M. Molero

Symmetrical Poisson–Boltzmann and modified Poisson–Boltzmann theories

Symmetric pair and triplet ionic correlation functions in the electrolyte bulk are derived using Kirkwoods charging process. Four closures at the Poisson–Boltzmann level of approximation are considered for the pair distribution function and comparisons made with simulation thermodynamic properties for the following primitive model electrolytes: (i) 1 : 1 and 2 : 2 valencies with equal and unequal ion radii, (ii) 1 : 3 valency with equal ion radii. The modified Poisson–Boltzmann theory is extended to treat electrolytes with both unequal radii and/or unsymmetrical valencies.

Effective interaction between colloidal particles using a symmetric Poisson—Boltzmann theory

Abstract A symmetric Poisson—Boltzmann theory is used to study the effective interaction between colloidal particles in a binary aqueous electrolyte. The conditions under which this effective interaction can become weakly attractive are investigated with respect to variations in the valence and radius of both the ions and colloidal particles.

Primitive model electrolytes in the modified Poisson–Boltzmann theory

A symmetric formulation of the modified Poisson–Boltzmann theory is applied to study the thermodynamics and structure of single primitive model electrolytes. Comparisons are made with Monte Carlo simulations for 1 : 1, 2 : 1, 3 : 1 and 2 : 2 electrolytes. Good agreement is found with the thermodynamic properties using the coupling parameter technique. The structural results are generally very good, with discrepancies occurring only at high concentrations for unsymmetrical valences or at high concentrations for large variations in ion size. The results are comparable in accuracy to those of the hypernetted chain (HNC) theory.

The primitive model of ionic fluids near its critical point in the Poisson–Boltzmann and modified Poisson–Boltzmann theories

The Poisson–Boltzmann (PB) and modified Poisson–Boltzmann (MPB) theories are used to investigate the primitive model of ionic fluids in the low density–large coupling regime where the liquid–vapor transition is situated. The PB and MPB spinodal curves for the restricted primitive model are calculated from the virial route and compared with those from the mean spherical approximation (energy route) and the hybrid hypernetted‐chain/mean spherical approximation (virial route). The effect of unequal ion sizes on the critical point and spinodal curves is also considered.

Individual ionic activity coefficients from a symmetric Poisson–Boltzmann theory

A symmetric Poisson–Boltzmann equation is used to calculate individual activity coefficients for 1 : 1 and 2 : 1 primitive model electrolytes. Comparisons are made with the mean-spherical approximation and hypernetted chain integral equation predictions and available Monte Carlo simulation results. A brief comparison is also made with the mean-spherical approximation and Monte Carlo results for the aqueous mixture KCl–KF.

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

Abstract 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.

Estimation of activity coefficients at different temperatures by using the mean spherical approximation

A method of estimating activity coefficients for a variety of electrolytes at different temperatures is presented. The MSA approximation is used to calculate the activity coefficients from experimental data available in the literature. This strategy provides suitable results within a wide range of temperatures, electrolyte stoichiometries and concentrations of investigated solutions.

Reorientation of Thiols during 2D Self-Assembly: Interplay between Steric and Energetic Factors

Reorientation of thiols during their 2D self-assembly is well established; however, little is known about its energetics and the factors that control its onset. We have developed a new strategy to determine the critical reorientational surface concentration (crsc) of thiols at the substrate/solution interface, which makes use of a cathodic stripping protocol. Its application to distinct homologous series of alkylthiols shows that the magnitude of the crsc and its variation with the molecular size is strongly dependent on the nature of the terminal group. Methyl-terminated alkylthiols reorient close to the saturation coverage of the lying-down phase, thus following their molecular size trend; whereas reorientation of alkylthiols bearing a negatively charged end group starts well below the monolayer coverage of the lying-down phase, with its onset being almost independent of the molecular size. Hydroxy-terminated alkylthiols show an intermediate behavior. A theoretical approach is developed to determine the reorientation equilibrium constant from the crsc value. The standard free energy of reorientation has been found to vary linearly with the alkyl chain length, and to increase upon replacing the terminal methyl group by a negatively charged one. A quantitative correlation between the reorientation equilibrium constant and the hydrophobicity of the molecule has been established. Overall, these findings have allowed us to disentangle the role of steric and energetic factors in the onset of the reorientation process of alkylthiols, demonstrating that their interplay can be finely tuned by varying either the alkyl chain length or the nature of the terminal group.

Influence of temperature on the reduction kinetics of Zn2+ at a mercury electrode

The reduction of Zn 2 at a mercury electrode has been studied by the a.c. impedance technique in the 263/318 K temperature range. Kinetic data have been analyzed in terms of a mechanism consisting of two consecutive charge transfer steps. The apparent activation enthalpies and charge transfer coefficients of both steps are found to be temperature independent. Differential capacity curves and potentials of zero charge of the NaClO4 supporting electrolyte solutions were measured at each temperature of interest. Double layer effects on the Zn 2 reduction rate constants were analyzed within the framework of the unequal distances of closest approach theory, and the corrected rate constants were found to be independent of supporting electrolyte concentration when allowance for changes in the Zn 2 activity coefficient was made. The implications of the activation parameter values on the nature of the rate determining steps are also discussed. # 2002 Elsevier Science B.V. All rights reserved.

Keto-enol tautomerism in the electrochemical reduction of parabanic acid

Abstract Convolution potential sweep voltammetry has been applied to the electrochemical reduction of parabanic acid in the 10–100 V/s scan rate range. The overall reduction corresponds to a quasireversible two-electron, two-proton transfer to give an enediol intermediate compound. This compound undergoes an enol-keto transformation in a following homogeneous chemical reaction. This chemical reaction appears to be acid-base catalyzed in the 2 M-4×10 −3 M H + concentration range. The catalytic and electrochemical constants were determined.