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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /Rheology
Conjugation andLigands
Acknowledgement
Characterization of colloidal materials byelectrochemical, electrokinetic and
rheological methods
Michael Bredol
Fachhochschule Munster - University of Applied Sciences
ADMAT intensive programme 2006
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /Rheology
Conjugation andLigands
Acknowledgement
Fields of Colloid Chemistry
Colloid chemistry is the chemistry of heterogeneoussystems and thus dominated by interfaces
� Colloidal systems with “large” components: classicalbehaviour� Very “small” particles are dominated by quantumphysics
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayersDebye-Huckel model
Electrokinetics
Kinetics /Rheology
Conjugation andLigands
Acknowledgement
Thermodynamic and kineticstability
� Dispersion colloids are instable: positive interfacetension� Stabilization necessary for technical applications� Ion adsorption on particles: electrostatic stabilization� Polymer adsorption on particles: entropicstabilization� Without stabilization: rapid (diffusion controlled)agglomeration� Molecular colloids (hydrophilic polymers, proteins. . . ) are thermodynamically stable� Association colloids (micelles, membranes . . . ) arethermodynamically stable
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayersDebye-Huckel model
Electrokinetics
Kinetics /Rheology
Conjugation andLigands
Acknowledgement
Particle, rigid adsorption layerand counter ion cloud
Debye-Huckel model: reduction into spherical systemaround one particleElectric potential at shear plane:
�–potential
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayersDebye-Huckel model
Electrokinetics
Kinetics /Rheology
Conjugation andLigands
Acknowledgement
Thickness of ion cloudElectrostatic analysis yields expression for thickness ofdiffuse counter ion cloud:
rD �� �
RTIF 2 I � 1
2 � z2i ci
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayersDebye-Huckel model
Electrokinetics
Kinetics /Rheology
Conjugation andLigands
Acknowledgement
Repulsive and attractiveforces
Attraction between particles: van-der-Waals–forcesRepulsion:� electrostatics, due to adsorbed ionsSum curve: potential barrier against agglomeration
Global minimum on close contact, second local minimumbefore barrier !
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayersDebye-Huckel model
Electrokinetics
Kinetics /Rheology
Conjugation andLigands
Acknowledgement
Stabilization and ion strength
Ion strength is most important parameter for electrostaticstabilization
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayersDebye-Huckel model
Electrokinetics
Kinetics /Rheology
Conjugation andLigands
Acknowledgement
Steric / entropic stabilization
� Adsorption of polymers with high molecular massresults in repulsive interaction� In contact area, number of conformations isrestricted, thus entropy reduced, resulting inrepulsion� Correct concentration is critical !
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayersDebye-Huckel model
Electrokinetics
Kinetics /Rheology
Conjugation andLigands
Acknowledgement
Coagulation in stabilized andnon-stabilized systems
� Unstabilized dispersions form irreversibly fluffysediments with low density� Stabilized dispersions form reversibly sediments withhigh density� Sediments from stable dispersions may beredispersed by simple stirring and/or shaking
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayersDebye-Huckel model
Electrokinetics
Kinetics /Rheology
Conjugation andLigands
Acknowledgement
Measurement of � –potentials
� Charging makes particles mobile in an electrical field� �–potential is measure for charging� Either Brownian motion or shear at the shear plane
are employd for measurement
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayersDebye-Huckel model
Electrokinetics
Kinetics /Rheology
Conjugation andLigands
Acknowledgement
Measurement of � –potentials
� Charging makes particles mobile in an electrical field� �–potential is measure for charging� Either Brownian motion or shear at the shear plane
are employd for measurement� Absolute measurements often on individual particles,but technical dispersions are very concentrated� Practically, relative measurements are often sufficient
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayersDebye-Huckel model
Electrokinetics
Kinetics /Rheology
Conjugation andLigands
Acknowledgement
Measurement of � –potentials
� Charging makes particles mobile in an electrical field� �–potential is measure for charging� Either Brownian motion or shear at the shear plane
are employd for measurement� Absolute measurements often on individual particles,but technical dispersions are very concentrated� Practically, relative measurements are often sufficient� Linear phenomena without electrochemistry (at theelectrodes) are desired� Electroacoustic methods allow for characterization ofconcentrated dispersions
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayersDebye-Huckel model
Electrokinetics
Kinetics /Rheology
Conjugation andLigands
Acknowledgement
Electrophoresis
Drift velocity vw is directly proportional to�–potential.
