Interfacial Properties Of Binary Mixtures Research Articles

Vapor‐liquid interfacial properties of binary mixtures from molecular simulation and density gradient theory

AbstractProperties of the vapor‐liquid interface of 16 binary mixtures were studied using molecular dynamics simulations and density gradient theory in combination with the PCP‐SAFT equation of state. All binary combinations of the heavy‐boiling components (cyclohexane, toluene, acetone, and carbon tetrachloride) with the light‐boiling components (methane, carbon dioxide, hydrogen chloride, and nitrogen) were investigated at 0.7 times the critical temperature of the heavy‐boiling component in the whole composition range. Data on the surface tension, the enrichment, the relative adsorption, and the interfacial thickness, as well as for the vapor‐liquid equilibrium and Henry's law constant are reported. The binary interaction parameters were fitted to experimental data in a consistent way for all systems and both methods. Overall, the results from both methods agree well for all investigated properties. The interfacial properties of the different studied systems differ strongly. We show that these differences are directly related to the underlying phase equilibrium behavior.

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.

Interfacial Properties of Linear Alkane/Nitrogen Binary Mixtures: Molecular Dynamics Vapor-Liquid Equilibrium Simulations.

Molecular dynamics simulations were used to investigate the vapor-liquid equilibria (VLE) and interfacial properties of binary mixtures of N2 with either ethane, propane, n-decane, or n-dodecane. Alkanes and N2 were modeled by using the TraPPE-UA and Rivera force fields, respectively. The typically used Lorentz-Berthelot combining rules resulted in liquid phases that are too N2-rich compared to experiment. To improve the accuracy of VLE predictions, the hydrocarbon-nitrogen interactions were fine-tuned, and these improved parameters were used to investigate interfacial properties. Scaling the interaction strength between nitrogen and -CH3 and -CH2- groups by factors of 0.95 and 0.85, respectively, relative to the Lorentz-Berthelot value, was found to minimize error in pressure-composition phase diagrams. These scaling parameters gave excellent agreement with experimental phase diagrams for mixtures of N2 with ethane, propane, or n-dodecane over a range of state points. For ethane/N2 and n-decane/N2 mixtures, trends in surface tension as a function of temperature and pressure are correctly reproduced, although the simulated values are slightly too high compared to experimental values. To assess how the accuracy of hydrocarbon-N2 interaction strength impacts interfacial property predictions, we have compared density profiles and surface tension using several different scaling factors. Using the Lorentz-Berthelot combining rules rather than optimized parameters gave the same qualitative trends, although some quantitative results, such as liquid-phase N2 mole fraction, were found to differ by a factor of ∼1.5. Using the optimized interaction parameters, interfacial behavior was examined by calculating density and free energy profiles. Nitrogen molecules preferentially adsorb at the interfacial region between the liquid and vapor phases. This interfacial adsorption becomes less energetically favorable as either the temperature, pressure, or length of the alkane chain increases.

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.

Modeling Interfacial Properties with Spot-DGT-ePC-SAFT for Binary Mixtures Including Ionic Liquid-Based Systems

In this work, spot-density gradient theory (DGT), with an approximate density profile in the vapor–liquid interfacial phase, was combined with ePC-soft-statistical associating fluid theory (SAFT) to describe the interfacial properties of binary mixtures. The developed model, which is termed as spot-DGT-ePC-SAFT, was first used for the mixtures containing common substances (e.g., alkane, benzene, CO2) to verify the model and compare the model performance with the rigorous DGT models. It shows that the surface tensions predicted with spot-DGT-ePC-SAFT are almost the same as those with the rigorous DGT, while spot-DGT costs much less calculation time. The developed spot-DGT-ePC-SAFT was further extended to ionic liquid (IL)–IL and IL–CO2 systems. Again, the predicted surface tensions agree well with the experimental data, indicating the reliability of the developed model for the IL-based systems.

Interfacial properties of binary mixtures of Lennard-Jones chains in planar interfaces by molecular dynamics simulation.

Binary mixtures of fully flexible linear tangent chains composed of bonded Lennard-Jones interaction sites (monomers) were studied using the molecular dynamics simulation in the NVT ensemble. Their interfacial properties were investigated in planar interfaces by direct simulation of an explicit liquid film in equilibrium with its vapor. A method for the calculation of long-range interactions in inhomogeneous fluids was implemented to take into account the potential truncation effects. Surface tension and the pressure tensor were calculated via the classical Irving-Kirkwood method; vapor pressure, orthobaric densities, density profiles, and Gibbs relative adsorption of the volatile component with respect to the heavy component were also obtained. The properties were studied as a function of the temperature, molar concentration of the heavy component, and the asymmetry of the mixture. According to the results of this work, the temperature loses influence on the surface tension, vapor pressure, and Gibbs relative adsorption curves as the molecular length of the heavy component increases. This suggests that the universal behavior observed in pure fluids of Lennard-Jones chains also holds for binary mixtures. The contribution of the long-range interactions turned out to account for about 60%, 20%, and 10% of the surface tension, vapor pressure, and orthobaric density final values, respectively. This contribution was even larger at high temperatures and for large molecules. Strong enrichment of the volatile component at the interface was observed in the asymmetric mixtures. One of these mixtures even showed a barotropic effect at elevated pressures and a class III phase behavior.

