuse through the system. As the temperature is decreased, the small colloids localize, akin to a metal-insulator transition, with the small colloids playing a role similar to electrons. Our results are summarized in terms of phase diagrams.

Solid–Liquid Phase Behaviors of Binary Mixtures of Various Partial Acylglycerols by Differential Scanning Calorimetry

AbstractMonoacylglycerol (MAG), diacylglycerol (DAG), and triacylglycerol (TAG) are impurities in biodiesel and a major cause of precipitation. Understanding the behavior of such acylglycerols is essential for predicting biodiesel cold flow properties (CFPs). The previous study on MAG/MAG binary mixtures shows that they tend to solidify by forming molecular compounds. In contrast, TAG/TAG mixtures, which have been studied extensively, are commonly eutectic or monotectic systems, in which each component solidifies separately. The present study focuses on binary mixtures of DAG/DAG and different acylglycerol pairs (MAG/DAG, TAG/MAG, and DAG/TAG), and determination of their solid–liquid phase behavior by differential scanning calorimetry. These mixtures are found to behave as eutectic or monotectic systems with no sign of compound formation. As DAG and TAG have lower contents than MAG in biodiesel and they are unlikely to form molecular compounds with MAG, it is suggested that DAG and TAG have little effect on the biodiesel CFPs.Practical Applications: Biodiesel has attracted much interest because its blending with conventional fossil diesel has become more standard with biofuel mandates. From an energy perspective, the solid–liquid phase behavior of acylglycerols will contribute to building prediction models for biodiesel CFPs.

Phase Behavior and Crystal Structure of Binary Mixtures of Droplets and Four-Patch Colloids

We report simulation studies on the phase behavior and crystal structures of anisotropic colloid–droplet binary mixtures. The droplets interact with the colloids at the patches of different geometr...

Phase Behavior of Selected Condensed Double-Decker Shaped Silsesquioxane Compounds

Bisfunctional dichlorosilanes were reacted with tetrasilanol of double-decker shaped octaphenylsilsesquioxane (DDSQ) to form fully condensed DDSQ compounds. The crystallographic and thermal characteristics of these compounds were examined. For compounds capped with dichlorosilanes bearing linear aliphatic moieties, as the number of carbon increases from methyl to n-butyl (from 1 to 4) the melting temperature, Tm, dropped from 546 K to 416 K. While for compounds capped with cycloaliphatic, as the moiety changes from cyclopentyl to cyclohexyl, the value of Tm increases from 533 K to 555 K. Surprising, the highest Tm observed was that when capping was done using diisopropyl dichlorosilane. A Tm of around 565 K was observed, which was even higher as compare to diphenyl dichlorosilane capped DDSQ, which had Tm of about 526 K. Phase behavior of binary and ternary mixtures of these condensed DDSQ was also investigated. To our surprise, mixtures of these compounds form eutectic. The eutectic points were calculated based on ideal binary and ternary eutectic from the thermal properties of pure components. Depending on the crystallography, experimental observations of eutectic match well with calculated values.Graphical abstract Melting and evaporation temperatures were identified for functionalized double-decker shaped silsesquioxanes, melting temperature suppression was observed after mixing DDSQ with different functional groups.

Molecular Simulations of Vapor-Liquid Equilibrium of Isocyanates.

The wide range of applications of the isocyanates across multiple industries sparks the interest in the study of their phase behavior. A molecular simulation is a powerful tool that can go beyond experimental investigations relying on a molecular structure of a chemical. The success of a molecular simulation relies on a description of the system, namely, force field, and its parameterization on reproducing properties of interest. In this work, we propose a united-atom force field based on the transferable potentials for phase equilibria (TraPPE) to model the vapor-liquid phase behavior of isocyanates. With Monte Carlo and molecular dynamics simulation methods and the introduced force field, we modeled vapor-liquid equilibrium for a family of linear mono-isocyanates, from methyl isocyanate to hexyl isocyanate, and hexamethylene diisocyanate. Additionally, we performed similar calculations for methyl, ethyl, and butyl isocyanates based on the all-atom GAFF-IC force field available in the literature for modeling isocyanate viscosities. We showed that the developed TraPPE-based force field generally overperformed the GAFF-IC force field and overall showed excellent performance in modeling phase behavior of isocyanates. Based on the simulated vapor pressures for the considered compounds, we estimated the Antoine equation parameters to calculate the vapor pressure in a range of temperatures. The predictions are of particular use in the investigation of thermodynamic properties for those isocyanates lacking experimental vapor pressure data. Results can also be employed in modeling the phase behavior of isocyanate mixtures to investigate their sensing and capturing. Furthermore, from the vapor-liquid equilibrium binodals, we predicted the critical properties of isocyanates which can be used in thermodynamic models based on an equation of state.

Mixtures of discotic and spherical soft particles: de-mixing, liquid crystal behaviour and relative solubility

Liquid crystals (LC) mixtures are ubiquitous in all real-life applications of LCs. This is due to the need of modulating the macroscopic properties of the LC phase used in specific devices. However, mixing particles of different shapes, not to mention different strength of interaction, often results in de-mixing (phase separation) with the formation of one phase rich in one type of particles separated from the other phase. In this work we investigate the phase behaviour of mixtures of discotic Gay-Berne (GB) particles with spherical Lennard-Jones (LJ) particles using molecular dynamics (MD) simulations. We have considered compositions ranging from the pure GB system up to a GB mole fraction of 0.1 and we have studied the systems at constant volume as a function of the temperature keeping all systems at the same packing fraction. We observe an interesting behaviour where GB-rich systems undergo a nematic-to-isotropic transition, with decreasing transition temperature as the fraction of LJ solutes is increased, keeping a single phase with a relatively large amount of LJ particles dissolved. Lowering the temperature eventually results in a phase separated hexagonal columnar in coexistence with an isotropic phase. In contrast, for LJ-rich systems the de-mixing occurs together with the transition to the nematic phase and the transition point raises again close to the values observed for the pure GB. A clear correlation is also observed between the orientational order parameter of the LC phase and the solubility of LJ particles within the LC phase itself.