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Ion distribution near a charged wall

The electrostatic interactions between charged objects in solution and their ion
atmosphere play an important role in biological processes and colloidal stability.
The folding of proteins and their biological activity, the compaction of genetic
materials, the adsorption of ions onto lipid membranes, or the self-assembly of
biomolecules are amongst the numerous examples in biology where a theoretical
description of the electrostatic interactions can lead to an improved understanding
of the molecular functions in cells and to a better efficacy of drugs for biomedical applications.

The Poisson-Boltzmann theory has been used for nearly one century to compute the distribution of ions surrounding charged surfaces. This theory considers pointlike ions interacting via their mean field in a continuum dielectric medium. More comprehensive models based on integral equation theories or Monte Carlo simulations have shown several discrepancies with the classical Poisson-Boltzmann approach due to correlation effects arising from the size of ions and from the spatial fluctuations of the electrostatic potential. These models have pointed out the importance of different sizes in the ion-ion interaction for monovalent systems. The composition of the electrical double layer can be altered compared to the classical theory because the increase of the effective excluded volume of ions results in a decrease of the system bulk entropy that favors the tendency of ions to be adsorbed; it can thus lead to a surface overcharging where an apparent charge is adsorbed onto a like-charged wall. Integral equation theories and particle simulations, however, lack the simple physical picture provided by a Poisson-Boltzmann type of approach.

We have therefore devised a generalized Poisson-Fermi formalism applicable to polydisperse systems of multiple ions. The distributions of ions are hence described in terms of bulk volume fractions of each ion species rather than just their bulk concentrations. An arbitrary number of excluded volumes can be taken into account. Size correlation effects, which have not been studied before in the framework of such a simple theory, are exemplified next to a charged wall: underscreening, saturated layer of mixed ions and ion stratification (Figure 1) are reported. At last, we have proposed a self-consistent way to compute the change of the effective dielectric constant as the ion volume fractions vary across the solution.

Figure 1. Spatial repartitions of ions with different valences next to a charged wall. The solution is made of three electrolytes: a 3:1 at 10 µM, a 2:1 at 10 mM, and a 1:1 at 300 mM. The wall has a charge density of σ=-0.4 C.m-2. All the counterions have an excluded volume of 1 nm3 and the coions 0.15 nm3. (A) Ion concentrations given by the generalized Poisson-Fermi equation. The inset is a schematic of the ion stratification next to the wall. (B) Ion concentrations given by the Poisson-Boltzmann theory. The numbers next to the curves on (A) and (B) indicate the ion valence. ci is the ion concentrations and κx the distance from the wall normalized to the Debye length. [Phys. Rev. E 78 (2008) 061506]

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