Diffusion of ions in unsaturated porous materials

In a salinity gradient, the diffusion of ions through the connected porosity of a porous and charged material is influenced by the charged nature of the interface between the pore water and the solid. This influence is exerted through the generation of a macroscopic electrical field termed the diffusion or membrane potential. This electrical field depends on the excess of counterions located in the pore space counterbalancing the charge density of the surface of the solid. In unsaturated porous materials, we have to consider (1) the effect of the charged nature of the air/water interface, (2) the increase of the counterion density as the counterions are packed in a smaller volume when the saturation of the nonwetting phase (air) increases, and (3) the influence of the water saturation upon the tortuosity of the water phase. The volume average of the Nernst-Planck equation is used to determine the constitutive equations for the coupled diffusion flux and current density of a multicomponent electrolyte in unsaturated conditions. We assume that water is the wetting phase for the solid phase. We neglect the electro-osmotic flow in the coupled constitutive equations and the deformation of the medium (the medium is assumed to be both isotropic and rigid). This model explains well the observed tendency of strong decreases of the apparent diffusion coefficient of ions with the decrease of the saturation of the water phase under steady-state conditions. This decrease is mainly due to the influence of the saturation upon the tortuosity of the water phase.     

A comparative study of particle swarm optimization and its variants for phase stability and equilibrium calculations in multicomponent reactive and non-reactive systems

Particle swarm optimization is a novel evolutionary stochastic global optimization method that has gained popularity in the chemical engineering community. This optimization strategy has been successfully used for several applications including thermodynamic calculations. To the best of our knowledge, the performance of PSO in phase stability and equilibrium calculations for both multicomponent reactive and non-reactive mixtures has not yet been reported. This study introduces the application of particle swarm optimization and several of its variants for solving phase stability and equilibrium problems in multicomponent systems with or without chemical equilibrium. The reliability and efficiency of a number of particle swarm optimization algorithms are tested and compared using multicomponent systems with vapor–liquid and liquid–liquid equilibrium. Our results indicate that the classical particle swarm optimization with constant cognitive and social parameters is a reliable method and offers the best performance for global minimization of the tangent plane distance function and the Gibbs energy function in both reactive and non-reactive systems.     

Dynamics of fluid mixtures in nanospaces

A multicomponent extension of our recent theory of simple fluids [U. M. B. Marconi and S. Melchionna, J. Chem. Phys. 131, 014105 (2009)] is proposed to describe miscible and immiscible liquid mixtures under inhomogeneous, nonsteady conditions typical of confined fluid flows. We first derive from a microscopic level the evolution equations of the phase space distribution function of each component in terms of a set of self-consistent fields, representing both body forces and viscous forces (forces dependent on the density distributions in the fluid and on the velocity distributions). Second, we numerically solve the resulting governing equations by means of the lattice Boltzmann method, whose implementation contains novel features with respect to existing approaches. Our model incorporates hydrodynamic flow, diffusion, surface tension, and the possibility for global and local viscosity variations. We validate our model by studying the bulk viscosity dependence of the mixture on concentration, packing fraction, and size ratio. Finally, we consider inhomogeneous systems and study the dynamics of mixtures in slits of molecular thickness and relate structural and flow properties.     

Nonempirical statistical theory for molecular evaporation from nonrigid clusters

We propose a nonempirical statistical theory to give the reaction rate and the kinetic energy distribution of fragments for molecular evaporation from highly nonrigid atomic and van der Waals clusters. To quantify the theory, an efficient and accurate method to evaluate the absolute value of classical density of states (the Thomas-Fermi density in phase space) and the flux at the so-called dividing surface is critically important, and we have devised such an efficient method. The theory and associated methods are verified by numerical comparison with the corresponding molecular dynamics simulation through the study of Ar(2) evaporation from Ar(8) cluster, in which evaporation is strongly coupled with structural isomerization dynamics. It turns out that the nonempirical statistical theory gives quite an accurate reaction rate. We also study the kinetic energy release (KER) arising from these evaporations and its Boltzmann-like distribution both for atomic and diatomic evaporations. This provides a general relation between the KER and temperature of the fragments.     

