Electrolytes at charged interfaces: Pair integral equation approximations for model 2–2 electrolytes (original) (raw)
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Electrolytes at charged interfaces: Integral equation theory for 2–2 and 1–1 model electrolytes
The Journal of Chemical Physics, 1995
The structure and thermodynamics of both 2{2 and 1{1 model electrolytes at a charged interface have been determined. The solvent is modeled as a structureless dielectric continuum. The structure is calculated from the`singlet' version of the Ornstein-Zernike integral equation, using as input the structure of the bulk electrolyte from a recent integral equation theory. The approximation in the theory is the wall-ion bridge function, which is investigated for two levels of approximation. Surface thermodynamic quantities calculated from this structural information are compared with the classical Gouy-Chapman-Stern approximation for the interfacial region, computer simulations, and selected experimental data. Higher order structure predicted by the integral equations indicates that caution should be used when interpreting results of the classical approximation.
The Journal of Chemical Physics, 2002
The structure of 3:3 and 1:3 electrolyte solutions at various concentrations and several cation/anion size ratios has been analyzed in terms of triplet and pair correlation functions, by means of simulation and a triplet integral equation theory derived from the inhomogeneous Ornstein-Zernike equation. The interaction model consists of a truncated and shifted Coulomb plus the Ramanathan-Friedman repulsive core. Concentration and size and charge asymmetry are found to induce changes in the triplet structure beyond those predicted by the simple superposition approximation, which are, however, correctly reproduced by the triplet integral equation.
Ion pairing in model electrolytes: A study via three-particle correlation functions
The Journal of Chemical Physics, 2003
A novel integral equations approach is applied for studying ion pairing in the restricted primitive model (RPM) electrolyte, i. e., the three point extension (TPE) to the Ornstein-Zernike integral equations. In the TPE approach, the three-particle correlation functions g [3] (r 1 , r 2 , r 3 ) are obtained. The TPE results are compared to molecular dynamics (MD) simulations and other theories. Good agreement between TPE and MD is observed for a wide range of parameters, particularly where standard integral equations theories fail, i. e., low salt concentration and high ionic valence. Our results support the formation of ion pairs and aligned ion complexes.
Pure and Applied Chemistry, 2000
A survey is given on our attempts to calculate equilibrium properties of molecular liquids (pure solvents and electrolyte solutions) with the help of spatial pair correlation functions, starting from classical molecular pair interactions. The selection of potential models, especially the influence of molecular polarizability, is discussed as well as the limitations of the different methods of calculation of molecular pair correlation functions (e.g., from molecular and site-site Ornstein-Zernike theories, from MC and from MD simulations). We have performed simulations and integral equation calculations for spatial distribution functions in pure solvents with very low dielectric constants as dioxane and tetrahydrofurane, up to solvents with a very high dielectric constant like
The Journal of Physical Chemistry B, 2012
Most theoretical studies of an electrical double layer, which is formed by an electrolyte in contact with a charged electrode, employ a primitive model in which the solvent is represented by a dielectric continuum. This implicit-solvent model is convenient because computations are comparatively simple. However, it suppresses oscillations in the density profiles of ionic species that result from the discreteness of the solvent molecules. Furthermore, the implicit-solvent model yields poor results for the capacitance. In comparison with experiment at fixed electrode charge density, it predicts a too small electrode potential, and the resultant capacitance is too large. This latter discrepancy can be compensated in part by postulating the existence of an often fictitious inner layer whose properties are parametrized to agree best with experiment. The use of an implicit solvent model and an inner layer helps in correlating experimental results but rests on a faulty microscopic picture. Unfortunately, explicit consideration of solvent molecules poses both theoretical and numerical difficulties and, as a result, studies using an explicit solvent model have been few and far between. In this study, we consider a simple nonprimitive or explicit solvent model in which each solvent molecule is represented by a dimer composed of touching positive and negative hard spheres, with a resulting dipole moment that is equal to that of a water molecule, and the ions are represented by charged hard spheres. The density profiles and charge−potential relationship of this model are examined using the classical density functional theory. We find that the introduction of an explicit solvent increases the electrode potential, at fixed electrode charge, without the need to postulate a parametrized inner layer. Because of the solvent polarity, the ion profiles become strong oscillatory and show local charge inversion near a highly charged electrode surface at all ion concentrations.
