Ion-dipole Mixture Research Articles

Modified Poisson–Boltzmann equations and macroscopic forces in inhomogeneous ionic fluids

We propose a field-theoretical approach based on the thermodynamic perturbation theory and within it derive a grand thermodynamic potential of the inhomogeneous ionic fluid as a functional of electrostatic potential for an arbitrary reference fluid system. We obtain a modified Poisson–Boltzmann (PB) equation as the Euler–Lagrange equation for the obtained functional. Applying Noether’s theorem to this functional, we derive a general mean-field expression for the stress tensor consistent with the respective modified PB equation. We derive a general expression for the macroscopic force acting on the dielectric or conductive body immersed in an ionic fluid. In particular, we derive a general mean-field expression for the disjoining pressure of an ionic fluid in a slit pore. We apply the developed formalism to describe three ionic fluid models of practical importance: nonpolarizable models (including the well-known PB and Poisson–Fermi equations), polarizable models (ions carry nonzero permanent dipole or static polarizability), and models of ion-dipole mixtures (including the well-known PB–Langevin equation). For these models, we obtain modified PB equations and respective stress tensors, which could be valuable for different applications, where it is necessary to estimate the macroscopic forces acting on the dielectric or conductive bodies (electrodes, colloids, membranes, etc) together with the local electrostatic potential (field) and ionic concentrations.

Inclusion of solvent polarizability in the modeling of ion-dipole mixtures within the mean spherical approximation (MSA)

The fluctuating polarization model of Høye and Stell [J. Chem. Phys. 72, 1597 (1980); J. Chem. Phys. 73, 461 (1980)] is introduced in the non-primitive mean spherical approximation (MSA) framework for an arbitrary mixture of hard spherical ions and polar polarizable solvent molecules. The thermodynamic properties are derived for this more realistic representation of an aqueous electrolyte. The obtained explicit formulas constitute a thermodynamically consistent set.

Statement of Retraction: Ewald summation for ion–dipole mixture under the dielectric confinement

Statement of Retraction: Ewald summation for ion–dipole mixture under the dielectric confinement

RETRACTED ARTICLE: Ewald summation for ion–dipole mixture under the dielectric confinement

The authors, editors and publisher of Molecular Simulation, have retracted the following article:Jiaxing Yuan (2019) Ewald summation for ion–dipole mixture under the dielectric confinement, Molecular Simulation, DOI: 10.1080/08927022.2019.1698740“This article was published without having recognized all of the contributors to the work. With the agreement of all authors, therefore, it is being retracted.”We have been informed in our decision-making by our policy on publishing ethics and integrity and the COPE guidelines on retractions.The retracted article will remain online to maintain the scholarly record, but it will be digitally watermarked on each page as “Retracted”.

A simulation assessment of the thermodynamics of dense ion-dipole mixtures with polarization.

Molecular dynamics (MD) simulations are employed to ascertain the relative importance of various electrostatic interaction contributions, including induction interactions, to the thermodynamics of dense, hot ion-dipole mixtures. In the absence of polarization, we find that an MD-constrained free energy term accounting for the ion-dipole interactions, combined with well tested ionic and dipolar contributions, yields a simple, fairly accurate free energy form that may be a better option for describing the thermodynamics of such mixtures than the mean spherical approximation (MSA). Polarization contributions induced by the presence of permanent dipoles and ions are found to be additive to a good approximation, simplifying the thermodynamic modeling. We suggest simple free energy corrections that account for these two effects, based in part on standard perturbative treatments and partly on comparisons with MD simulation. Even though the proposed approximations likely need further study, they provide a first quantitative assessment of polarization contributions at high densities and temperatures and may serve as a guide for future modeling efforts.

Effect of polarization on the solubility of gases in molten salts

The solubility of noble gases and water in molten salts is predicted by developing an analytical parameter-free description in terms of polarizable hard sphere (the gas particle) in a medium composed of charged hard spheres of comparable size (the salt). The chemical potential of solute contains contributions from excluded volume, polarization, and dispersion forces. The polarization of the gas particle is calculated explicitly within the framework of the mean spherical approximation for the ion-dipole mixture. An additional contribution originating from the polarization of the salt is proposed. This effect has been overlooked in previous theoretical studies. Its magnitude is estimated in an approximate way. The description is found to give predictions that are in good agreement with experimental solubility data for noble gases and water in molten KCl and RbCl. The results suggest that volume exclusion and salt polarization may constitute the main two opposing factors affecting this phenomenon.