Determination of Dicyandiamide with Non-Aqueous Polar Eluent

Abstract

Epoxy powder paints are characterized by their dicyandiamide content. Dicyandiamide is determined on a amino bonded phase column with non-aqueous but polar eluent consisting of methanol and acetonitrile (5/95, v/v). The elution order of the components is that of normal phase but the eluent is more like reversed phase. This method is essentially faster than the previous methods in the analysis of dicyandiamide in multicomponent powder extracts.     

Solidification of small p-H2 clusters at zero temperature

We have determined the ground-state energies of para-H(2) clusters at zero temperature using the diffusion Monte Carlo method. The liquid or solid character of each cluster is investigated by restricting the phase through the use of proper importance sampling. Our results show inhomogeneous crystallization of clusters, with alternating behavior between liquid and solid phases up to N = 55. From there on, all clusters are solid. The ground-state energies in the range N = 13-75 are established, and the stable phase of each cluster is determined. In spite of the small differences observed between the energy of liquid and solid clusters, the corresponding density profiles are significantly different, a feature that can help to solve ambiguities in the determination of the specific phase of H(2) clusters.     

Extraordinary Supercapacitor Performance of a Multicomponent and Mixed‐Valence Oxyhydroxide

We report a novel multicomponent mixed‐valence oxyhydroxide‐based electrode synthesized by electrochemical polarization of a de‐alloyed nanoporous NiCuMn alloy. The multicomponent oxyhydroxide has a high specific capacitance larger than 627 F cm^?3 (1097±95 F g^?1) at a current density of 0.25 A cm^?3, originating from multiple redox reactions. More importantly, the oxyhydroxide electrode possesses an extraordinarily wide working‐potential window of 1.8 V in an aqueous electrolyte, which far exceeds the theoretically stable window of water. The realization of both high specific capacitance and high working‐potential windows gives rise to a high energy density, 51 mWh cm^?3, of the multicomponent oxyhydroxide‐based supercapacitor for high‐energy and high‐power applications.     

Atomistic simulations of 2D bicomponent self-assembly: from molecular recognition to self-healing

Supramolecular two-dimensional engineering epitomizes the design of complex molecular architectures through recognition events in multicomponent self-assembly. Despite being the subject of in-depth experimental studies, such articulated phenomena have not been yet elucidated in time and space with atomic precision. Here we use atomistic molecular dynamics to simulate the recognition of complementary hydrogen-bonding modules forming 2D porous networks on graphite. We describe the transition path from the melt to the crystalline hexagonal phase and show that self-assembly proceeds through a series of intermediate states featuring a plethora of polygonal types. Finally, we design a novel bicomponent system possessing kinetically improved self-healing ability in silico, thus demonstrating that a priori engineering of 2D self-assembly is possible.     

On measurement of thermal diffusion coefficients in multicomponent mixtures

We investigate the steady-state separation of the individual components of an incompressible multicomponent liquid mixture in a narrow two-dimensional thermogravitational column. Analytic working equations for measuring thermal diffusion coefficients analogous to the existing equations for a binary mixture are derived. Similar to the binary results, we find that when compositional variation has negligible effect on fluid density and vertical diffusive flux can be ignored, molecular diffusion does not affect steady-state separation. However, when compositional effects on density are taken into account, molecular diffusion does affect the bulk convective flow and the steady-state separation of the components. There may be also two distinct trends in the velocity and separation profiles. With one or more negative thermal diffusion coefficients, there may be more than one convection cell resulting in oscillatory behavior of separation. The working equations presented can be used to measure thermal diffusion coefficients of multicomponent mixtures. Such measurements have not yet been reported in the literature.     