Interfacial properties of binary mixtures of simple fluids and their relation to the phase diagram.

Interfacial properties of binary fluid mixtures were studied using both molecular dynamics (MD) simulations and density gradient theory (DGT). The focus of the study is on the relation of the interfacial properties to the phase diagram of the mixture. Two binary Lennard-Jones mixtures were investigated in a wide range of states: a highly asymmetric mixture (type III), which exhibits vapour-liquid equilibria (VL1E and VL2E), liquid-liquid equilibria (L1L2E), a three-phase equilibrium (VL1L2E), and supercritical fluid-fluid equilibria (F1F2E), and, as a reference, an ideal mixture (type I). The studied interfacial properties are: the surface tension, the relative adsorption, the width of the interfacial region, and the enrichment of the low-boiling component, on which we set a focus. Enrichment was observed at VL1 interfaces; and, to a small extent, also at L1L2 interfaces; but not at the supercritical F1F2 interfaces. The large enrichment found at VL1 interfaces of the type III mixture can be interpreted as a wetting transition: approaching the VL1L2E three-phase line from the VL1 side, the enrichment gets stronger and can be interpreted as precursor of the second liquid phase L2. However, the actual existence of a three-phase line in the phase diagram is no prerequisite for an enrichment. The enrichment is found to be highly temperature-dependent and increases with decreasing temperature.

Interfacial properties of binary Lennard-Jones mixtures by molecular simulation and density gradient theory.

A systematic study of interfacial properties of binary mixtures of simple fluids was carried out by molecular dynamics (MD) simulation and density gradient theory (DGT). The fluids are described by the Lennard-Jones truncated and shifted (LJTS) potential with truncation radius of 2.5 diameters. The following interfacial properties were studied: surface tension, relative adsorption, enrichment, and interfacial thickness. A recently developed equation of state for the LJTS fluid, the Perturbed Lennard-Jones truncated and shifted equation of state (PeTS EOS) was used as the basis for DGT. Six binary mixtures (components 1 + 2) were studied at a constant temperature, which was chosen such that the high-boiling component 1 is subcritical, while the low-boiling component 2 is either subcritical or supercritical. Furthermore, a parameter ξ in the combination rule for the unlike dispersive interaction was varied such that the resulting mixtures showed three types of behavior: high-boiling azeotrope, ideal, and low-boiling azeotrope. The parameters of the LJTS potential, including ξ, were also used in the PeTS EOS without any adjustment. Despite this simple approach, excellent agreement between the results of the PeTS EOS and the MD results for the phase equilibrium and the interfacial properties is observed. Enrichment at the interface is only found for the low-boiling component 2. The enrichment increases with decreasing concentration of component 2 and is favored by high boiling point differences of the pure components 1 and 2 and positive deviations from Raoult's law for the mixture 1 + 2.

Interfacial properties of binary mixtures of square-well molecules from Monte Carlo simulation.

We determine the interfacial properties of mixtures of spherical square-well molecules from direct simulation of the vapor-liquid interface. We consider mixtures with the same molecular size and intermolecular potential range but different dispersive energy parameter values. We perform Monte Carlo simulations in the canonical ensemble to obtain the interfacial properties of mixtures of square-well molecules. In particular, we determine the pressuretensor using the mechanical (virial) route and the vapor-liquid interfacial tension evaluated using the Irving-Kirkwood method. In addition to the pressuretensor and the surface tension, we also obtain density profiles, coexistence densities, and interfacial thickness as functions of pressure, at a given temperature. This work can be considered as the extension of our previous work [F. J. Martínez-Ruiz and F. J. Blas, Mol. Phys. 113, 1217 (2015)] to deal with mixtures of spherical molecules that interact through a discontinuous intermolecular potential. According to our results, the main effect of increasing the ratio between the dispersive energy parameters of the mixture, ϵ22/ϵ11, is to sharpen the vapor-liquid interface and to increase the width of the biphasic coexistence region. Particularly interesting is the presence of a relative maximum in the density profiles of the more volatile component at the interface. This maximum is related with adsorption or accumulation of these molecules at the interface, since there are stronger attractive interactions between these molecules in comparison with the rest of intermolecular interactions. Also, the interfacial thickness decreases and the surface tension increases as ϵ22/ϵ11 is larger, a direct consequence of the increasing of the cohesive energy of the system.

