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    Colloid: Electrical Double Layer: Kausar Ahmad

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    Ahmed Asfora
    The document discusses the electrical double layer (EDL) that forms at the interface between a charged colloid particle and the surrounding liquid medium. It describes three theories of the EDL structure: Helmholtz's model of a single ion layer, Gouy's diffuse double layer model, and Stern's combination model. The repulsive force between colloid particles due to their overlapping EDLs is responsible for stability and is a function of particle size, ionic strength, and interparticle distance according to DLVO theory.

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    Colloid: Electrical Double Layer: Kausar Ahmad

    Uploaded by

    Ahmed Asfora
    14 views14 pages
    The document discusses the electrical double layer (EDL) that forms at the interface between a charged colloid particle and the surrounding liquid medium. It describes three theories of the EDL structure: Helmholtz's model of a single ion layer, Gouy's diffuse double layer model, and Stern's combination model. The repulsive force between colloid particles due to their overlapping EDLs is responsible for stability and is a function of particle size, ionic strength, and interparticle distance according to DLVO theory.

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    The document discusses the electrical double layer (EDL) that forms at the interface between a charged colloid particle and the surrounding liquid medium. It describes three theories of the EDL structure: Helmholtz's model of a single ion layer, Gouy's diffuse double layer model, and Stern's combination model. The repulsive force between colloid particles due to their overlapping EDLs is responsible for stability and is a function of particle size, ionic strength, and interparticle distance according to DLVO theory.

    Copyright:

    © All Rights Reserved
    Download as PPTX, PDF, TXT or read online from Scribd
    Download as pptx, pdf, or txt
    14 views14 pages

    Colloid: Electrical Double Layer: Kausar Ahmad

    Uploaded by

    Ahmed Asfora
    The document discusses the electrical double layer (EDL) that forms at the interface between a charged colloid particle and the surrounding liquid medium. It describes three theories of the EDL structure: Helmholtz's model of a single ion layer, Gouy's diffuse double layer model, and Stern's combination model. The repulsive force between colloid particles due to their overlapping EDLs is responsible for stability and is a function of particle size, ionic strength, and interparticle distance according to DLVO theory.

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    © All Rights Reserved
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    of 14

    COLLOID:

    ELECTRICAL DOUBLE LAYER

    Kausar Ahmad
    http://staff.iium.edu.my/akausar
    akausar@iium.edu.my

    Physical Pharmacy 2 1
    Contents
    • Electrical Double Layer theories

    • Repulsive effect of Electrical Double Layer

    Physical Pharmacy 2 2
    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

    Physical Pharmacy 2 3
    GOUY DIFFUSE DOUBLE LAYER

    • Double layer is diffused


    • outer ionic layer having an electric density falling
    off according to an exponential law.

    Physical Pharmacy 2 4
    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.

    Physical Pharmacy 2 5
    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.

    Physical Pharmacy 2 6
    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

    Physical Pharmacy 2 8
    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

    Physical Pharmacy 2 9
    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’?
    Physical Pharmacy 2 10
    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 = V A + V R

    • The forces on colloidal particles in a dispersion are due


    to electrostatic repulsion and London-type Van der Waals
    forces Physical Pharmacy 2 11
    ENERGY OF INTERACTION &
    DISTANCE BETWEEN PARTICLES

    Physical Pharmacy 2 12
    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.

    Physical Pharmacy 2 13
    REFERENCES

    RJ Hunter, Foundations of Colloid Science


    Volume 2, Clarendon Press Oxford (1989)

    …and as indicated on slides…

    Physical Pharmacy 2 14

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