Zeta Potentialmoha

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    Zeta Potential Measurement

    Submitted by:

    Jit Pal (2010 TTF3690)

    Mahadev Bar (2010TTF3695)

    Submitted to:

    Dr Mangala Joshi

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    Three of the fundamental states of matter are solids, liquids and gases.

    If one of these states is finely dispersed in another then colloidal system

    comes.

    Particles may adhere to one another and form aggregates of successively

    increasing size. An initially formed aggregate is called a floc and the process

    of its formation flocculation.If the aggregate changes to a much denser form,it is said to undergo coagulation. An aggregate usually separates out either by

    sedimentation (if it is more dense than the medium) or by creaming (if it less

    dense than the medium).

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    Flocculation

    Coagulation

    Sedimentation

    Flocculation

    Sedimentation

    Coagulation

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    The scientists Derjaguin, Verwey, Landau and Overbeekdeveloped a

    theory in the 1940s which dealt with the stability of colloidal systems.

    Stability of the particle depends on its total potential energy function

    VT

    = VA

    + VR

    + VS

    VS

    = the potential energy due to the solvent

    VA

    = -A/(12 D2) where A is Hamaker constant & D is the particle separation

    VR

    = 2 a 2exp(-D) where a is particle radius, is solvent permeability, is

    function of ionic composition & is Zeta potential

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    The stability of a colloidal system is

    determined by the sum of these Vander

    Waals attractive (VA) and electrical

    double layer repulsive (VR) forces.

    If the zeta potential is reduced

    secondary minimum is created

    adhesion between particles exists.

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    Ionisation of Surface Groups

    Differential loss of ions from the crystal lattice

    Adsorption of charged species

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    Ionisation of Surface Groups:

    Dissolution of acidic or basic groups on the surface of the particle will show

    the negative or positive charged surface.The surface charge can be reduced to

    zero by suppressing the surface ionisation by changing the pH of the solution.

    Origin of surface charge by

    ionisation of acidic groups

    Origin of surface charge by ionisation

    of basic groups

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    Differential loss of ions from the crystal lattice:As an example, consider a crystal of silver iodide placed in water. Solution of

    ions occurs. If equal amounts of Ag+ and I- ions were to dissolve, the surface

    would be uncharged. In fact silver ions dissolve preferentially, leaving a

    negatively charged surface.

    Adsorption of charged species:Surface charge also depends upon the type of surfactant absorbed.

    Cl-Cl-

    Cl-

    Cl-

    Cl-

    Cl-Cl

    -

    Cl-

    RNH2+

    RNH2+

    RNH2+

    RNH2+

    RNH2+

    RNH2+

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    The liquid layer surrounding the particle exists as two parts; an inner region

    (Stern layer) where the ions are strongly bound and an outer (diffuse) region

    where they are less firmly associated. The potential at this boundary is the

    zeta Potential.

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    Based on any one of the four electrokinetics effects.

    Electro-osmosisStreaming current/potentialElectrophoresis

    Sedimentation potential

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    Electro-osmosis

    Streaming current/potential

    The solid is fixed as plug or diaphragm.

    Motion of ions in the diffuse layer is caused by

    an externally applied electrical potential.

    In consequence a motion of the electrolyte solution in the capillaries of the plug is

    produced

    It is the inversion of electro-osmosis.

    Electrolyte solution is forced through the capillaries of the plug by external pressure.

    Current or Potential resulting from motion of ions in the diffuse layer is measured

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    In case of Electrophoresis and Sedimentation potential solid particles move in an

    electrolyte solution due to either an external electrical field or a mechanical force.

    Transport rate or sedimentation potential is measured.

    Electrophoresis and Sedimentation potential

    Helmholtz and Smoluchowski relating the mechanical and electrical

    force equations for single capillary fiber plugs

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    Where

    D = volume flow

    U = voltage

    V = volume

    t = timel = length of capillary

    q = cross-sectional area of

    capillary

    = influence constant

    0 = relative dielectric

    constant = viscosity

    p = hydrodynamic pressure

    R = electrical resistance

    The streaming current

    Is = (..0.q.p)/.l.

    Or,

    Is / p= (..0.q)/.l.

    Capillary filled with electrolyte solution

    The electro-osmotic volume flow

    D = dv/dt = (..0.q.U)/l

    The streaming potential

    Us= (..0.q.p)/.l.R

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    The Equations are valid on assuming that

    a) The charge distribution in solution obeys Poissons law,

    b) The electrical potential across the surface is constant,

    c) The radius of the capillary is large compared with the thickness of the

    electrochemical double layer,

    d) Streaming in the capillary is laminar (streaming potential),

    e) The externally applied potential gradient is constant throughout the

    capillary (electro-osmosis).