应用相图边界理论勾画多元恒温截面相图

根据相图的边界理论,可以确定恒温截面图中紧邻相区及其边界关系,进而可以根据有限的实验数据,比较直接地勾画出整个恒温截面图。本文以5元相图和8元相图为例加以说明。     

Modeling of phase equilibrium and vapor adsorption on carbon black based on a combination of a lattice theory and equation of state

A thermodynamic approach is developed in this paper to describe the behavior of a subcritical fluid in the neighborhood of vapor-liquid interface and close to a graphite surface. The fluid is modeled as a system of parallel molecular layers. The Helmholtz free energy of the fluid is expressed as the sum of the intrinsic Helmholtz free energies of separate layers and the potential energy of their mutual interactions calculated by the 10-4 potential. This Helmholtz free energy is described by an equation of state (such as the Bender or Peng-Robinson equation), which allows us a convenient means to obtain the intrinsic Helmholtz free energy of each molecular layer as a function of its two-dimensional density. All molecular layers of the bulk fluid are in mechanical equilibrium corresponding to the minimum of the total potential energy. In the case of adsorption the external potential exerted by the graphite layers is added to the free energy. The state of the interface zone between the liquid and the vapor phases or the state of the adsorbed phase is determined by the minimum of the grand potential. In the case of phase equilibrium the approach leads to the distribution of density and pressure over the transition zone. The interrelation between the collision diameter and the potential well depth was determined by the surface tension. It was shown that the distance between neighboring molecular layers substantially changes in the vapor-liquid transition zone and in the adsorbed phase with loading. The approach is considered in this paper for the case of adsorption of argon and nitrogen on carbon black. In both cases an excellent agreement with the experimental data was achieved without additional assumptions and fitting parameters, except for the fluid-solid potential well depth. The approach has far-reaching consequences and can be readily extended to the model of adsorption in slit pores of carbonaceous materials and to the analysis of multicomponent adsorption systems.     

Phase Behavior of Ternary Systems of the Type H2O?Oil?Nonionic Amphiphile (Microemulsions)

Liquid multicomponent systems of the type H₂O-oil-amphiphile-electrolyte are of growing interest, both in industry and in research. In applications technology, there are two problems to solve: 1. To prepare stable homogeneous solutions of H₂O and nonpolar liquids with as little amphiphile as possible which can be diluted with H₂O in all proportions without phase separation; e.g., concentrated solutions of drugs, herbicides, or insecticides. 2. To prepare stable mixtures of an aqueous, an amphiphile, and an oil phase with as little amphiphile as possible which are employed in tertiary oil recovery and in the pharmaceutical industry. Furthermore, such systems may be used for performing chemical reactions in heterogeneous liquid mixtures with continuously variable properties. In research, such systems are of interest for both experimental and theoretical studies of critical phenomena, especially near so-called tricritical points. Last but not least, their properties may stimulate further research in the field of associated solutions. In this paper we summarize the results of our studies on the phase behavior of ternary systems with nonionic amphiphiles, in particular with respect to the evolution of liquid three-phase bodies. The results suggest that the tricritical points in such systems should be regarded as kinds of pivot points from which the phase behavior evolves. If this were an approach to reality, the phase behavior would be governed in more or less good approximation by universal scaling laws, irrespective of the particular microstructure of the solutions. Finally, we discuss the effect of electrolytes on the phase behavior both in a quaternary phase tetrahedron and a pseudoternary phase prism representation. Although in practice most systems consist of mixtures of oils, amphiphiles, and electrolytes, an understanding of the phase behavior of truly ternary and quaternary systems with chemically well-defined components permits at least qualitative predictions with respect to the phase behavior of the multicomponent mixtures encountered in practice.     

