The electrical double layer theory deals with the distribution of ions that occurs at the interface between a charged solid surface (such as a colloid) in contact with an aqueous electrolyte solution. The theory proposes that ions of the opposite charge are attracted to the solid surface (known as the potential determining ions), forming a tightly bound layer, giving the surface a net positive or negative charge. Nearby, counter ions of the opposite charge form a diffuse layer in the solution. The difference in electric potential between the charged surface and the neutral point in the diffuse layer is known as the Nernst potential. The electrical double layer consists of a fixed Stern layer near the surface and a diffuse layer extending into the solution.

1. Electrical double layer of colloids
Colloids:
Colloids, a mixture in which one substance is divided into minute particles and
dispersed throughout a second substance. The dispersed system consists of particulate
matter, known as the dispersed medium. The mixture is also called a Colloidal
system, colloidal solution or colloidal dispersion.
Introduction:
 Helmholtz (1879) explained qualitatively the electrical properties of colloids on
the concept of electrical double layer at solid-liquid interface.
 All the molecules or particles carry out electric charge due to their properties.
 Study of reaction in which charged particles cross the interface between two
phases of matter, such as interface between a solid and a liquid.
 Electrochemistry and the surface and colloidal chemistry can be combine with
this phenomena.
Electrical double layer theory:
The theory of the electric double layer deals with this distribution of ions and
hence with magnitude of the electric potentials that occur in the locality of the
charged surface. Consider a solid charged surface in contact with an aqueous solution
of electrolyte.

2. Suppose that some of the cations are absorbed onto the solid surface giving it a
positive charge and these are called potential determining ions. Then the counter
anions are attracted to the positively charged surface by electrical forces and forms a
region called tightly bound layer. In this layer these are fewer anions than cations
absorbed on to the solid surface and hence the potential is still positive.
At the particular distance from the surface, the concentration on anions and cations
are equal and form an electrically neutral region. The system as a hole is electrically
neutral even though they are regions of unequal distribution of anions and cations.
The potential at the sold surface is called Nernst potential and its defined as the
difference in potential between actual solid surface and the electrically neutral region
of the solution.
 When electrode immersed in an electrolytic solution, charge accumulation will
occur.
 Particle size should be greater than 1nm.
 Charge separation always occur at the interface of the electrodes in the
solution.
 The excess charge on the electrode surface is accumulated by an accumulation
of the excess irons of the opposite charge in the solution.
 EDL is a transition region between two phases consists of,
1. An inner molecular layer
2. An outer diffuse region
3. A layer intermediate between inner molecular layer and the outer
diffuse layer.

3. Layers of EDL
According to Stern, an electrical double layer consists of two parts;
1) One part of the double layer, known as fixed part (STERN LAYER)
remains almost fixed to the solid surface. It has positive of negative
ions. There is a sharp fall of potential.
2) The second part of the double layer, known as diffuse part (DIFFUSE
LAYER) extends to some distance into the liquid phase. This layer
contains ions of both signs. Its net charge is equal and opposite to that
on the fixed part. There is a gradual fall of potential into the bulk of the
liquid where is charge distribution is not uniform.
Concepts Based on EDL
 Zeta potential
 DLVO Theory
Zeta potential: -
Zeta potential is defined as the potential difference between the dispersion
medium and the stationary layer of fluid attached to the particle and it is measured by
a zeta potential analyzer.
 It is a parameter of equilibrium on interface.
 It depends on:
a) Properties of liquid and surface.
b) Electrostatic repulsion between particle.
 High zeta potential value – stronger repulsion, the more stable colloidal
system.

4. DLVO Theory: -
 Was named by the scientists named as Derjaguin, Landau, Verwey, and
Overbeek.
 Very important for suspension of solids.
Assumptions of DLVO theory
 Dispersion in dilute.
 Only two forces act on the dispersed particles. Those are Vanderwaals forces
and the electrostatic forces.
 The electric charge and the other properties are uniformly distributed over the
solid surfaces.
 The distribution of ions determined by the electrostatic forces, Brownian motion
and the entropic dispersion.
VT = VR + VA
Where,
VA = Sum of the vanderwaals attractive
VR = Electrical double layer repulsive forces
VT = Total energy of the total layer

5. Applications of EDL: -
 Uses of zeta potential to study colloid – electrolyte interactions.
 Intravenous fat emulsion.
 Drug Targeting and Delivery Systems
 To make the EDLC
REFERENCES
 Internet web address:
1) http://soft-matter.seas.harvard.edu/index.php/DLVO_theory
2) http://image.slidesharecdn.com/emulsionstability-130803053556-
phpapp02/95/emulsion-stability-10-638.jpg?cb=1375508255
3) http://glossary.periodni.com/glossary.php?en=electrical+double+layer
4) http://web.nmsu.edu/~snsm/classes/chem435/Lab14/double_layer.html
 Book referred:
 Advanced physical chemistry by Dr. J.N. Gurtu & Dr. A. Gurtu, twelfth
edition 2010
 Physical Pharmacy by Alfred Martin, Sixth edition