Recent Advances in the Use of MSA Approach to Electrolyte and Charged Colloid Solutions

In this paper, a comprehensive review of recent advances in the use of mean spherical approximation (MSA) to the primitive electrolyte model for ionic fluids, the non-primitive electrolyte model for ion–dipole mixtures, the one-Yukawa EOS for neutral fluids, and the two-Yukawa EOS for charged colloid systems is introduced. A reasonable mathematical derivation and theoretical explanation are proposed why the dielectric constant of the vacuum cannot be used in the long-range electrostatic term in some successful EOSs for real electrolyte solutions, although the dipolar solvent is treated as explicit molecule in these theories. All the studies show that the MSA approach is of significance both in theoretical research and practical use.

Scaling for mixtures of hard ions and dipoles in the mean spherical approximation

Using new scaling parameters βi, we derive simple expressions for the excess thermodynamic properties of the mean spherical approximation (MSA) for the ion-dipole mixture. For the MSA and its extensions we have shown that the thermodynamic excess functions are a function of a reduced set of scaling matrices Γχ. We show now that for factorizable interactions like the hard ion–dipole mixture there is a further reduction to a diagonal matrices βχ. The excess thermodynamic properties are simple functions of these new parameters. For the entropy density we get S=−{k/3π}(F[βα])α∈χ, where F is a simple algebraic functional (such as the cube of the modulus of a vector in function space) of the scaling matrices of irreducible representations χ of the closure of the Ornstein–Zernike equation. The new scaling parameters βi are also simply related to the chemical potentials of the components. The analysis also provides a new definition of the Born solvation energy for arbitrary concentrations of electrolytes.

Comparison of perturbation theory and mean spherical approximation for polar fluids and ion–dipole mixtures based on molecular simulation data

A comprehensive study on various internal energies, pressures and chemical potentials for the pure dipolar hard sphere fluids, pure Stockmayer fluids, the Lennard–Jones and Stockmayer mixtures, the Stockmayer and Stockmayer mixtures and the ion–dipole mixtures is reported based on the perturbation theory (PT) and mean spherical approximation (MSA). Compared with the results of molecular simulations, it is shown that the PT is superior to MSA in most cases.

Description of vapour-liquid phase equilibria of the H2O-NaCl system between 100 and 900°C with a thermodynamic model based on the Mean Spherical Approximation

The Mean Spherical Approximation (MSA) of the ion-dipole mixture is used to describe the vapour-liquid equilibria of the H 2 O-NaCl system. Comparison with experimental data reveals that the H 2 O-NaCl mixture cannot be solely modelled by the MSA ion-dipole model. Discrepancies have been significantly reduced by taking into account NaCl ion pair formation in vapours at all temperatures and in high-temperature liquids (T>387 degrees C). A van der Waals interaction term must be included at lower temperatures (T 2 O-NaCl system.

Monte Carlo simulation of an ion-dipole mixture as a model of an electrical double layer

Canonical Monte Carlo simulations were performed for a nonprimitive model of an electrical double layer. The ions and the solvent molecules are modeled as charged and dipolar hard spheres, respectively, while the electrode as a hard, impenetrable wall carrying uniform surface charge. We found that the ion-dipole model gives a reasonable description of the double layer for partially charged ions with small to moderate dipole moments, or equivalently for an “effective” dielectric constant. Density, polarization and mean electrostatic potential profiles are reported. Strong layering structure, and at higher charges, charge inversion in the second layer were found. With appropriate choices of charge and solvent parameters, states corresponding to the primitive or the solvent primitive model can be produced, and the results agreed well with literature data. At higher effective charges and dipole moments, the dipolar solvent has difficulties in preventing the ions from clustering. More realistic models of water and other solvents are necessary to study the double layer.

