Transcript Electrical Double Layer

COLLOID:
ELECTRICAL DOUBLE LAYER
Kausar Ahmad
http://staff.iium.edu.my/akausar
[email protected]
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Contents
• Electrical Double Layer theories
• Repulsive effect of Electrical Double Layer
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HELMHOLTZ EDL
• Helmholtz in 1879 introduced the concept of the
electrical double layer.
• Charge on the particles of a lyophobic colloid due to
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
• Double layer is diffused
• 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 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 EDL, 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
REPULSIVE EFFECT OF EDL
•
Repulsive effect from the EDL is
responsible for stability.
•
Repulsive potential energy is a function of
(from Verwey and Overbeek):
 Distance between droplets
 The reciprocal of the effective radius of the
double layer
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REPULSIVE POTENTIAL ENERGY
• From Verwey and Overbeek
VR = 4.62 x 10-6 (r/2) e-kHo
VR Repulsive potential 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 EDL potential
 Boltzmann constant
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ATTRACTIVE POTENTIAL ENERGY
• 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 Derjaguin, Landau, Verwey and Overbeek
• Describes the stability of hydrophobic suspension
• Electrostatic repulsive potential energy, VR, and the
attractive potential energy, VA, gives the total potential
energy of interaction:
VT = 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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ENERGY OF INTERACTION &
DISTANCE BETWEEN PARTICLES
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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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REFERENCES
RJ Hunter, Foundations of Colloid Science
Volume 2, Clarendon Press Oxford (1989)
…and as indicated on slides…
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