MINISTRY OF EDUCATION AND SCIENCE OF UKRAINEAVIATION UNIVERSITYOF ECOLOGICAL SAFETYOF CHEMISTRY AND CHEMICAL TECHNOLOGY
TASKthe discipline: Physical and colloid chemistry: Double electric layer. Mechanism of formation and theory of structure
Done by student of IES 304Litvinby Maksimuk MR
2014
Content
Introduction
1.Theories of double electrical layer structure
1.1Helmholtz Theory
.2Gouy-Chapman Theory
1.3Stern Theory
1.4Grahame Theory
.5Bockris/Devanthan/Mьller Theory
.6Trasatti/Buzzanca Theory
.7Conway Theory
.8Marcus Theory
1.9Modern Theory of Electrical Double Layer
.Mathematical description
3.Methods of study
References
Introduction
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), comprises ions 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 [1].
1. Schematic of double layer in a liquid at contact with a negatively-charged solid. Depending on the nature of the solid, there may be another double layer (unmarked on the drawing) inside the solid
DL is most apparent in systems with a large surface area to volume ratio, such as colloid or porous bodies with particles or pores (respectively) on the scale of micrometres to nanometres. However, DL is important to other phenomena, such as the electrochemical behavior of electrodes.DL plays a fundamental role in many everyday substances. For instance, milk exists only because fat droplets are covered with a DL that prevent their coagulation into butter. DLs exist in practically all heterogeneous fluid-based systems, such as blood, paint, ink and ceramic and cement slurry.DL is closely related to electrokinetic phenomena and electroacoustic phenomena.
1. Theories of double electrical layer structure
1.1 Helmholtz theory
First quantitative theory of the electric double layer was developed by Helmholtz in 1879. At that time the existence of ions in solution was not known and Helmholtz consider double layer as a capacitor, outer armature of which is positioned in liquid parallel to the surface at a distance of the molecular order from it [2] .an electronic conductor is brought in contact with a solid or liquid ionic conductor (electrolyte), a common boundary (interface) among the two phases appears. Hermann von Helmholtz was the first to realize that charged electrodes immersed in electrolytic solutions repel the coions of the charge while attracting counterions to their surfaces. Two layers of opposite polarity form at the interface between electrode and electrolyte. 1853 he showed that an electrical double layer (DL), that is essentially a molecular dielectric, stored charge electrostatically. Below the electrolyte's decomposition voltage the stored charge is linearly dependent on the voltage applied.illustration of the potential development in the area and in the further course of a Helmholtz double layer is shown on Figure 1.1.
1.1. Simplified illustration of the potential development in the area and in the further course of a Helmholtz double layer
This early model predicted a constant differential capacitance independent from the charge density depending on the dielectric constant of the electrolyte solvent and the thickness of the double-layer. model, while a good foundation for the description of the interface, does not consider important factors including diffusion/mixing of ions in solution, the possibility of adsorption onto the surface and the interaction between solvent dipole moments and the electrode [1].
1.2 Gouy-Chapman theory
Louis Georges Gouy in 1 910 and David Leonard Chapman in 1 91...