A global phase diagram for the ion-dipole model in the mean spherical approximation

This work is a theoretical study of the phase relations of the ion–dipole mixture, as estimated by the mean spherical approximation. Phase diagrams have been calculated for different values of the model parameters (ion charge, dipole moment, dipole, and ion size) and classified according to the nomenclature of van Konynenburg and Scott. The different types of topologies have been mapped onto a global phase diagram in the space of only two dimensionless parameters, denoted respectively α1 and α2. The α1 parameter measures the ratio of force fields produced by dipoles and ions, whereas the α2 parameter is the ratio of the dipole diameter to the ion diameter. For mixtures of equal-sized ions and dipoles the following sequence of phase topology has been recognized with increasing α1 parameter; type III, type IIIm, type IV, type II, type II with double azeotropy, type IV with double azeotropy, and type III-HA. More complex topologies are obtained for α2 values below and above 1 (gas–gas equilibria, heteroazeotropy, azeotropy). It is shown that the global phase diagram provides insights on the phase topology of water–salt and water–salt–gas systems.

Ionic fluids. Structure and thermodynamics of ion-dipole mixtures

Abstract

The contribution of dipole alignment of the force between macroions in an ion-dipole fluid

The mean spherical approximation integral equation is solved for a pair of large charged hard spheres in an ion-dipole mixture. In this work, the effect of the molecular nature of the solvent is taken into account approximately. Extra terms, not included in the DLVO theory, are found and are significant. The effect of dipole alignment near the macroions is repulsive at large separation but can be attractive at short separations.

Improved Monte Carlo simulations of the structure of ion-dipole mixtures

Monte Carlo simulations of ion-diople mixtures are performed for a few state points for which results reported earlier were suspected to be unreliable. Some extended long runs are made with a larger cut-off radius to ensure well-equilibrated systems. Comparison with previous simulation data and theories is made. The results show that some data reoprted earlier are inaccurate and there is a beter agreement than supposed between the Monte Carlo simulations and restricted hypernetted-chain theory.

Application of the mean spherical approximation to describe the Gibbs solvation energies of monovalent monoatomic ions in polar solvents

Abstract : The mean spherical approximation for the ion-dipole mixture is used to study the solvation energies of monovalent monoatomic ions in polar solvents. From the structure of the analytical solution it is inferred that there are two parameters that should be used to describe the solvation: the dielectric constant and a polarization parameter which plays the role of a mean field parameter to account for non-sphericity, chemical interactions, and other effects related to failure of the hard sphere model for the system. The model was successfully fitted to data for the Gibbs solvation energy of five alkali metal cations and their halide ions in 17 different solvents including water. On the basis of this analysis, it was shown that a single value of the polarization parameter describes the data for the cations in each solvent, a quite different value being appropriate for the anions. The polarization parameters were found to be linearly correlated with empirical parameters characterizing the solvents basicity in the case of cations, and its acidity in the case of anions.

A mean field analysis of an ion—dipole mixture against a charged hard wall with specific dipole adsorption: classification of differential capacity plots

A classification is given of some differential capacity curves based upon a mean field analysis of a hard sphere ion—dipole mixture adjacent to a uniformly charged plane hard wall with specific dipole adsorption.

A mean field analysis of an ion–dipole mixture against a charged hard wall with specific adsorption. Part 2.—Non-linear results

A study is made of a hard-sphere ion–dipole mixture against a charged hard wall with specific adsorption of the dipoles modelled by Baxter's sticky potential. The theory is evaluated in the mean field approximation and a comparison made with the experimental differential capacitance results of the system Hg/LiAsF6 with solvent tetrahydrofuran. An explanation is given of the differential capacitance results over the experimental range of electrolyte concentrations and surface charge. The variation of the differential capacitance with temperature is also given. With specific dipole adsorption a classification of differential capacitance shapes can be given on the basis of the relative sizes of the molecular radii.

Monte Carlo simulation of an ion-dipole mixture

We demonstrate on a simple model of an ion-dipole mixture that the convergence of a Monte Carlo simulation at low ionic concentration may be quite slow. For a one molar concentration 105 trial moves per particle are needed for a 864 particle system to obtain a precise estimate of both the ion-dipole and ion-ion energies and the static pair correlation functions. Our results show that recently published Monte Carlo results (1989, Molec. Phys., 66, 299) on a similar system are far from being converged and question the conclusion drawn in this publication on the failure of the reference hypernetted chain equation.

Mean field analysis of an ion-dipole mixture next to a plane charged wall

Mean field analysis of an ion-dipole mixture next to a plane charged wall