What is an Emulsion?
A dispersion of one liquid in a second, immiscible liquid.
There are two phases: Dispersed phase and continuous phase.
Two distinct type of emulsion: O/W and W/O.
O/W – Oil is the dispersed phase while water is the
continuous phase. Example: Mayonnaise, milk.
W/O – Water is the dispersed phase while oil is the continuous
phase. Example: Butter.
There are 2 intermolecular forces which affect the stability of
Van der Waals force is an attractive force, which tends to
destabilize the emulsion.
Electrostatic force is a repulsive force, which gives stability to the
When the total attractive forces > total repulsive forces, the
emulsion is unstable.
When total repulsive forces > total attractive forces, the emulsion
Often, emulsions are inherently unstable.
The small suspended droplets first coalesce into less stable, larger
droplets. This coalescence then continues until the emulsion breaks
into two layers.
This phenomenon can occur over a relatively short period of
If a long emulsion lifetime is required, therefore, a stabilizer
needs to be added.
This stabilizer is usually a surfactant.
Electrostatic stabilization of colloids:
A mechanism in which the attraction Van der Waals
forces are counterbalanced by the repulsive
Coulomb forces acting between negatively charged
Electrical Double Layer.
A phenomenon, which plays a fundamental role in the mechanism
of the electrostatic stabilization of colloids.
It has a combination of charged surface.
Unequal distribution of co-ions and counter-ions near the surface.
Colloidal particles gain negative electric charge when negatively
charged ions of the dispersion medium are adsorbed on the particles
A negatively charged particle attracts the positive counter-ions
surrounding the particle.
Electric Double Layer is the layer surrounding a particle of the
dispersed phase and including the ions adsorbed on the particle
surface and a film of the counter-charged dispersion medium
The Electric Double Layer is electrically neutral.
It consists of three parts:
Surface charge - charged ions (commonly negative)
adsorbed on the particle surface.
Stern layer – counter-ions (charged opposite to the surface
charge) attracted to the particle surface and closely
attached to it by the electrostatic force.
Diffuse layer - a film of the dispersion medium (solvent)
adjacent to the particle. Diffuse layer contains free ions
with a higher concentration of the counter-ions. The ions of
the diffuse layer are affected by the electrostatic force of
the charged particle.
It is sensitive to electrolytes and temperature.
The electrical potential within the Electric Double Layer has
the maximum value on the particle surface (Stern layer).
The potential drops with the increase of distance from the
surface and reaches 0 at the boundary of the Electric Double
When a colloidal particle moves in the dispersion medium, a
layer of the surrounding liquid remains attached to the
The boundary of this layer is called slipping plane (shear
The value of the electric potential at the slipping plane is
called Zeta potential, which is very important parameter in
the theory of interaction of colloidal particles.
Deryagin, Landau, Vewey and Overbeek developed a theory of the
stability of colloidal systems (DLVO theory) in the 1940s.
Assumptions of DLVO theory:
1. Dispersion is dilute.
2. Only two forces act on the dispersed particles: Van der Waals force and
3. The electric charge and other properties are uniformly distributed over
the solid surface.
4. The distribution of the ions is determined by the electrostatic force,
Brownian motion and entropic dispersion.
The theory states that the colloidal stability is determined by the potential
energy of the particles (VT) summarizing two parts: potential energy of the
attractive interaction due to van der Waals force VA and potential energy of
the repulsive electrostatic interaction VR: VT = VA + VR
DLVO Theory suggest that’s that electrical double-layer
repulsion will stabilize the emulsion, when the electrolyte
concentration phase is less than a certain value.
• The minimum of the potential energy
determines the distance between two particles
corresponding to their stable equilibrium.
• The two particles form a loose aggregate,
which can be easily re-dispersed.
• The strong aggregate may be formed at a
shorter distance corresponding to the primary
minimum of the potential energy (not shown
in the picture).
• In order to approach to the distance of the
primary minimum the particle should
overcome the potential barrier.
It is the electrical potential at the hydrodynamic
plane of shear.
The particles interact according to the magnitude of
the zeta potential, not their surface charge.
Zeta potential tells us about the effectiveness of the
For electrically stabilized dispersion, the higher the
value of zeta potential, the more stable the
dispersion is likely to be.
Stability dividing line is generally considered to be
Particles with the zeta potential more than +30mV
or less than -30mV formed a stable dispersion.
Small changes in pH or concentration of ions (ionic
strength) can lead to dramatic changes in the zeta
Potential Energy Curve for Stable
and Unstable Emulsion.
1. Proceedings of the Symposium on the
Electrochemical Double Layer, By Carol
Korzeniewski, B. E. Conway.
2. Colloids in agrochemicals: colloids and interface
science, Volume 5, By Tharwat F. Tadros
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2. See Kah Ling
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4. Yeap Chiao Li 104512