Thermodynamic properties of gold–water nanolayer mixtures using molecular dynamics

The physical behavior of a fluid in contact with solid layers is still not fully understood. The present work focuses on the study and understanding of thermodynamic and structural properties of gold–water nanolayer mixtures using molecular dynamics simulations. Two different systems are considered, where approximately 1,700 water molecules are confined between gold nanolayers with separations of 7.4 and 6.2 nm, respectively. Novelties of the present work are in the use of accurate force fields for modeling the inter- and intra-molecular interactions of the components, and providing comprehensive thermodynamic properties of the mixtures. The results are validated by examination of the pure fluid and pure solid properties. Results indicate that the thermodynamics of the system does not behave as an ideal mixture. The structure of the pure fluid is also analyzed and compared against the structure of the confined fluid in the mixture. Anisotropicity is observed in the fluid structure close to the surface of the nanolayer. Higher ordering and higher flux are detected in the fluid molecules close to the fluid–solid interface. Unusual thermodynamic behavior, anisotropicity, liquid layering, and higher interfacial fluid flux could be just some of the factors leading to the enhanced energy transport observed in mixtures involving at least one nanoscale component, such as nanofluids.     

Phase separation of highly dissymmetric binary ionic mixtures

We study the phase separation of binary ionic mixtures involving two species of classical point ions in a rigid uniform neutralizing background of degenerate electrons. The thermodynamic properties of the ionic fluid are calculated on the basis of the HNC integral equation for the three partial pair distribution functions. We develop a systematic technique which allows the properties of mixtures of arbitrary composition to be expressed in terms of infinitely dilute solutions. Phase diagrams and critical parameters are determined for 12 different binary systems involving ionic charge ratios between 2 and 8. The dependence of the critical temperature on the ionic charges, on the pressure and an ionic quantum corrections is examined in detail.     

Thermodynamic Curvature of the Binary van der Waals Fluid

The thermodynamic Ricci curvature scalar R has been applied in a number of contexts, mostly for systems characterized by 2D thermodynamic geometries. Calculations of R in thermodynamic geometries of dimension three or greater have been very few, especially in the fluid regime. In this paper, we calculate R for two examples involving binary fluid mixtures: a binary mixture of a van der Waals (vdW) fluid with only repulsive interactions, and a binary vdW mixture with attractive interactions added. In both of these examples, we evaluate R for full 3D thermodynamic geometries. Our finding is that basic physical patterns found for R in the pure fluid are reproduced to a large extent for the binary fluid.     

Fundamentals of surface thermodynamics

The nonequilibrium behavior for mixtures of fluids in interfaces is discussed. In particular, a thermodynamic field theory is given for media in thin, curved regions (interfaces with finite thickness), which separates two media with different physical properties. The moving interface is considered as semipermeable and a generalized transport equation and specific balance equations are derived. A systematic investigation of constitutive equations is made and in the limit as the thickness of the interface goes to zero it is shown that all relevant interfacial relations can be found.     

Phase diagrams of liquid helium mixtures and metamagnets: Experiment and mean field theory

The phase behavior at low temperature, in particular the critical and tricritical properties, of liquid ³He⁴He mixtures and certain types of metamagnets — a class of highly anisotropic antiferromagnets, such as FeCl₂ etc. — is investigated. Since both systems exhibit two successive phase transitions, one of which is a λ-type of transition while the other is a first order transition, special attention is given to the similarity in the behavior of the two systems. In part A first the experimental and earlier theoretical work are briefly reviewed. Then the phase behavior of each system is calculated on the basis of a simple model treated in mean field approximation. The results obtained are in general qualitative agreement with experiment. For the helium mixtures an isotopic mixture of hard-spheres following Fermi and Bose statistics is used, while for the metamagnet a two sublattice spin $\text{1}\text{2}$ Ising model with nearest neighbor and next-nearest neighbor interactions is employed. A simple physical argument for the analogous behavior of the two models is given. In part B, in an attempt to obtain a deeper understanding of the similarity of the two systems, the Hamiltonians of the two models are extended to include symmetry breaking fields: a particle source field for the helium mixtures and a staggered magnetic field for the metamagnet. The phase diagrams and critical behavior of the two models in an extended thermodynamic space that includes the symmetry breaking fields are discussed in mean field theory.We find that although the two models (treated in the mean field approximation) are very similar, there are differences near T = 0°K and in some of the critical exponents.     

