Electrokinetic properties of colloid electrical double layer kausar ahmad kulliyyah of pharmacy
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Electrokinetic properties of colloid: Electrical Double Layer Kausar Ahmad Kulliyyah of Pharmacy. Contents. Electrical double layer theories Repulsive effect of electrical double layer Potential energy of interaction. Helmholtz double layer.

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Electrokinetic properties of colloid electrical double layer kausar ahmad kulliyyah of pharmacy

Physical Pharmacy 2

Electrokinetic properties of colloid:Electrical Double LayerKausar AhmadKulliyyah of Pharmacy


Contents

Contents

Electrical double layer theories

Repulsive effect of electrical double layer

Potential energy of interaction

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Helmholtz double layer

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 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 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

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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.


Electrical double layer

Electrical double layer

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Repulsive effect of double layer

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

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From Verwey and Overbeek

VR = 4.62 x 10-6 (r/2) e-kHo

VRRepulsive energy

rParticle 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


Attractive force

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

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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Contents

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Interaction potentials

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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.


Reference

Reference

RJ Hunter, Foundations of Colloid Science Volume 2, Clarendon Press Oxford (1989)‏

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