Ion-specific thermodynamics of multicomponent electrolytes: A hybrid HNC/MD approach
Chemical Physics, 2009
Using effective infinite dilution ion-ion interaction potentials derived from explicit-water molecular dynamics (MD) computer simulations in the hypernetted-chain (HNC) integral equation theory we calculate the liquid structure and thermodynamic properties, namely the activity and osmotic coeffcients of various multicomponent aqueous electrolyte mixtures. The electrolyte structure expressed by the ion-ion radial distribution functions is for most ions in excellent agreement with MD and implicit solvent Monte-Carlo (MC) simulation results. Calculated thermodynamic properties are also represented consistently among these three methods. Our versatile HNC/MD hybrid method allows for a quick prediction of the thermodynamics of multicomponent electrolyte solutions for a wide range of concentrations and an efficient assessment of the validity of the employed MD force-fields with possible implications in the development of thermodynamically consistent parameter sets.
Solid State Ionics, 2019
A simple lattice model of a solid electrolyte presented as a xy-slab geometry system of mobile cations on a background of energetic landscape of the host system and a compensating field of uniformly distributed anions is studied. The system is confined in the z-direction between two oppositely charged walls, which are in parallel to xy-plane. Besides the long-range Coulomb interactions appearing in the system, the short-range attractive potential between cations is considered in our study. We propose the mean field description of this model and extend it by taking into account correlation effects at short distances. Using the free energy minimization at each of z-coordinates, the corresponding set of non-linear equations for the chemical potential is derived. The set of equations was solved numerically with respect to the charge density distribution in order to calculate the cations distribution profile and the electrostatic potential in the system along z-direction under different conditions. An asymmetry of charge distribution profile with respect to the midplane of the system is observed. The effects of the short-range interactions and pair correlations on the charge and electric field distributions are demonstrated.
The Journal of Chemical Physics, 2013
We extend the osmotic ensemble Monte Carlo (OEMC) molecular simulation method (Moucǩa et al. J. Phys Chem. B 2011, 115, 7849−7861) for directly calculating the aqueous solubility of electrolytes and for calculating their chemical potentials as functions of concentration to cases involving electrolyte hydrates and mixed electrolytes, including invariant points involving simultaneous precipitation of several solutes. The method utilizes a particular semigrand canonical ensemble, which performs simulations of the solution at a fixed number of solvent molecules, pressure, temperature, and specified overall electrolyte chemical potential. It avoids calculations for the solid phase, incorporating available solid chemical potential data from thermochemical tables, which are based on well-defined reference states, or from other sources. We apply the method to a range of alkali halides in water and to selected examples involving LiCl monohydrate, mixed electrolyte solutions involving water and hydrochloric acid, and invariant points in these solvents. The method uses several existing force-field models from the literature, and the results are compared with experiment. The calculated results agree qualitatively well with the experimental trends and are of reasonable accuracy. The accuracy of the calculated solubility is highly dependent on the solid chemical potential value and also on the force-field model used. Our results indicate that pairwise additive effective force-field models developed for the solution phase are unlikely to also be good models for the corresponding crystalline solid. We find that, in our OEMC simulations, each ionic force-field model is characterized by a limiting value of the total solution chemical potential and a corresponding aqueous concentration. For higher values of the imposed chemical potential, the solid phase in the simulation grows in size without limit. t of the remaining species are fixed. An example is an aqueous solution of s ions with a fixed
It is known that none of the available simple molecular interaction models of aqueous electrolytes based on SPC/E water and their associated force fields are able to reproduce the concentration dependence of important thermodynamic properties of even the simplest electrolyte, NaCl, at ambient conditions over the entire experimentally accessible concentration range [Moucka, F.; Nezbeda, I.; Smith, W. R. J. Chem. Phys. 2013, 138, 154102]. This paper explores the possibility of improving their performance by incorporating concentration-dependent experimental data for the total ionic chemical potential and the density into the fitting procedure, in addition to experimental values of solubility and solid chemical potential. We describe a general parameter estimation methodology for a studied class of models that incorporates the aforementioned experimental data. When the entire concentration range is considered, although the resulting force field is a slight improvement over others currently available in the literature, overall quantitative agreement with the experimental data over this range remains unsatisfactory. This indicates an inherent limitation of such simple molecular interaction models and strongly suggests that more complex mathematical forms of such models are required to quantitatively predict the properties of aqueous electrolyte solutions when the entire concentration range is of interest. Our parameter estimation methodology is also applicable to such cases.
Revisiting electrolyte thermodynamic models: Insights from molecular simulations
AIChE Journal, 2018
Pitzer and electrolyte non-random two-liquid (eNRTL) models are the two most widely used electrolyte thermodynamic models. For aqueous sodium chloride (NaCl) solution, both models data satisfactorily up to salt saturation concentration, i.e., ionic strength around 6 molal. However, beyond 6 molal, the model extrapolations deviate significantly and diverge from each other. We examine this divergence by calculating the mean ionic activity coefficient over a wide range of concentration based on molecular simulations and Kirkwood-Buff (KB) theory. We show that the asymptotic behavior of the activity coefficient predicted by the eNRTL model is consistent with the molecular simulation results and supersaturation experimental data.