Phase and interface behaviors in type-I and type-V Lennard-Jones mixtures: theory and simulations

Density gradient theory (DGT) and molecular-dynamics (MD) simulations have been used to predict subcritical phase and interface behaviors in type-I and type-V equal-size Lennard-Jones mixtures. Type-I mixtures exhibit a continuum critical line connecting their pure critical components, which implies that their subcritical phase equilibria are gas liquid. Type-V mixtures are characterized by two critical lines and a heteroazeotropic line. One of the two critical lines begins at the more volatile pure component critical point up to an upper critical end point and the other one comes from the less volatile pure component critical point ending at a lower critical end point. The heteroazeotropic line connects both critical end points and is characterized by gas-liquid-liquid equilibria. Therefore, subcritical states of this type exhibit gas-liquid and gas-liquid-liquid equilibria. In order to obtain a correct characterization of the phase and interface behaviors of these types of mixtures and to directly compare DGT and MD results, the global phase diagram of equal-size Lennard-Jones mixtures has been used to define the molecular parameters of these mixtures. According to our results, DGT and MD are two complementary methodologies able to obtain a complete and simultaneous prediction of phase equilibria and their interfacial properties. For the type of mixtures analyzed here, both approaches have shown excellent agreement in their phase equilibrium and interface properties in the full concentration range.     

Monte Carlo simulation of equilibrium reactions at modified vapor-liquid interfaces

The equilibrium conversion of a chemical reaction is known to be affected by its local environment. Various factors may alter reaction equilibria, including shifts in pressure or temperature, solvation, adsorption within porous materials, or the presence of an interface. Previously, reactive Monte Carlo simulations have been used to predict the equilibrium behavior of chemical reactions at vapor-liquid interfaces. Here, a route is tested for tuning the interfacial conversion of a Lennard-Jones dimerization reaction by adding surfactants to the vapor-liquid interface. Several temperatures are explored as well as several different surfactant models. Even with the addition of a small concentration of surfactants, the simulations predict significant shifts in the conversion at the interface. In general, the shifts in the conversion tend to be related to the values of the interfacial tension.     

Theoretical study of the three-phase contact line and its tension in adsorbed colloid-polymer mixtures

We perform a theoretical study of the three-phase contact line and the line tension in an adsorbed colloid-polymer mixture near a first-order wetting transition, employing an interface displacement model. We use a simple free-energy functional to describe a colloid-polymer mixture near a hard wall. The bulk phase behavior and the substrate-adsorbate interaction are modeled by the free-volume theory for ideal polymers. The large size of the colloidal particles and the suppression of the van der Waals interaction by optical matching of colloid and solvent justify the planar hard wall model for the substrate. Following the Fisher-Jin scheme, we derive from the free-energy functional an interface potential V(l) for these mixtures. For a particle diameter of 10-100 nm, the calculations indicate a line tension tau approximately 10(-12)-10(-13) N at room temperature. In view of the ultralow interfacial tension in colloid-polymer mixtures, gamma approximately 10(-7) Nm, this leads to a rather large characteristic length scale taugamma in the micrometer range for the three-phase contact zone width. In contrast with molecular fluids, this zone could be studied directly with optical techniques such as confocal scanning laser microscopy.     

Equilibrium calculations of viscosity and thermal conductivity across a solid-liquid interface using boundary fluctuations

We calculate viscosity and thermal conductivity in systems of Lennard-Jones particles consisting of coexisting solid and liquid with different interface wetting properties using the recently developed equilibrium boundary fluctuation theory. We compare the slip length and equivalent liquid length obtained from these calculations with those obtained from nonequilibrium molecular dynamics. The equilibrium and nonequilibrium calculations of the slip length and the sum of the thermal equivalent lengths are in good agreement. We conclude that for both interfacial properties, the nonequilibrium simulations were probing the linear response. The significant dependence of the intrinsic equivalence length on the interfacial temperature difference used to generate the thermal gradient is explained as a consequence of the different thermodynamic states of the two interfaces.     

Bulk and interfacial wetting behavior of binary mixtures induced by associating between unlike-pair molecules

The statistical associating fluid theory of Wertheim is applied to describe binary mixtures with associating between unlike-pair molecules. The phase behavior of this binary mixture would fall into five different types (I, II, III, V, and VI) of the classification scheme of van Konynenburg and Scott by varying the associating strength and the energy parameters. Both interfacial wetting behavior and wetting transitions are carefully examined in all the vapor-liquid-liquid (gamma-beta-alpha) three-phase-coexisting regions of the binary mixtures. The global wetting behavior and wetting transitions are delineated by scanning the parameter space. In certain regions, the middle beta phase exhibits interfacial phase transitions sequentially, nonwetting --> partial-wetting --> nonwetting, at the interface separating lower alpha and upper gamma phases along with increasing temperature.     

