Boltzmann distribution, solution potential-determining ions

Figure 2.13 illustrates what is currently a widely accepted model of the electrode-solution interphase. This model has evolved from simpler models, which first considered the interphase as a simple capacitor (Helmholtz), then as a Boltzmann distribution of ions (Gouy-Chapman). The electrode is covered by a sheath of oriented solvent molecules (water molecules are illustrated). Adsorbed anions or molecules, A, contact the electrode directly and are not fully solvated. The plane that passes through the center of these molecules is called the inner Helmholtz plane (IHP). Such molecules or ions are said to be specifically adsorbed or contact adsorbed. The molecules in the next layer carry their primary (hydration) shell and are separated from the electrode by the monolayer of oriented solvent (water) molecules adsorbed on the electrode. The plane passing through the center of these solvated molecules or ions is referred to as the outer Helmholtz plane (OHP). Beyond the compact layer defined by the OHP is a Boltzmann distribution of ions determined by electrostatic interaction between the ions and the potential at the OHP and the random jostling of ions and... 

Here s(r) is the dielectric constant. We use s(r) to emphasize that the constant is a function of position for example, it is different inside a solvent and inside a solute. The electrostatic potential to be determined and p(r) is the charge distribution of the solute. Equation (25) is an exact equation of the electrostatic potential in a dielectric. If the solvent contains some dissolved salt (i.e., the concentration of positive and negative charges is the same) and we assume that the distribution of salt ions follows a Boltzmann distribution, then the electrostatic potential of the system is described by the Poisson-Boltzmann equation... 

The ion and electrical potential distributions in the electrical double layer can be determined by solving the Poisson-Boltzmann equation [2,3]. According to the theory of electrostatics, the relationship between the eleetrieal potential ij/ and the local net charge density per unit volume at any point in the solution is deseribed by the Poisson equation ... 

The use of molecular dynamics to study the electric double-layer structure started a little over a decade ago, with the hope of determining more accurate structures because the classical description of an electric double layer based on the Poisson-Boltzmann equation is accurate only for low surface potential and dilute electrolytes. The Poisson-Boltzmann equation only considers the electrostatic interactions between the charged surface and ions in the solution, but not the ion-ion interactions in the solution and the finite molecule size, which can be taken into account in molecular dynamics simulations. It was shown [6, 7] that the ion distribution in the near-wall region could be significantly different from the prediction of classical theory. Typical molecular dynamics simulation results of counterion and co-ion concentrations in a nanochannel are shown in Fig. 2a. The ion distribution obtained... 

Later, in a model where each cylindrical polymer rod was confined to a concentric, cylindrical, electroneutral shell whose volume represents the mean volume available to the macromolecule, the concept was extended to the macroscopic system itself which was considered as an assembly of electroneutral shells at whose periphery the gradient of potential goes to zero and the potential itself has a constant value. Closed analytical expressions which represent exact solutions of the Poisson-Boltzmann equation can be given for the infinite cylinder model. These solutions, moreover, were seen to describe the essence of the problem. The potential field close in to the chain was found to be the determining factor and under most practical circumstances a sizable fraction of the counter-ions was trapped and held closely paired to the chain, in the Bjerrum sense, by the potential. The counter-ions thus behave as though distributed between two phases, a condensed phase near in and a free phase further out. The fraction which is free behaves as though subject to the Debye-Hiickel potential in the ordinary way, the fraction condensed as though bound . 

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