Moder� n systems use laser tracking of individual particles.
vw ��E
6 � 0
�
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayersDebye-Huckel model
Electrokinetics
Kinetics /Rheology
Conjugation andLigands
Acknowledgement
Sedimentation potential
Popular method due to ease of measurement. Works alsoin� turbid and coloured dispersions.
ES �� �
0 lgr3N3 �� ������� 0 � �
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayersDebye-Huckel model
Electrokinetics
Kinetics /Rheology
Conjugation andLigands
Acknowledgement
Streaming potential
Important method in fibre and paper industry
ESt�P �
�6 � 75 � 107 � 4 ��� �
Automated systems collect fiber plug on funnel withelectrodes
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayersDebye-Huckel model
Electrokinetics
Kinetics /Rheology
Conjugation andLigands
Acknowledgement
Electroosmosis
A�
current I drives a volume stream:
dVdt �
�I
4 ��� �Not as convenient as streaming potential, since highvoltage necessary
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayersDebye-Huckel model
Electrokinetics
Kinetics /Rheology
Conjugation andLigands
Acknowledgement
Electroacoustics
Coupling acoustic pressure with electric effect is a ACmethod� and works well with concentrated dispersions.
Colloid vibration potential; inverse effect: electroacousticsound amplitude ESA
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayersDebye-Huckel model
Electrokinetics
Kinetics /Rheology
Conjugation andLigands
Acknowledgement
ESA-setup
ESA measurements are very fast and can be performedonline or during titration
Alternatively, probes can be inserted into stirreddispersions (large particles, unstable dispersions)
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayersDebye-Huckel model
Electrokinetics
Kinetics /Rheology
Conjugation andLigands
Acknowledgement
ESA-example: charge oncolloidal silica
A useful way to characterize colloids is to assess thedependence�
of�-potential on pH, here for colloidal silica
(LUDOX):
-10
-8
-6
-4
-2
0
2
3 4 5 6 7 8 9
zeta
-Pot
entia
l / m
V�
pH
The pH, at which no charging is measured, is called thepoint of zero charge or the isoelectrical point
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayersDebye-Huckel model
Electrokinetics
Kinetics /Rheology
Conjugation andLigands
Acknowledgement
ESA-example: adsorption ofbenzoic acid
Adsorption on colloidal particles can be followed by theESA method. Titratrion of silica with benzoate at pH=9:
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
1
0.001 0.01
zeta
-Pot
entia
l / m
V�
c(Benzoat) / (mol/l)
The surface charge is effectively reduced due toadsorption of benzoic acid replacing negative groups
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayersDebye-Huckel model
Electrokinetics
Kinetics /Rheology
Conjugation andLigands
Acknowledgement
ESA-example: coating of YF3
Surface state (e.g. hydrolisis) and coatings can befollowed by ESA as well:
Application: protection of sensitive surfaces of powderparticles (protection against water, oxygen, radiation)
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /RheologyBasics
Flow characteristics
Examples
Conjugation andLigands
Acknowledgement
Shear stress
� Colloidal systems show Non-Newtonian flow: controlthrough interface chemistry� Basic quantities and situation:
Viscosities span several orders of magnitude (glaciers togases)
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /RheologyBasics
Flow characteristics
Examples
Conjugation andLigands
Acknowledgement
Laminar flow
Situation for laminar flow (small Reynold number)
Viscosity may be defined as the coefficient of transport ofmomentum p in a gradient of velocities v :
dpx
Adt � � dvx
dz � ! � kgm s � Pa s
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /RheologyBasics
Flow characteristics
Examples
Conjugation andLigands
Acknowledgement
Viscosity measurementDispersions under test are sheared between rotator andstator.
