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Ionic size effects: generalized Boltzmann distributions, counterion stratification and modified Debye length. (English) Zbl 1286.35233

Authors’ abstract: Near a charged surface, counterions of different valences and sizes cluster; and their concentration profiles stratify. At a distance from such a surface larger than the Debye length, the electric field is screened by counterions. Both recent studies using a variational mean-field approach that includes ionic size effects and Monte Carlo simulations suggest that counterion stratification is determined by the ionic valence-to-volume ratios. Central in the mean-field approach is a free-energy functional of ionic concentrations in which the ionic size effects are included through the entropic effect of solvent molecules. The corresponding equilibrium conditions define the generalized Boltzmann distributions relating the ionic concentrations to the electrostatic potential. This paper presents a detailed analysis and numerical calculations for such a free-energy functional to understand the dependence of the ionic charge density on the electrostatic potential through the generalized Boltzmann distributions, the role of ionic valence-to-volume ratios in the counterion stratification and the modification of Debye length due to the effect of ionic sizes.

MSC:

35Q60 PDEs in connection with optics and electromagnetic theory
82B80 Numerical methods in equilibrium statistical mechanics (MSC2010)
65C05 Monte Carlo methods
78A30 Electro- and magnetostatics
57N80 Stratifications in topological manifolds
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