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    EXTENSION OF HELMHOLTZ -SMOLUCHOWSKI

    EQUATIONS TO BUNDLES OF CAPILLARIES

    The value of LD and QD cannot be measured directly, but the quotient LD/QD , which is

    required for the calculation of zeta-potential, can be determined by two different

    method.

    correlate the geometrical parameters of a single capillary for n capillaries arranged in a

    series of bundle

    the cross-section area and the length will be average cross-section area and length.

    So,LD = l /n

    QD = q

    Now the streaming current will be

    Is = n .Is= (..0.QD.p)/. LD

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    FAIRBROTHER AND MASTIN METHOD

    Assuming that the fibers to be insulators, electrical conductance occur only in

    capillaries filled with electrolyte solution.

    The specific electrical conductance x of an electrolyte solution

    x = LM/QM .R

    LD/QDratio determined by measuring the electrical resistance R of the fiber plug.

    whereLM = distance of electrodes

    QM = cross section area of the electrodes.

    The ratio LM/QM is called resistance capacity C (cell constant)

    Substituting LD

    and QD

    in equation of Is by LM

    and QM

    so,

    Is=(..0.p)/.x.R

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    This model is proposed by Goring and Mason .

    Cell constant (CD) can obtained by geometrical

    considerations.

    Length of capillaries in the plug( LD ) >length of the plug (LM)

    Thus, LD is given by

    LD = LM/ cos

    The sum of the cross sections of the capillaries QD is given by

    QD =VD/LDWhere,

    QD = sum of the cross-sections of cpillaries

    VD = total available volume for streaming solution.

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    CAPILLARY BUNDLE MODEL

    Introducing the specific volume() a of the swollen fibers, VD can be expressed asthe difference between the total volume VM of the plug and the volume fraction of

    the fibers:

    VD = VM(1 - d)

    Where,d = packing density.

    VM is given by

    VM = QM . LM

    So,CD = LM/QM (1-.d).cos2 = LM/QM.

    Or,

    IS/P = .0 .QM/ .LM.

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    Zeta potential measurement devices based on steaming potential and steaming

    current are consists of

    a) Measuring cell with electrodes,

    b) Device for producing and measuring hydrostatic pressure,

    c) Measuring device for streaming potential streaming current,

    d) Conductivity meter.

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    For streaming potential, various electrodes like Ag/AgC , platinum , gold have

    been used weather for streaming current, reversible electrodes with low

    asymmetry potential are required.

    The fiber sample, soaked

    in the measuring solution

    (To avoid air bubbles),

    The ratios Us/p or Is/p

    are determined for

    Different driving pressures

    The measuring cell is inserted

    Into the measuring device,

    and the measuring solution is

    Pressed through the plug.

    Introduced into the measuring

    Cell, which is confined by

    Perforated electrodes

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    Compared with streaming potential, elecro-osmosis has not been used so

    frequently for fibers.

    Methodical work was especially carried out by Fairbrother and Mastin , Mason and

    co-workers, Stackelberg and co-workers , and Androsow and coworkers .

    They obtain reproducible zeta-potentials of fibers in electrolyte solutions and

    solutions of surface active agents.

    Zeta potential measurement equipment based on Electro-Osmosis generally

    consists of:

    a) Cell for uptake of the fiber plug,b) Unpolarizable electrodes and direct current supply,

    c) Measuring devices for current and resistance or voltage drop in the plug,

    d) Device for determinig velocity of fluid motion.

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    The zeta-potential is calculated using following equation .

    = (D ..R0.1N KCL.X 0.1N KCL )/ I..0 R

    To move the solution through the fibre

    plug An external electrical potential (100

    to 400 V) is applied by Ag/ AgSO4-

    electrodes.

    The moving velocity can be measured byobserving the meniscus in the measuring

    capillary.

    Electro-osmosis equipment used by

    Stackelburg and co-workers

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    Zeta-potential is calculated using following equations

    IS/P = .0 .QM/ .LM.

    Electro-osmosis equipment used by

    Biefer and Mason

    They use a measuring cell like measurement of streaming potential.

    External electrical potential is applied by Ag/AgC1 electrodes.

    These electrodes will work reversibly only at KC1 concentrations up to 1.10 4 molar

    At higher concentrations, gas bubbles arise and disturb the measurements

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    There are many other principle which may successfully use for measuring zeta

    potential of Fibre such as-

    Non-stationary zeta-potential measurement method

    Streaming current detector methodZeta-potential measurement by ultrasonic waves

    Ultrasonic measuring principle was tested with dispersed pulp particles and found

    a relatively good relation between vibration potential and zeta-potential

    determined by streaming potential measurements.

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