� Measurement of torsional force together with fixedgeometry yields stress applied� Rotational speed defines shear rate� Flow curves are determined by sweeping shear rates� Viscoelastic properties are accessible by oscillatingrotation
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /RheologyBasics
Flow characteristics
Examples
Conjugation andLigands
Acknowledgement
Basic flow types
Newtonian flow: viscosity is independent of shear rate
Pseudoplastic flow: viscosity decreases with shear rate(“shear thinning”)Dilatant flow: viscosity increases with shear rate
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /RheologyBasics
Flow characteristics
Examples
Conjugation andLigands
Acknowledgement
Thixotropic behaviour
Often flow equilibrium is not reached immediately:thixotropic"
behaviour
Area of hysteresis is characteristic for ratio of relaxationtime and shear rate change
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /RheologyBasics
Flow characteristics
Examples
Conjugation andLigands
Acknowledgement
Flow curves
Alternative plot: stress over shear rateNewtonian flow then is depicted by straight lines
Critical yield stresses are displayed clearly in these plots
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /RheologyBasics
Flow characteristics
Examples
Conjugation andLigands
Acknowledgement
Flow modifiers
Flow control through additives relies on weak interaction,e.g. hydrogen bridges.Typical inorganic system (inks, varnishes. . . ):
Characteristics may be modified by intercalation ofcomplex ions and/or organic molecules
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /RheologyBasics
Flow characteristics
Examples
Conjugation andLigands
Acknowledgement
Polymeric flow modifiers
Typical: polymers (if necessary, branched) withh# ydrophilic groups (OH, COOH, NH2, SO3H, . . . ) alongthe back bone
Examples: cellulose / starch, polymaleic acid, polyacrylicacid
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /RheologyBasics
Flow characteristics
Examples
Conjugation andLigands
Acknowledgement
Superabsorbers
Example for branching:
Weakly interconnected polyacrylic acid (partially as salt)is used in “superabsorbers”
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /RheologyBasics
Flow characteristics
Examples
Conjugation andLigands
Acknowledgement
ViscoelasticityElastic (energy storage) and inelastic (flow) response toexternal stress
Description by complex viscosity with inelastic flow in thereal part and elastic response in the imaginary part
� %$ �'& �)( i %$*$ �'& �& is the frequency of oscillation (sign reversal of stress)
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /RheologyBasics
Flow characteristics
Examples
Conjugation andLigands
Acknowledgement
Example: ball pen
Inks in ball pens need a precisely defined yield stress andshould not exhibit thixotropic hysteresis
Typical modifiers: modified montmorillonites
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /RheologyBasics
Flow characteristics
Examples
Conjugation andLigands
Acknowledgement
Example: wall paintApplying wall paint produces a typical shear rate andstress
Shear thinning (flow under stress, but not at rest) andsome thixotropic component (film smoothening) have tobe definedTypical modifiers: cellulose (cheap), polyacrylic acids(expensive, but better defined)
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /Rheology
Conjugation andLigands
Coated particles
Layers and films
Film formation
Resisitivity
Acknowledgement
Ligands
Colloidal+ components in a system can be coupled bysurface groups or adsorbed linkers� Adsorption by covalent bonding or electrostatic
attraction� Bifunctional entities offer very flexible chemistry� Spacers may deliberatly decouple components� Bifunctional silanes offer branching into inorganic orhybrid network� Biochemical applications focus on drug delivery,signalling . . .
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /Rheology
Conjugation andLigands
Coated particles
Layers and films
Film formation
Resisitivity
Acknowledgement
Bio–conjugates
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /Rheology
Conjugation andLigands
Coated particles
Layers and films
Film formation
Resisitivity
Acknowledgement
Y2O2S coated with Fe2O3
Functionalization and protection of surfaces are crucialf,or several applications
Example: red emitting TV phosphor with colour filterprotecting sulfidic substrate
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /Rheology
Conjugation andLigands
Coated particles
Layers and films
Film formation
Resisitivity
Acknowledgement
LaOBr:Tb coated with ZnS
Protection: Thick coatings to prevent hydrolisis
Example: green emitting projection TV phosphor coatedwith sulfidic barrier
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /Rheology
Conjugation andLigands
Coated particles
Layers and films
Film formation
Resisitivity
Acknowledgement
Application of colloids in films
A huge number of applications uses colloidal systems inlayers and films� Film formation typically proceeds from liquid
precursors� Dipping, spinning, spraying, flooding, printing . . . offerbroad range of technological choices
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /Rheology
Conjugation andLigands
Coated particles
Layers and films
Film formation
Resisitivity
Acknowledgement
Application of colloids in films
A huge number of applications uses colloidal systems inlayers and films� Film formation typically proceeds from liquid
precursors� Dipping, spinning, spraying, flooding, printing . . . offerbroad range of technological choices� During film consolidation, internal film structures arepreformed� Curing to solid films can be thermal or photochemicalprocesses� Self assembly is nature’s choice to fabricate complexfilm structures under ambient conditions
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /Rheology
Conjugation andLigands
Coated particles
Layers and films
Film formation
Resisitivity
Acknowledgement
Dip–coating
Prototype of film formation process is dip–coating
Sequence: dipping �.- withdrawal ��- dripping �/-consolidation (evaporation, drying)Wet film thickness depends on viscosity and withdrawalspeed. Control of atmosphere necessary !
