Double layer (surface science)

Summary

In surface science, a double layer (DL, also called an electrical double layer, EDL) is a structure that appears on the surface of an object when it is exposed to a fluid. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The DL refers to two parallel layers of charge surrounding the object. The first layer, the surface charge (either positive or negative), consists of ions which are adsorbed onto the object due to chemical interactions. The second layer is composed of ions attracted to the surface charge via the Coulomb force, electrically screening the first layer. This second layer is loosely associated with the object. It is made of free ions that move in the fluid under the influence of electric attraction and thermal motion rather than being firmly anchored. It is thus called the "diffuse layer". Interfacial DLs are most apparent in systems with a large surface-area-to-volume ratio, such as a colloid or porous bodies with particles or por

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Related publications (49)

Trace concentration - Huge impact: Nitrate in the calcite/Eu(III) system

Moritz Schmidt, Kislon Voitchovsky

The interactions of trivalent lanthanides and actinides with secondary mineral phases such as calcite is of high importance for the safety assessment of deep geological repositories for high level nuclear waste (HLW). Due to similar ionic radii, calcium-bearing mineral phases are suitable host minerals for Ln(III) and An(III) ions. Especially calcite has been proven to retain these metal ions effectively by both surface complexation and bulk incorporation. Since anionic ligands (e.g., nitrate) are omnipresent in the geological environment and due to their coordinating properties, their influence on retentive processes should not be underestimated. Nitrate is a common contaminant in most HLW forms as a result of using nitric acid in fuel reprocessing. It is also formed by microbial activity under aerobic conditions. In this study, atomic force microscopy investigations revealed a major influence of nitrate upon the surface of calcite crystals. NaNO3 causes serious modifications even in trace amounts (

Elsevier2014

Interaction forces between silica surfaces with adsorbed poly (amidoamine) (PAMAM) dendrimers were measured with the colloidal probe technique based on the atomic force microscope (AFM). At small separations, attractive non-DLVO forces are observed. These forces likely originate from interactions between oppositely charged patches on the surfaces, resulting from the heterogeneous distribution of the adsorbed positively charged dendrimers on the negatively charged surface. At large separation distances, the interaction forces are repulsive and consistent with DLVO theory. These forces can be interpreted quantitatively as originating from the overlap of diffuse layers of positively charged surfaces in terms of the Poisson-Boltzmann theory. From the resulting diffuse layer potentials, one can extract the effective charge of the dendrimers in the adsorbed state. This effective charge is about half of the ion condensation value given by Poisson-Boltzmann theory for a charged spherical macromolecule in solution for the different dendrimer generations investigated. This reduction probably originates from the smaller volume available for the diffuse layer of adsorbed macromolecules, but charge neutralization may also play an essential role.

American Chemical Society2009

Interaction forces between ionizable surfaces across an electrolyte solution on the Poisson-Boltzmann level are discussed within the constant regulation approximation. The chemical response of each surface is expressed in terms of two parameters, namely, the diffuse layer potential and the regulation parameter p. Both parameters are easily available because they arise naturally within classical equilibrium models for a single noninteracting surface. This approximation, thus, eliminates the need to treat the more intricate problem of two chemical adsorption equilibria coupled to the overlapping double layers between the surfaces. The ensuing simplicity makes this approach extremely versatile for the analysis of experimental data. The classical boundary condition of constant potential corresponds to p = 0, and that of constant charge corresponds to p = 1. While this approximation is rigorously correct at large separations, we find that it remains excellent down to contact in many realistic situations, such as in symmetric or asymmetric systems involving metal oxides or silica described by the 1-pK basic Stern model.

2004

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