Transport of volume in a binary liquid

In this paper, the transport of volume in a binary liquid mixture is theoretically investigated in three steps, with strong implications for the measurement of mutual diffusivities in non-dilute mixtures. In a first step, the velocity of volume transport is determined from the transport velocities of the two components and the thermodynamic relation of state of the liquid mixture in equilibrium. The role played by Galilean invariance and the choice of a rigid frame of reference for reckoning current densities is highlighted. The divergence of the volume-transport velocity field is found to involve the isothermal compressibility and the thermal expansivity of the liquid together with the spatiotemporal variations of pressure and temperature. In a second step, a linear-response relation is introduced between the interdiffusion current density and the gradient of composition; this relation phenomenologically defines the mutual diffusivity of the binary liquid in a manifestly Galilean-invariant way. In a third step, it is examined whether the practical measurement of that diffusivity in a constant-volume container entails a vanishing mass-transport or volume-transport velocity. From a singular-perturbation analysis of the hydrodynamic equations, it is shown that the mass-transport velocity vanishes in the limit of a diffusion of composition that is much slower than the diffusion of momentum. As a consequence, the volume-transport velocity does not vanish during interdiffusion even though the law of additive volumes of the components holds. The physical meaning of the non-vanishing volume velocity is interpreted by means of the thermodynamic results obtained in the second step. Some of the conclusions carry over to multicomponent liquid mixtures.     

Thermodynamic properties of multicomponent mixtures near the liquid-vapor critical point

A new method has been proposed to describe the physical properties of multicomponent mixtures near their critical points. The method is based on the transition from the experimental thermodynamic variables to scaling fields, is applicable to a mixture with any number of the components, and is, thus, universal. For the previously studied methane-propane-pentane mixture, it has been shown that the anomalies of the specific heat at a constant volume and derivative (?P/?T)_ρ,x can be quantitatively described in this approach in a wide vicinity of a critical point, including noncritical isochores.     

The empirical Kestin model and the second order transport properties of binary mixtures of Ne,Ar, Kr and Xe

It is showed from the experimental α_T data for the dilute binary mixtures of Ne, Ar, Kr and Xe at 340 K that the law of corresponding states proposed by Kestin, Ro and Wakeham is able to predict the mentioned second order kinetic property altogether with the first order transport and thermodynamic properties of the examined mixtures.     

Nonlocal fluid density distribution in the vicinity of the critical point

The differential equation for the component local density of a binary fluid was obtained as a result of an isoperimetric problem of the system free energy minimization under condition of particle number stability. The obtained expression can be used in the calculation of liquid's profiles as well as in the investigation of thermodynamic properties in the vicinity of the critical point. An isoperimetric problem solution for the model binary fluid of the system free energy minimization under condition of particle number stability was obtained in different cases: fluid within flat parallel layer under a gravitational field, under a gravitational field and wall potential as well as under a wall potential for another equation of state.     

On thermal diffusion in binary and ternary Lennard-Jones mixtures by non-equilibrium molecular dynamics

Molecular simulation appears to be an alternative to experiment for the estimation of transport and thermodynamics properties of fluid mixtures, which is of primary importance in the evaluation of the initial state of a petroleum reservoir. In this study, a non-equilibrium molecular dynamics algorithm has been applied to mixtures of Lennard-Jones spheres in order to compute the thermal diffusion process. The pertinence of such an approach to simple alkane mixtures is shown. The separate influences on the thermal diffusion of the molecular features in binary equimolar mixtures are then summarized. Simulations on binary non-equimolar mixtures have been performed as well. The results indicate an increase in the thermal diffusion process with increasing molar fraction of the lightest component. Moreover, this increase is enhanced with increasing difference in the number of carbons between the two alkanes. Then, a simple method, which yields results consistent with simulations, is proposed to predict thermal diffusion for the whole range of molar fractions starting only from the equimolar value. Finally, for ternary mixtures, the law of the corresponding states is shown to be valid when the appropriate mixing rules are applied, which allows the estimation of thermal diffusion in such mixtures from equivalent binary mixtures.     