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /Rheology
Conjugation andLigands
Coated particles
Layers and films
Film formation
Resisitivity
Acknowledgement
Integration withsemiconductor devices
Spin–coating0 is standard technique in semiconductorprocessing
Example: nano-dispersions with luminescentnanoparticles on LED-chip
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /Rheology
Conjugation andLigands
Coated particles
Layers and films
Film formation
Resisitivity
Acknowledgement
Photonic structures
During film formation and consolidation, photonicstr1 uctures may crystallize from colloidal particles:
Applications for electrooptical applications to bedeveloped (e.g. photonic switches)
Generated polymeric templates may be converted tometamaterials of semiconductors, metals . . .
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /Rheology
Conjugation andLigands
Coated particles
Layers and films
Film formation
Resisitivity
Acknowledgement
Layer-by-layer self assemblyCombining acidic and basic surface ligands allow formolecular recognition during fim formation
Example: layer-by-layer self assembly of quantum dot /silica composites
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /Rheology
Conjugation andLigands
Coated particles
Layers and films
Film formation
Resisitivity
Acknowledgement
DC-conductivities
Sensor for nitric oxides
Excess conductivitiesConcentration of nitric oxides derived from excess chargecarries after adsorption
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /Rheology
Conjugation andLigands
Coated particles
Layers and films
Film formation
Resisitivity
Acknowledgement
Impedance spectroscopy
Example: Mixed e 2 Ag 3 conductor
Non-electron conductivity contributions can becharacterized by AC methods.
Frequency-dependent conductivity measurement(“Impedance Spectrscopy”) at small potential yieldsresistive and capacitive contributions withoutelectrochemical effects.
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /Rheology
Conjugation andLigands
Coated particles
Layers and films
Film formation
Resisitivity
Acknowledgement
Interpreting results
Left: Complex plane, frequency as parameterRight: Absolute impedance
Extraction of results by fit of measured data to eletricalmodel
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /Rheology
Conjugation andLigands
Coated particles
Layers and films
Film formation
Resisitivity
Acknowledgement
Non-local phenomenaDiffusion and reaction are included by special elements:
Warburg-impedance (imaginary impedance due todiffusion)
Typical form of Nyquist plot in the presence ofWarburg-impedance
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /Rheology
Conjugation andLigands
Coated particles
Layers and films
Film formation
Resisitivity
Acknowledgement
Application in corrosionprotection
Non-desctructive testing of inverted and coated metalsubstrates for quality assessment.Applications also for foils (packaging) between suitedelectrodes.
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /Rheology
Conjugation andLigands
Coated particles
Layers and films
Film formation
Resisitivity
Acknowledgement
Application in fuel cells
Ion conductivities of membranes as well as three phasegas4 electrodes can be investigated by impedancespectroscopy
Measurements can als be done under load (galvanosticmode in contrast to potentiostatic mode)
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Characterization ofcolloidal materials
byelectrochemical,
electrokinetic andrheologicalmethods
Michael Bredol
Kinetics / doublelayers
Kinetics /Rheology
Conjugation andLigands
Acknowledgement
Acknowledgements
Many people help to sustain colloid chemical activities atF5 achhochschule Munster� Thanks are due to BMBF and Bundesland NRW for
grants and financial help� Scientific coworkers in the lab of Physical Chemistrydo most of the work� Project and thesis students generated a lot ofexperimental data� DAAD funded generally the incorporation of foreignstudents into a binational course programme� EU / Erasmus provides funds for intensiveprogrammes