Dezsö Boda

Monte Carlo, density functional theory, and Poisson-Boltzmann theory study of the structure of an electrolyte near an electrode

Monte Carlo (MC) and density functional theory (DFT) results are reported for an electrolyte, consisting of charged hard spheres of diameter 3 A with the solvent modeled as a dielectric continuum, near a charged flat uniformly charged electrode. These results are more interesting than the earlier MC results of Torrie and Valleau [J. Chem. Phys. 73, 5807 (1980); J. Phys. Chem. 86, 3251 (1982)] for 4.25 A spheres because the popular Gouy–Chapman (GC) theory is less successful for this system. The DFT results are in good agreement with the MC results. Both the MC and DFT results show particularly interesting features when the counterions are divalent. For such divalent counterions, the diffuse layer potential passes through a maximum magnitude, then declines, and ultimately has a sign that is opposite to that of the electrode charge. The consequences of this behavior are discussed. In contrast, the well-known GC theory consistently overestimates the magnitude of the diffuse layer potential, does not have any...

On the influence of ionic association on the capacitance of an electrical double layer

The concept of ionic association is applied to give an explanation of the anomalous temperature dependence of the capacitance of the electrical double layer. It is shown that, except in the intermediate region where the temperature derivative of the capacitance changes sign, a Bjerrum-like correction of the mean spherical approximation, with Ebelings expression for the association constant, satisfactorily reproduces the computer simulation data at high and low temperatures. Including polarization effects that arise from ionic pairs improves the description in this region.

Monte Carlo study of the selectivity of calcium channels: improved geometrical model

An extended model of a calcium channel is described involving a channel with a finite length. With this new geometry the channel is still selective but less so than in an infinite cylinder geometry (Boda, D., Busath, D. D., Henderson, D., and Sokolowski, S., 2000, J. phys. Chem. B, 104, 8903). The selectivity of the channel depends on the width and length of the channel filter but is not significantly affected by changes in the size of the entry vestibules. Interestingly, changes in the size of the entry vestibules do affect the details of the concentration profiles of some of the ions.

The capacitance of the solvent primitive model double layer at low effective temperatures

The effect of the solvent, modeled as a hard sphere fluid, on the behavior of the double layer at low temperatures is studied by means of Monte Carlo simulations. We find that the density profiles are strongly affected by the addition of solvent molecules. The profiles pass from near monotonic functions that exhibit partial drying to oscillatory functions that exhibit partial wetting as the bulk density of the solvent is increased in the system. However, the capacitance, as in the case of the primitive model, increases at low effective temperatures, while after a maximum it decreases at higher effective temperatures as the temperature increases. We find that the adsorption of the ions decreases and changes sign from positive to negative with the decreasing temperature at low electrode charges. Some of these features can be explained by means of a contact value theorem.

Monte Carlo simulation of the electric double layer : dielectric boundaries and the effects of induced charge

To model the double layer near an electrode, theories and simulations must include the different dielectric coefficients of the electrode, the commonly-postulated ‘inner’ layer, and the electrolyte. Recently, Boda et al. [D. Boda, D. Henderson, K.-Y. Chan, D.T. Wasan. Phys. Rev. E, 69, 046702, (2004)] developed a technique to include inhomogeneous dielectric coefficients in arbitrary geometries in a simulation. Here, Monte Carlo simulation results based on this method are reported for the density profiles of 1:1, 2:2 and 2:1 aqueous electrolytes. The simulations include two dielectric boundaries, one from an inner layer of low dielectric coefficient and one from an uncharged metal electrode. In addition, an extension of a Poisson–Boltzmann (PB) type theory due to Onsager and Samara [L. Onsager, N.N.T. Samara. J. chem. Phys., 2, 528, (1934)] is developed and compared with our simulation results. This approach works best for 1:1 salts at low concentrations.

