Electrical Double-Layer Formation


Costas Tsouris Separations and Materials Research Group, Oak Ridge National Laboratory

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When a surface is immersed or created in an aqueous solution, a discontinuity is formed at the interface where such physicochemical variables as electrical potential and electrolyte concentration change significantly from the aqueous phase to another phase. Because of the different chemical potentials between the two phases, charge separation often occurs at the interfacial region. This interfacial region, together with the charged surface, is usually known as the electrical double layer (EDL). This layer, which can extend as far as 100 nm in a very dilute solution to only a few angstroms in a concentrated solution, plays an important role in electrochemistry, colloid science, and surface chemistry. Applications of the electrical double layer can be found in energy-storing devices in the form of capacitors, electrosorption of metal ions, purification of drinking water, nanomaterial processing, and sensor assembly.

The importance of the EDL has led to numerous studies, and many models were proposed in the past. The best known is the Gouy–Chapman model, in which ions are considered as point charges and water is considered as a continuum. The electrical potential distribution and the concentration profiles can be predicted simply by using the Poisson–Boltzmann (P–B) equation. This theory, which has been proven very successful at low electrolyte concentrations and low surface charge densities, has been applied to many EDL problems, such as heavy metal adsorption and ion transport through membrane channels. For higher concentrations of ions and higher surface charge densities, a better description of the EDL can be obtained by using primitive models, in which water is still considered as a continuum but ions are considered as hard spheres with a point charge embedded at their centers. Various theoretical approaches such as molecular dynamics, modified P–B equations, integral equation theories, and Monte Carlo simulations have been employed to study the primitive model. These studies confirm that the primitive model and the Gouy–Chapman model provide similar results for dilute solutions and low surface charge densities. However, the agreement between these two models becomes worse when a high ion concentration or a high surface charge density is encountered, especially if an asymmetric electrolyte is present in the solution. A more realistic EDL model would be the nonprimitive model, in which water is considered as individual molecules instead of a continuum. For example, in the solvent primitive model (the simplest form of a nonprimitive model), water molecules are considered as neutral hard spheres. Because of the steric effect caused by water molecules, the ion concentration profile in the EDL shows strong oscillations, which cannot be observed in the primitive model. In this study, the results of Monte Carlo simulations used to compare the primitive and nonprimitive EDL models with the classical Gouy–Chapman model in 10- and 20-Å slit-type nanopores are reported.