On the critical non-additivity driving segregation of asymmetric binary hard sphere fluids

A previously proposed version of thermodynamic perturbation theory, appropriate for singular pair interactions between particles, is applied to binary mixtures of hard spheres with non-additive diameters. The critical non-additivity Δ_C required to drive fluid–fluid phase separation is determined as a function of the ratio ξ ≤ 1 of the diameters of the two species. Δ_C(ξ) is found to decrease with ξ and to go through a minimum for ξ ? 0.015 before increasing sharply as ξ → 0, irrespective of the total packing-fraction η of the mixture. These results are the basis of an estimate of the range of size ratios for which a binary mixture of additive hard spheres exhibit a fluid–fluid miscibility gap. This range is conjectured to be 0.01 ? ξ ? 0.1.     

A multi-zone chemistry mapping approach for direct numerical simulation of auto-ignition and flame propagation in a constant volume enclosure

A direct numerical simulation (DNS) coupling with multi-zone chemistry mapping (MZCM) is presented to simulate flame propagation and auto-ignition in premixed fuel/air mixtures. In the MZCM approach, the physical domain is mapped into a low-dimensional phase domain with a few thermodynamic variables as the independent variables. The approach is based on the fractional step method, in which the flow and transport are solved in the flow time steps whereas the integration of the chemical reaction rates and heat release rate is performed in much finer time steps to accommodate the small time scales in the chemical reactions. It is shown that for premixed mixtures, two independent variables can be sufficient to construct the phase space to achieve a satisfactory mapping. The two variables can be the temperature of the mixture and the specific element mass ratio of H atom for fuels containing hydrogen atoms. An aliasing error in the MZCM is investigated. It is shown that if the element mass ratio is based on the element involved in the most diffusive molecules, the aliasing error of the model can approach zero when the grid in the phase space is refined. The results of DNS coupled with MZCM (DNS-MZCM) are compared with full DNS that integrates the chemical reaction rates and heat release rate directly in physical space. Application of the MZCM to different mixtures of fuel and air is presented to demonstrate the performance of the method for combustion processes with different complexity in the chemical kinetics, transport and flame–turbulence interaction. Good agreement between the results from DNS and DNS-MZCM is obtained for different fuel/air mixtures, including H₂/air, CO/H₂/air and methane/air, while the computational time is reduced by nearly 70%. It is shown that the MZCM model can properly address important phenomena such as differential diffusion, local extinction and re-ignition in premixed combustion.     

Magnetic behavior in UFe?Zn?? and URu?Zn?? single crystals

The magnetic, electrical transport and thermodynamic properties of the compounds UFe?Zn?? and URu?Zn?? were studied on single-crystalline specimens over wide ranges of temperature and magnetic field. The results indicate that the two ternaries are paramagnetic moderately enhanced heavy fermion systems. Their physical behavior is governed predominantly by the hybridization of uranium 5f orbitals with electronic states of ligands, which brings about considerable delocalization of the 5f states.     

Spurious character of singularities associated with phase transitions in cylindrical pores

Phase transitions of simple fluids and binary fluid mixtures confined into long cylindrical pores are re-examined, such as capillary condensation/evaporation and wetting transitions. While a large part of the literature ignores the fact that due to the quasi-one-dimensional character of these systems a singular behavior associated with a sharp phase transition cannot occur, we pay attention to the extent in which these phase transitions are smoothed out (in relation to the magnitude of the pore cross-sectional area). We argue that the finiteness of the pore length is an important parameter which controls the physical phenomena that are observed in simulations (and presumably also experiments explaining the distinction between the apparent “pore critical temperature” and the “hysteresis critical temperature”). We illustrate our arguments with recent findings from simulations of a lattice gas/Ising system and of the Asakura-Oosawa model of colloid-polymer mixtures.     