Stability of thin polymer films: influence of solvents

The interface and surface properties and the wetting behavior of polymer-solvent mixtures are investigated using Monte Carlo simulations and self-consistent field calculations. We carry out Monte Carlo simulations in the framework of a coarse-grained bead-spring model using short chains (oligomers) of N(P)=5 beads and a monomeric solvent, N(S)=1. The self-consistent field calculations are based on a simple phenomenological equation of state for compressible binary mixtures and we employ Gaussian chain model. The bulk behavior of the polymer-solvent mixture belongs to type III in the classification of van Konynenburg and Scott [Phil. Trans. R. Soc. London, Ser. A 298, 495 (1980)]. It is characterized by a triple line on which the polymer-liquid coexists with solvent-vapor and a solvent-rich liquid. The solvent is not homogeneously distributed across the dense polymer film but tends to accumulate at the surface and the polymer-vapor interface. This solvent enrichment at the interface and surface becomes more pronounced upon increasing the vapor pressure and alters the surface and interface tensions. This effect gives rise to a nonmonotonic dependence of the contact angle on the vapor pressure and one might observe reentrant wetting. The results of the Monte Carlo simulations and the self-consistent field calculations qualitatively agree. The profiles of drops are investigated by Monte Carlo simulations and a pronounced solvent enrichment is observed at the wedge formed by the substrate and the liquid-vapor interface at the three-phase contact line.     

Phase equilibria and interfacial tension of fluids confined in narrow pores

Correlation between phase behaviors of a Lennard-Jones fluid in and outside a pore is examined over wide thermodynamic conditions by grand canonical Monte Carlo simulations. A pressure tensor component of the confined fluid, a variable controllable in simulation but usually uncontrollable in experiment, is related with the pressure of a bulk homogeneous system in equilibrium with the confined system. Effects of the pore dimensionality, size, and attractive potential on the correlations between thermodynamic properties of the confined and bulk systems are clarified. A fluid-wall interfacial tension defined as an excess grand potential is evaluated as a function of the pore size. It is found that the tension decreases linearly with the inverse of the pore diameter or width.     

Entropic wetting and the free isotropic-nematic interface of hard colloidal platelets

We study bulk and interfacial properties of a model suspension of hard colloidal platelets with continuous orientations and vanishing thickness using both density functional theory, based on either a second virial approach or fundamental measure theory (FMT), and Monte Carlo (MC) simulations. We calculate the bulk equation of state, bulk isotropic-nematic (IN) coexistence, and properties of the (planar) free IN interface and of adsorption at a planar hard wall, where we find complete wetting of the nematic phase at the isotropic-wall interface upon approaching bulk IN coexistence. We investigate in detail the asymptotic decay of correlations at large distances. In all cases, the results from FMT and MC agree quantitatively. Our findings are of direct relevance to understanding interfacial properties of dispersions of colloidal platelets.     

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.     

Monte Carlo versus molecular dynamics simulations in heterogeneous systems: an application to the n-pentane liquid-vapor interface

The Monte Carlo (MC) and molecular dynamics (MD) methodologies are now well established for computing equilibrium properties in homogeneous fluids. This is not yet the case for the direct simulation of two-phase systems, which exhibit nonuniformity of the density distribution across the interface. We have performed direct MC and MD simulations of the liquid-gas interface of n-pentane using a standard force-field model. We obtained density and pressure components profiles along the direction normal to the interface that can be very different, depending on the truncation and long range correction strategies. We discuss the influence on predicted properties of different potential truncation schemes implemented in both MC and MD simulations. We show that the MD and MC profiles can be made in agreement by using a Lennard-Jones potential truncated via a polynomial function that makes the first and second derivatives of the potential continuous at the cutoff distance. In this case however, the predicted thermodynamic properties (phase envelope, surface tension) deviate from experiments, because of the changes made in the potential. A further readjustment of the potential parameters is needed if one wants to use this method. We conclude that a straightforward use of bulk phase force fields in MD simulations may lead to some physical inconsistencies when computing interfacial properties.     

Observation of a sequence of wetting transitions in the binary water+ethylene glycol monobutyl ether mixture

A homemade pendant drop/bubble tensiometer was assembled and applied to perform the surface-interfacial tension measurements for the binary water+ethylene glycol monobutyl ether (C4E1) mixture over the temperature range from 50 to 128 degrees C at 10 bar. The symbol CiEj is the abbreviation of a nonionic polyoxyethylene alcohol CiH2i+1(OCH2CH2)jOH. The wetting behavior of the C4E1-rich phase at the interface separating the gas and the aqueous phases was systematically examined according to the wetting coefficient calculated from the experimental results of surface/interfacial tensions. It was found that the C4E1-rich phase exhibits a sequence of wetting transitions, nonwetting-->partial wetting-->complete wetting, at the gas-water interface in the water+C4E1 system along with increasing the temperature, consistent with the conjecture of Kahlweit and Busse [J. Chem. Phys. 91, 1339 (1989)]. In addition, the relationship of the mutual solubility and the interfacial tension of the interface separating the C4E1-rich phase and the aqueous phase is discussed.     