Density-Functional Theory for Mixtures of AB Random Copolymer and CO2

We propose a density-functional theory (DFT) to describe inhomogeneous mixtures of AB random copolymer and carbon dioxide (CO_2). The statistical sequence of monomer in the polymer chain backbone is modeled by a transition matrix in a Markov-step growth process. The parameters of the theory are determined by fitting the bulk experimental data. We apply the DFT to the interfacial properties of binary mixtures of CO_2 with poly(methyl methacrylate co ethyl methacrylate) (P(MMA-co-EMA)), poly(methyl methacrylate co ethyl acrylate) (P(MMA-co-EA) and poly(styrene co ethyl acrylate) (P(S-co-EA)). The dependence of CO_2 solubility and interfacial tension on the copolymer composition and pressure is examined. We find that higher fractions of EA or EMA result in higher solubility of CO_2 at a given pressure, but also results in higher interfacial tension at a fixed CO_2 content in the polymer-rich phase. Using the classical nucleation theory as a rough estimate, we examine the effect of the copolymer composition on the free energy barrier of bubble nucleation in random copolymer–CO_2 mixtures.

Interfacial properties of mixtures containing supercritical gases

The paper aims to the prediction of interfacial properties of binary mixtures using only pure component interfacial properties and bulk-properties of the mixture, like phase equilibria. This approach follows strictly the basic idea from Van der Waals. In principle, this goal can be reached by the application of the density gradient theory in combination with an equation of state. This paper focuses on the thermodynamic description of the phase and the surface properties for binary systems, if a supercritical component, like nitrogen or carbon dioxide (CO 2), is present in the mixture. Whereas nitrogen can be treated as non-polar component, carbon dioxide has a strong quadrupole moment. Using PCP-SAFT-EOS in combination with the density gradient theory the prediction of the surface tensions of mixtures close to experimental data is possible.

Electrolyte effect on mixed micelle and interfacial properties of binary mixtures of cationic and nonionic surfactants

In the present work, the adsorption behavior at the liquid–air interface and micellization characteristics of mixtures of cetyltrimethylammonium bromide (CTAB) and p-(1,1,3,3-tetramethylbutyl) polyoxyethylene (TritonX-100) in aqueous media containing different concentrations of NaBr were investigated by surface tension and potentiometry measurements. From plots of surface tension ( γ) as a function of solution composition and total surfactant concentration, we determined the critical micelle concentration (CMC), minimum surface tension at the CMC ( γ CMC ), surface excess ( Γ max ), and mean molecular surface area ( A min ). On the basis of regular solution theory, the compositions of the adsorbed film ( Z) and micelles ( X M ) were estimated, and then the interaction parameters in the micelles ( β M ) and in the adsorbed film phase ( β σ ) were calculated. For all mole fraction ratios, the results showed synergistically enhanced ability to form mixed micelles as well as surface tension reduction. Furthermore β was calculated by considering nonrandom mixing and head group size effects. It was observed that, for both the planar air/aqueous interface and micellar systems, the nonideality decreased as the amount of electrolyte in the aqueous medium was increased. This was attributed to a decrease of the surface charge density caused by increasing the concentration of bromide ions.

Interfacial properties of binary mixtures

Interfacial properties are important in many engineering applications throughout the chemical industry (i.e. distillation and extraction columns, piping used for adsorption and fluid flow). The variation of interfacial properties (interfacial tension, interfacial profiles, interfacial thickness and relative Gibbs adsorption of one component) with temperature and pressure strongly influences the mass transport between fluid phases. In this contribution the gradient theory is applied for modelling the interfacial properties of planar interface in different binary systems between two fluid phases. The Cahn–Hilliard theory was combined with the Peng–Robinson EOS or the original SAFT EOS in order to describe both the phase behaviour and the interfacial properties. The paper focuses on the experimental and theoretical investigation of the interfacial tension between the coexisting liquid phases. In the present study we find that for mixtures containing associating components the simultaneous correlation of both phase equilibria and interface properties with the SAFT EOS in combination with the Cahn–Hilliard theory is possible. For systems where only vapour–liquid equilibria are present, the simpler PR EOS can be used for the simultaneous modelling of phase and interfacial properties.

Density functional theory: Structure and interfacial properties of binary mixtures

AbstractBased on Density Functional Theory (DFT) of inhomogeneous fluid mixtures we model the liquid‐vapour interface of some binary mixtures. Attractive contributions to the Helmholtz free energy are treated by a mean‐field approximation (MF). The local density approximation (LDA) is applied to the repulsive reference system described by a hard sphere equation of state. A cut‐off and shifted Lennard‐Jones‐12‐6 potential divided according to the WCA‐prescription is used as interaction pair potential. Binary Lennard‐Jones parameter are introduced and fitted to experimental liquid‐vapour compositions to describe interactions between different species. The calculations give good results in the case of simple fluid mixtures like Ar‐N2 and CO‐N2. Similar results are obtained for the nonpolar mixture n‐heptane‐toluene. The predicted surface tensions γ agree very well with experimental data. Computed partial density profiles clearly show the enrichment of the component with the lower surface tension in the interphase. The relative Gibbs surface excess concentration is calculated from partial density profiles and compared with experimental values. The influence of activity coefficients on Gibbs adsorption is discussed.