A new mechanistic Parachor model to predict dynamic interfacial tension and miscibility in multicomponent hydrocarbon systems

Widely used traditional Parachor model fails to provide reliable interfacial tension predictions in multicomponent hydrocarbon systems due to the inability of this model to account for mass transfer effects between the fluid phases. In this paper, we therefore proposed a new mass transfer enhanced mechanistic Parachor model to predict interfacial tension and to identify the governing mass transfer mechanism responsible for attaining the thermodynamic fluid phase equilibria in multicomponent hydrocarbon systems. The proposed model has been evaluated against experimental data for two gas-oil systems of Rainbow Keg River and Terra Nova reservoirs. The results from the proposed model indicated good IFT predictions and that the vaporization of light hydrocarbon components from crude oil to gas phase is the governing mass transfer mechanism for the attainment of fluid phase equilibria in both the gas-oil systems used. A multiple linear regression model has also been developed for a priori prediction of exponent in the mechanistic model by using only the reservoir fluid compositions, without the need for experimental measurements. The dynamic nature of interfacial tensions observed in the experiments justifies the use of diffusivities in the mechanistic model, thus enabling the proposed model predictions to determine dynamic gas-oil miscibility conditions in multicomponent hydrocarbon systems.     

Liquid–liquid phase behaviour,liquid–liquid density,and interfacial tension of multicomponent systems at 298 K

Experimental liquid–liquid phase diagrams are presented for the multicomponent systems isooctane–benzene–(80 mass% methanol + 20 mass% water)–5 mass% isobutyl alcohol (2-methyl-1-propanol) and isooctane–benzene–(80 mass% methanol + 20 mass% water)–15 mass% isobutyl alcohol, at 298.15 K. The density and interfacial tension of conjugate phases of concentration located in the isothermal binodal have been determined at 298.15 K for the partially miscible systems: isooctane–benzene–methanol, isooctane–benzene–(80 mass% methanol + 20 mass% water), isooctane–benzene–(80 mass% methanol + 20 mass% water)–5 mass% isobutyl alcohol, and isooctane–benzene–(80 mass% methanol + 20 mass% water)–15 mass% isobutyl alcohol. The experimental tie-line data define the binodal or coexistence curve of the two studied multicomponent systems and depending on the initial isobutyl alcohol concentration the liquid–liquid phase diagram is either of type II, with low alcohol concentration, or type I, with high concentration of alcohol, which is a clear indication that the solubility of the partially miscible systems is greatly enhanced via the co-solvency phenomenon. It is observed that both the density of each conjugate phase and the interfacial tension of each tie-line are valuable indicators of the degree of solubility of the multicomponent systems. Furthermore the experimental tie-lines data were correlated with the NRTL and UNIQUAC solution models with satisfactory quantitative results.     

Efficient methods to study phase equilibria in multinary aqueous systems

We report the results of developing efficient universal isothermal methods for the determination of compositions of all phases (several solid phases and a liquid) involved in invariant equilibria in multicomponent aqueous systems using chemical analysis (a combination method) or not (an optimized sections method) to determine the composition of an equilibrium liquid phase.     

On a partial differential equation method for determining the free energies and coexisting phase compositions of ternary mixtures from light scattering data

In this paper we present a method for determining the free energies of ternary mixtures from light scattering data. We use an approximation that is appropriate for liquid mixtures, which we formulate as a second-order nonlinear partial differential equation. This partial differential equation (PDE) relates the Hessian of the intensive free energy to the efficiency of light scattering in the forward direction. This basic equation applies in regions of the phase diagram in which the mixtures are thermodynamically stable. In regions in which the mixtures are unstable or metastable, the appropriate PDE is the nonlinear equation for the convex hull. We formulate this equation along with continuity conditions for the transition between the two equations at cloud point loci. We show how to discretize this problem to obtain a finite-difference approximation to it, and we present an iterative method for solving the discretized problem. We present the results of calculations that were done with a computer program that implements our method. These calculations show that our method is capable of reconstructing test free energy functions from simulated light scattering data. If the cloud point loci are known, the method also finds the tie lines and tie triangles that describe thermodynamic equilibrium between two or among three liquid phases. A robust method for solving this PDE problem, such as the one presented here, can be a basis for optical, noninvasive means of characterizing the thermodynamics of multicomponent mixtures.     