Phase and interfacial behavior of partially miscible symmetric Lennard-Jones binary mixtures

We have carried out extensive equilibrium molecular-dynamics simulations to study quantitatively the topology of the temperature versus density phase diagrams and related interfacial phenomena in a partially miscible symmetric Lennard-Jones binary mixture. The topological features are studied as a function of miscibility parameter, alpha = epsilonAB/epsilonAA. Here epsilonAA = epsilonBB and epsilonAB stand for the parameters related to the attractive part of the intermolecular interactions for similar and dissimilar particles, respectively. When the miscibility varies in the range 0 < alpha < 1, a continuous critical line of consolute points Tcons(rho)--critical demixing transition line--appears. This line intersects the liquid-vapor coexistence curve at different positions depending on the values of alpha, yielding mainly three different topologies for the phase diagrams. These results are in qualitative agreement to those found previously for square-well and hard-core Yukawa binary mixtures. The main contributions of the present paper are (i) a quantitative analysis of the phase behavior and (ii) a detailed study of the liquid-liquid interfacial and liquid-vapor surface tensions, as function of temperature and miscibility as well as its relationship to the topological features of the phase diagrams.     

十二烷基硫酸钠对两性咪唑类离子液体表面活性剂1-磺丙基-3-十二烷基咪唑内盐界面聚集行为的影响

利用界面扩张流变技术,研究了两性咪唑类离子液体表面活性剂1-磺丙基-3-十二烷基咪唑内盐(C₁₂imSP)的界面聚集行为,探讨传统表面活性剂十二烷基硫酸钠(SDS)对C₁₂imSP界面聚集行为的影响机制。 结果表明,少量SDS的加入可以填补界面上疏松的C₁₂imSP分子间的空位,界面上形成表面活性剂混合吸附膜,界面张力显著降低;提高SDS的浓度,其分子从体相向界面层的扩散交换占优势,界面层分子逐渐达到饱和吸附,此后体系中有混合胶束形成。 体相胶束中富集的SDS分子对C₁₂imSP分子的“收纳”作用及进一步的“挽留”作用,加之C₁₂imSP分子本身相对较大的空间位阻效应导致界面上的C₁₂imSP分子一旦通过扩散作用被交换至体相,其很难再回复到表面层,即界面膜以SDS分子为主。 通过调节体系中SDS的含量,可以实现对混合体系SDS/C₁₂imSP/NaCl(0.1 mol/L)界面聚集行为的调控,进而实现对界面膜性质的调控。     

Wall-liquid and wall-crystal interfacial free energies via thermodynamic integration: A molecular dynamics simulation study

A method is proposed to compute the interfacial free energy of a Lennard-Jones system in contact with a structured wall by molecular dynamics simulation. Both the bulk liquid and bulk face-centered-cubic crystal phase along the (111) orientation are considered. Our approach is based on a thermodynamic integration scheme where first the bulk Lennard-Jones system is reversibly transformed to a state where it interacts with a structureless flat wall. In a second step, the flat structureless wall is reversibly transformed into an atomistic wall with crystalline structure. The dependence of the interfacial free energy on various parameters such as the wall potential, the density and orientation of the wall is investigated. The conditions are indicated under which a Lennard-Jones crystal partially wets a flat wall.     

Verification of Onsager's reciprocal relations for evaporation and condensation using non-equilibrium molecular dynamics

Non-equilibrium molecular dynamic (NEMD) simulations have been used to study heat and mass transfer across a vapor-liquid interface for a one-component system using a Lennard-Jones spline potential. It was confirmed that the relation between the surface tension and the surface temperature in the non-equilibrium system was the same as in equilibrium (local equilibrium). Interfacial transfer coefficients were evaluated for the surface, which expressed the heat and mass fluxes in temperature and chemical potential differences across the interfacial region (film). In this analysis it was assumed that the Onsager reciprocal relations were valid. In this paper we extend the number of simulations such that we can calculate all four interface film transfer coefficients along the whole liquid-vapor coexistence curve. We do this analysis both for the case where we use the measurable heat flux on the vapor side and for the case where we use the measurable heat flux on the liquid side. The most important result we found is that the coupling coefficients within the accuracy of the calculation are equal. This is the first verification of the validity of the Onsager relations for transport through a surface using molecular dynamics. The interfacial film transfer coefficients are found to be a function of the surface temperature alone. New expressions are given for the kinetic theory values of these coefficients which only depend on the surface temperature. The NEMD values were found to be in good agreement with these expressions.     

