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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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