Transcript Electrical Double Layer

Electrokinetic properties of
colloid:
Electrical Double Layer
Kausar Ahmad
Kulliyyah of Pharmacy
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Contents
• Electrical double layer theories
• Repulsive effect of electrical double layer
• Potential energy of interaction
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Helmholtz double layer
• Helmholtz in 1879 introduced the concept of the
electrical double layer.
• The charge on the particles of a lyophobic colloid
was due to an unequal distribution of ions at the
particle-water interface.
• If ions of one charge were closely bound to the
particle, ions of opposite charge would line up
parallel to them, forming a double layer of charges
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Gouy diffuse double layer
• Gouy proposed that the double layer is diffused,
with the outer ionic layer having an electric density
falling off according to an exponential law.
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Stern diffuse double layer
Stern compromised Helmholtz and Gouy.
The double layer is in two parts:
1. Helmholtz layer - one layer approximately a single
ion in thickness, remains essentially fixed to the
interfacial surface. In this layer, there is a sharp drop of
potential.
2. Gouy layer – this layer extends some distance into
the liquid dispersing phase and is diffuse, with a
gradual fall in potential into the bulk of the liquid.
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Electric double layer
• is a region of molecular dimension at the boundary of two
substances across which an electrical field exists.
• The substances must each contain electrically charged particles,
such as electrons, ions, or molecules with a separation of electrical
charges (polar molecules).
• In the electrical double layer, oppositely charged particles attract
each other and tend to collect at the surface of each substance but
remain separated from one another by the finite size of each particle
or by neutral molecules that surround the charged particles.
• The electrostatic attraction between the two opposite and separated
charges causes an electrical field to be established across the
interface.
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Electrical double layer
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Repulsive effect of
double layer
•
The repulsive effect from the double
layer is responsible for stability.
•
Verwey and Overbeek proposed that the
repulsive energy is a function of:
 Distance between droplets
 The reciprocal of the effective radius of the
double layer
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From Verwey and Overbeek
VR = 4.62 x 10-6 (r/2) e-kHo
VR Repulsive energy
r Particle radius
 Valence of counter ions
Ho distance between two particles
 = (ez/2 – 1) / (ez/2 + 1); Z = ueo/kT, o is the double layer potential
 Boltzmann constant
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Attractive force
• A small attractive Van der Waals force operating
between the droplets, can be given by:
VA = -Ar/12H0
• A is a constant depending on the polarisability of the
molecules of which the droplet is composed and is
known as the Hamaker constant;
A ca. 10-19 J to 10-20 J.
• Exercise: what happen if you have big ‘r’?
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DLVO Theory
• From Deryagin, Landau, Verwey and Overbeek
• Describes the stability of hydrophobic suspension
• Combination of electrostatic repulsive energy, VR, and
the attractive potential energy, VA, gives the total
potential energy of interaction:
Vtotal = VA + VR
• The forces on colloidal particles in a dispersion are due
to electrostatic repulsion and London-type Van der
Waals forces
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Interaction potentials
http://griergroup.uchicago.edu/~grier/leshouches2/leshouches2.html
• polystyrene sulfate
spheres in deionized
water at 25oC.
• Curves are labelled
by the spheres' radii.
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Reference
RJ Hunter, Foundations of Colloid Science
Volume 2, Clarendon Press Oxford (1989)
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