Stabilization of polymer blend structure by diblock copolymers

The stabilizing (emulsifying) effect of a symmetric diblock copolymer in the mixture of two immiscible homopolymers is considered. The equilibrium value of the interfacial area per copolymer chain is calculated via minimization of the free energy of the mixture for a fixed number of copolymer chains adsorbed to the interface. The size and concentration of droplets of the minor component are determined for the equilibrium state. The particles' radius is shown to be inversely proportional to the copolymer concentration, the coefficient of proportionality being dependent on the Flory-Huggins parameter and chain length. The penetration of homopolymer segments into the copolymer layer on the interface is taken into account and proved to be important for stabilization of the droplets by symmetric copolymers. The conditions of the validity of the presented approach are discussed in detail.     

Three-phase interactions and interfacial transport phenomena in coacervate/oil/water systems

Complex coacervation is an associative liquid/liquid phase separation resulting in the formation of two liquid phases: a polymer-rich coacervate phase and a dilute continuous solvent phase. In the presence of a third liquid phase in the form of disperse oil droplets, the coacervate phase tends to wet the oil/water interface. This affinity has long been known and used for the formation of core/shell capsules. However, while encapsulation by simple or complex coacervation has been used empirically for decades, there is a lack of a thorough understanding of the three-phase wetting phenomena that control the formation of encapsulated, compound droplets and the role of the viscoelasticity of the biopolymers involved. In this contribution, we review and discuss the interplay of wetting phenomena and fluid viscoelasticity in coacervate/oil/water systems from the perspective of colloid chemistry and fluid dynamics, focusing on aspects of rheology, interfacial tension measurements at the coacervate/solvent interface, and on the formation and fragmentation of three-phase compound drops.     

Phase behavior and interfacial properties of nonadditive mixtures of Onsager rods

Within a second virial theory, we study bulk phase diagrams as well as the free planar isotropic-nematic interface of binary mixtures of nonadditive thin and thick hard rods. For species of the same type, the excluded volume is determined only by the dimensions of the particles, whereas for dissimilar ones it is taken to be larger or smaller than that, giving rise to a nonadditivity that can be positive or negative. We argue that such a nonadditivity can result from modeling of soft interactions as effective hard-core interactions. The nonadditivity enhances or reduces the fractionation at isotropic-nematic (IN) coexistence and may induce or suppress a demixing of the high-density nematic phase into two nematic phases of different composition (N(1) and N(2)), depending on whether the nonadditivity is positive or negative. The interfacial tension between coexisting isotropic and nematic phases shows an increase with increasing fractionation at the IN interface, and complete wetting of the IN(2) interface by the N(1) phase upon approach of the triple-point coexistence. In all explored cases bulk and interfacial properties of the nonadditive mixtures exhibit a striking and quite unexpected similarity with the properties of additive mixtures of different diameter ratio.     

Capillary condensation of a binary mixture in slit-like pores

We investigate the capillary condensation of two model fluid mixtures in slit-like pores, which exhibit different demixing properties in the bulk phase. The interactions between adsorbate particles are modeled by using Lennard-Jones (12,6) potentials and the adsorbing potentials are of the Lennard-Jones (9,3) type. The calculations are performed for different pore widths and at different concentrations of the bulk gas, by means of density functional theory. We evaluate the capillary phase diagrams and discuss their dependence on the parameters of the model. Our calculations indicate that a binary mixture confined to a slit-like pore may exhibit rich phase behavior.     

The influence of mixed cationic–anionic surfactants on the three-phase contact parameters in silica–solution systems

The formation of thin wetting films on silica surface from aqueous solution of (a) tetradecyltrimetilammonium bromide (C₁₄TAB) and (b) surfactant mixture of the cationic C₁₄TAB with the anionic sodium alkyl- (straight chain C₁₂–, C₁₄– and C₁₆–) sulfonates, was studied using the microscopic thin wetting film method developed by Platikanov. Film lifetimes, three-phase contact (TPC) expansion rates, receding contact angles and surface tension were measured. It was found that the mixed surfactants caused lower contact angles, lower rates of the thin aqueous film rupture and longer film lifetimes, as compared to the pure C₁₄TAB. This behavior was explained by the strong initial adsorption of interfacial complexes from the mixed surfactant system at the air/solution interface, followed by adsorption at the silica interface. The formation of the interfacial complexes at the air/solution interface was proved by means of the surface tension data. It was also shown, that the chain length compatibility between the anionic and cationic surfactants controls the strength of the interfacial complex and causes synergistic lowering in the surface tension. The film rupture mechanism was explained by the heterocoagulation mechanism between the positively charged air/solution interface and the solution/silica interface, which remained negatively charged.