Monte Carlo and Density Functional Theory Study of the Electrical Double Layer: The Dependence of the Charge/Voltage Relation on the Diameter of the Ions

According to the well known Gouy–Chapman theory (GCT) for ions modeled as charged hard spheres that are of equal diameter, the diffuse layer potential (DLP), the potential at the distance of closest approach, is independent of the ionic diameter. In this paper, the question of the dependence of the diffuse layer DLP on the ion diameter is examined by means of a Monte Carlo (MC) simulation for three values of the diameter, d=2, 3 and 4.25 Å, for three ion concentrations, 0.1, 1 and 2 M, for the cases of 1:1 and 2:1 salts. For 1:1 salts at the lower concentrations, the dependence of the MC DLP on d is found to be rather weak. Thus, for these salts, the GCT is reasonably successful on this point even though the actual GCT values for the DLP are less satisfactory. For 1:1 salts, density functional theory (DFT), which is generally more accurate than the GCT, gives a dependence of the DLP on d that, at large electrode charge, is too great. The MC results for 2:1 salts show an appreciable dependence of the DLP on d at higher concentrations, especially when the divalent ions are the counterions. For these salts the GCT fails to describe both the actual values of the DLP and its d dependence; here DFT is more successful. Interestingly, at high concentrations, for 2:1 salts the MC and DFT (but not the GCT) values of the DLP can have the opposite sign to that of the electrode charge.

Thermodynamics and structural properties of the dipolar Yukawa fluid

We report computer simulations and a theoretical study of the thermodynamics and structure of a dipolar Yukawa system. A comparison between the analytical mean spherical approximation (MSA) solution, perturbation theory and Monte Carlo simulation data of pressure, internal energy and dielectric constant is given. In the perturbation theory, the MSA solution of hard core Yukawa fluid is used as a reference system. It was found that the MSA solution is reasonable only at lower dipole moments, while the perturbation theory gives good results at low and high values of dipole moment. Liquid–vapor coexistence data of dipolar Yukawa fluid are also obtained by Monte Carlo simulation and by both MSA and perturbation theory. It was found that at high dipole moments the liquid–vapor equilibrium disappears while chain-like structures appear in the low density fluid phase. The appearance of chain-like structures of dipolar Yukawa fluid is discussed in comparison with the Stockmayer fluid.

The application of density functional theory and the generalized mean spherical approximation to double layers containing strongly coupled ions

Density functional theory (DFT) is the most successful simple theory for ions near an electrode (the double layer). However, most previous applications of DFT have been for ions that are relatively weakly coupled. Interesting effects have been found in simulations for ions that are strongly coupled. Specifically, drying of the electrode with a resultant large increase in the magnitude of the adsorption is observed. Further, the capacitance decreases with increasing coupling. The DFT formalism requires the direct correlation function of the bulk electrolyte as input. If the bulk electrolyte is treated by means of the mean spherical approximation (MSA), DFT fails to account for these phenomena. However, if the bulk electrolyte is treated by means of a generalized MSA, partial success results. The electrolyte dries the electrode but the lowering of the capacitance is predicted only weakly. Further refinements are necessary for full success.

Simulation and density functional study of a simple membrane. II. Solvent effects using the solvent primitive model

The simple membrane, supporting charge densities σ1 and σ2=−σ1 on its inner and outer surfaces that we studied previously is extended to include solvent molecules, modeled as hard spheres. In addition to the electrostatic potential, the membrane can interact with the surrounding electrolyte by a short-range van der Waals-type potential that can be attractive or repulsive. The fluid beyond the outer surface is four component electrolyte consisting of the hard sphere solvent, two species of cations, and one species of anions. The membrane is impermeable to one of the cation species so that the fluid in the membrane and beyond the inner surface is a three component electrolyte. Monte Carlo simulations and density functional theory are used to study the density profiles of the electrolyte and the charge-electrostatic potential relationship for the membrane surfaces. The presence of the solvent molecules leads to oscillatory profiles. The potential versus charge relationship is strongly influenced by the solve...

Phase separation in fluid additive hard sphere mixtures

Some years ago, Lebowitz and Rowlinson ‘established’ that phase separation did not occur in fluid mixtures of additive hard spheres. Their analysis was based on the thermodynamic functions of this mixture, obtained from the solution of the Percus-Yevick (PY) equation. At the time, these results were thought to be accurate for all states for which additive hard sphere mixtures were fluid. Recent theoretical work and simulations have shown that the PY radial distribution function between a pair of large spheres is seriously in error when the ratio of the diameters of the large and small spheres becomes large and when the concentration of the large spheres is very small. Thus, a reexamination of the question of phase stability in fluid additive hard sphere mixtures is worthwhile. Indeed, it is found that phase separation is probable in fluid additive hard sphere mixtures when the concentration of the large particles is very dilute. Generally, the coexisting phase is solid. However, under some circumstances, ...