The influence of hydrogen bonds upon diffusion of simple amines and binary mixtures of ammonia

Abstract

Self diffusion coefficients of monomethylamine and trimethylamine and intradiffusion coefficients of some model mixtures containing ammonia have been measured up to pressures of 200 MPa at temperatures between the melting pressure curve and 423 K by pulsed field gradient Spin Echo NMR. Compared to water and the lower alcohols the self diffusion coefficients of pure fluid ammonia shows no clear influence from hydrogen bonds in contrast to other thermodynamic properties. Therefore the methylated substance monomethylamine has been studied to see weather a more structured charge distribution on the molecular surface compared to ammonia is needed to hinder fast rearrangements of the hydrogen bonds. For comparison also trimethylamine has been studied where no hydrogen bonds can be formed. Additionally binary mixtures of ammonia with methanol, benzene, trimethylamine and acetonitrile have been studied to see effects of intermolecular interactions.     

Molecular rotation in liquid crystals selective deuterium spin-lattice relaxation times in the nematic mesophase of 4-cyano-4′-d17-n-octylbiphenyl

The Gibbs ensemble is used to simulate the liquid–liquid equilibria of binary mixtures containing dipolar and non-polar components.The interactions of the dipolar fluid are calculated using the Keesom intermolecular potential. The liquid–liquid coexistence properties are reported for different pressures and different combinations of dipolar/non-polar molecules. The critical properties of the mixtures are estimated. The ability of a dipole to induce phase separation is influenced by the dispersion energy of the molecule. Phase separation is enhanced if the dipolar molecule is also the component with the greatest dispersion energy.     

Studies of a primitive model of mixtures of nonpolar molecules with water

The paper presents calculations of the properties of binary mixtures of hard spheres and directionally associating hard spheres, a simple model for mixtures of nonpolar molecules with water that was developed by Nezbeda and his coworkers. Extensive results from Monte Carlo simulations in the isobaric, isothermal ensemble are presented for the density, configurational energy and chemical potentials in the mixtures for fluid states over a range of temperatures, pressures and compositions. A species exchange technique is used to compute the chemical potential difference between components in the mixtures. The results obtained are compared with the predictions of first-order thermodynamic perturbation theory (TPT). It is found that this theory provides an accurate picture of the system over most of the conditions considered. Calculations are also made of vapour–liquid coexistence for the model using TPT and calculations of solid–fluid coexistence for the model using TPT and existing results for the free energy of the pure component solids. It is found that the vapour–liquid coexistence for the model is pre-empted by the solid–fluid coexistence, as had previously been found for the pure component directionally associating hard sphere system.     

Structure of liquid solutions near the critical point

Specific features of the structure of the critical state of binary liquid solutions leading to an anomalous behavior of the Rayleigh line due to a dramatic increase in concentration and density fluctuations are considered. It is shown that an experimental treatment must deal with two fluctuation regions near the critical point of solvent vaporization. In the first region, one can achieve a sufficient degree of accuracy by using theories like self-consistent field theory. In the second region, which is closer to the critical point than the first region, scaling theory of second-order phase transitions may be applied. It is found that the anomalous behavior of the Rayleigh line associated with kinetic coefficients is determined by the equilibrium thermodynamic properties and by the radius of fluctuation correlation (r_c). A general theory is developed for calculating thermodynamic potentials, especially the chemical potential and its concentration derivative in the fluctuation region. The results of these calculations are compared with the experimental data briefly described in the paper. Translated fromZhurnal Struktumoi Khimii, Vol. 39, No. 4, pp. 655–668, July–August, 1998.     

Time dependent correlation functions and mode-mode coupling theories

We critically discuss the various tools and methods which are used to describe the role of long wave length hydrodynamical processes in the analysis of time-dependent correlation functions. We also review the various physical problems (long time behavior of Green-Kubo integrands, 2 dimensional hydrodynamics, transport properties of the Van der Waals fluid, critical phenomena …) where these methods have received fruitful applications.