Lecture

Electric Double Layer: Helmholtz Model

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In course

CH-341: Physical chemistry of interfaces

Acquire an understanding of interfacial phenomena, micro-heterogeneous colloidal solution systems and dynamic electrochemistry. ...

Description

This lecture covers the concept of the electric double layer (DL) on charged surfaces in solution, focusing on the Helmholtz model. It explains how surfaces become charged in solutions, attracting counterions to form the DL. The lecture discusses the assumptions and limitations of the Helmholtz model, neglecting ion diffusion. It also introduces the Gouy-Chapman model, which considers ion thermal motion, leading to a diffuse DL. The Stern model, combining the Helmholtz and diffuse layers, is presented, along with the concept of the Outer Helmholtz Plane. Various calculations and examples related to the DL, Debye length, and surface potential are discussed, emphasizing their importance in understanding electrokinetic phenomena.

Instructor

Rainer Beck

Professor titulaire EPFL, 2006 Privat Docent EPFL, 1997 Habilitation and venia legendi, Universität Karlsruhe, 1996 Ph.D. in Physical Chemistry, Oregon State University, 1990 Degree in Physics, Universität Stuttgart, 1985

Official source

Ontological neighbourhood

Chemistry

Physical chemistry: Electrochemistry

Related concepts (32)

Double layer (surface science)

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.

Surface charge

A surface charge is an electric charge present on a two-dimensional surface. These electric charges are constrained on this 2-D surface, and surface charge density, measured in coulombs per square meter (C•m−2), is used to describe the charge distribution on the surface. The electric potential is continuous across a surface charge and the electric field is discontinuous, but not infinite; this is unless the surface charge consists of a dipole layer. In comparison, the potential and electric field both diverge at any point charge or linear charge.

Electrokinetic phenomena

Electrokinetic phenomena are a family of several different effects that occur in heterogeneous fluids, or in porous bodies filled with fluid, or in a fast flow over a flat surface. The term heterogeneous here means a fluid containing particles. Particles can be solid, liquid or gas bubbles with sizes on the scale of a micrometer or nanometer. There is a common source of all these effects—the so-called interfacial 'double layer' of charges. Influence of an external force on the diffuse layer generates tangential motion of a fluid with respect to an adjacent charged surface.

Debye length

In plasmas and electrolytes, the Debye length (Debye radius or Debye–Hückel screening length), is a measure of a charge carrier's net electrostatic effect in a solution and how far its electrostatic effect persists. With each Debye length the charges are increasingly electrically screened and the electric potential decreases in magnitude by 1/e. A Debye sphere is a volume whose radius is the Debye length. Debye length is an important parameter in plasma physics, electrolytes, and colloids (DLVO theory).

Zeta potential

Zeta potential is the electrical potential at the slipping plane. This plane is the interface which separates mobile fluid from fluid that remains attached to the surface. Zeta potential is a scientific term for electrokinetic potential in colloidal dispersions. In the colloidal chemistry literature, it is usually denoted using the Greek letter zeta (ζ), hence ζ-potential. The usual units are volts (V) or, more commonly, millivolts (mV).

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