Contributions to the Gelling Process in the Bi-Sr-Ca-Cu-O System, Via Acetate Route

Metal acetates represent suitable precursors in the sol-gel method. Generally, acetates are used in multicomponent systems with alkoxides or other carboxylic salts. However, Bi−Sr−Ca−Cu−O superconductors have been obtained by the sol-gel method starting only from acetate mixture. This work was intended to study the gelling process of acetates involved in the formation of bismuth-based superconducting phases, both individually and in the bi-and multicomponent systems. Probably, copper acetate is mainly responsible for the gel formation in these systems. The gelling process was monitored by measuring the pH, density and viscosity changes. The gels obtained were characterized by UV-VIS and IR spectra, DTA/TGA and powder X-ray diffraction.     

Composition of Coexisting Liquid Phases Determined by Rayleigh Interferometry

Rayleigh interferometry is a precise macroscopic gradient technique that has been utilized for the determination of multicomponent diffusion coefficients. Because concentration gradients in multicomponent systems drive a diffusion-based partial separation of different solutes, this interferometric technique may be potentially used for the determination of solute concentrations. We have therefore theoretically examined how Rayleigh interferometry can be applied for the determination of composition of ternary aqueous mixtures. The effect of cross-term diffusion coefficients on the accuracy of this method is also discussed. Furthermore, since the poly(vinyl)alcohol+poly(ethylene)glycol+water system undergoes liquid–liquid phase separation (LLPS), we have experimentally characterized its LLPS boundary at 25 °C. The corresponding tie-lines were characterized by determining the composition of the two coexisting liquid phases using Rayleigh interferometry.     

Dissolution of polycyclic aromatic hydrocarbons from a non-aqueous phase liquid into a surfactant solution using a rotating disk apparatus

The mass transfer of two polycyclic aromatic hydrocarbons (PAHs), naphthalene and phenanthrene from a multicomponent non-aqueous phase liquid (NAPL) into a nonionic surfactant solution, Brij 35 was investigated using a rotating apparatus. Few experimental methods have been applied to the study of solubilization kinetics in organic liquids because in those systems, the interfacial area during mixing is more difficult to maintain and measure. This challenge was overcome by permeating the NAPL through a membrane. Mass transfer experiments were conducted in the absence and presence of surfactant, and the concentrations of naphthalene and phenanthrene in the bulk aqueous phase were determined in samples collected at different time intervals from the time of initial contact of the NAPL phase with the aqueous solution phase. Experiments in pure water demonstrated that the rotating apparatus behaves as in much the same way as the Levich's rotating disk. The mass transfer coefficients and the dissolution of PAHs into the surfactant solution were measured at different doses of Brij 35. As the surfactant concentration increased, the mass transfer coefficients for both PAHs from the NAPL decreased.     

Mechanism of Proton Relaxation for Enzyme‐Manipulated,Multicomponent Gold–Magnetic Nanoparticle Chains

Longitudinal and transverse relaxation times of multicomponent nanoparticle (NP) chains are investigated for their potential use as multifunctional imaging agents in magnetic resonance imaging (MRI). Gold NPs (ca. 5 nm) are arranged linearly along double‐stranded DNA, creating gold NP chains. After cutting gold NP chains with restriction enzymes (EcoRI or BamHI), multicomponent NP chains are formed through a ligation reaction with enzyme‐cut, superparamagnetic NP chains. We evaluate the changes in relaxation times for different constructs of gold–iron oxide NP chains and gold–cobalt iron oxide NP chains using 300 MHz ¹H NMR. In addition, the mechanism of proton relaxation for multicomponent NP chains is examined. The results indicate that relaxation times are dependent on the one‐dimensional structure and the amount of superparamagnetic NP chains present in the multicomponent constructs. Multicomponent NP chains arranged on double‐stranded DNA provide a feasible method for fabrication of multifunctional imaging agents that improve relaxation times effectively for MRI applications.