characterization of colloidal materials by … · characterization of colloidal materials by...

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


byelectrochemical,


Michael Bredol


Kinetics /Rheology


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


byelectrochemical,


Michael Bredol

Kinetics / doublelayersDebye-Huckel model

Electrokinetics

Kinetics /Rheology


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


byelectrochemical,


Michael Bredol


Electrokinetics

Kinetics /Rheology


Acknowledgement

Particle, rigid adsorption layerand counter ion cloud

Debye-Huckel model: reduction into spherical systemaround one particleElectric potential at shear plane:

�–potential


byelectrochemical,


Michael Bredol


Electrokinetics

Kinetics /Rheology


Acknowledgement

Thickness of ion cloudElectrostatic analysis yields expression for thickness ofdiffuse counter ion cloud:

rD ��

RTIF 2 I � 1

2 � z2i ci


byelectrochemical,


Michael Bredol


Electrokinetics

Kinetics /Rheology


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 !


byelectrochemical,


Michael Bredol


Electrokinetics

Kinetics /Rheology


Acknowledgement

Stabilization and ion strength

Ion strength is most important parameter for electrostaticstabilization


byelectrochemical,


Michael Bredol


Electrokinetics

Kinetics /Rheology


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 !


byelectrochemical,


Michael Bredol


Electrokinetics

Kinetics /Rheology


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


byelectrochemical,


Michael Bredol


Electrokinetics

Kinetics /Rheology


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


byelectrochemical,


Michael Bredol


Electrokinetics

Kinetics /Rheology


Acknowledgement



are employd for measurement� Absolute measurements often on individual particles,but technical dispersions are very concentrated� Practically, relative measurements are often sufficient


byelectrochemical,


Michael Bredol


Electrokinetics

Kinetics /Rheology


Acknowledgement



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


byelectrochemical,


Michael Bredol


Electrokinetics

Kinetics /Rheology


Acknowledgement

Electrophoresis

Drift velocity vw is directly proportional to�–potential.

Moder� n systems use laser tracking of individual particles.

vw ��E

6 � 0

�


byelectrochemical,


Michael Bredol


Electrokinetics

Kinetics /Rheology


Acknowledgement

Sedimentation potential

Popular method due to ease of measurement. Works alsoin� turbid and coloured dispersions.

ES ��

0 lgr3N3 �� 0 � �


byelectrochemical,


Michael Bredol


Electrokinetics

Kinetics /Rheology


Acknowledgement

Streaming potential

Important method in fibre and paper industry

ESt�P �

�6 � 75 � 107 � 4 ��

Automated systems collect fiber plug on funnel withelectrodes


byelectrochemical,


Michael Bredol


Electrokinetics

Kinetics /Rheology


Acknowledgement

Electroosmosis

A�

current I drives a volume stream:

dVdt �

�I

4 �� Not as convenient as streaming potential, since highvoltage necessary


byelectrochemical,


Michael Bredol


Electrokinetics

Kinetics /Rheology


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


byelectrochemical,


Michael Bredol


Electrokinetics

Kinetics /Rheology


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)


byelectrochemical,


Michael Bredol


Electrokinetics

Kinetics /Rheology


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


byelectrochemical,


Michael Bredol


Electrokinetics

Kinetics /Rheology


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


byelectrochemical,


Michael Bredol


Electrokinetics

Kinetics /Rheology


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)


byelectrochemical,


Michael Bredol


Kinetics /RheologyBasics

Flow characteristics

Examples


Acknowledgement

Shear stress

� Colloidal systems show Non-Newtonian flow: controlthrough interface chemistry� Basic quantities and situation:

Viscosities span several orders of magnitude (glaciers togases)


byelectrochemical,


Michael Bredol




Examples


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


byelectrochemical,


Michael Bredol




Examples


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


byelectrochemical,


Michael Bredol




Examples


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


byelectrochemical,


Michael Bredol




Examples


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


byelectrochemical,


Michael Bredol




Examples


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


byelectrochemical,


Michael Bredol




Examples


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


byelectrochemical,


Michael Bredol




Examples


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


byelectrochemical,


Michael Bredol




Examples


Acknowledgement

Superabsorbers

Example for branching:

Weakly interconnected polyacrylic acid (partially as salt)is used in “superabsorbers”


byelectrochemical,


Michael Bredol




Examples


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)


byelectrochemical,


Michael Bredol




Examples


Acknowledgement

Example: ball pen

Inks in ball pens need a precisely defined yield stress andshould not exhibit thixotropic hysteresis

Typical modifiers: modified montmorillonites


byelectrochemical,


Michael Bredol




Examples


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)


byelectrochemical,


Michael Bredol


Kinetics /Rheology


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 . . .


byelectrochemical,


Michael Bredol


Kinetics /Rheology


Coated particles

Layers and films

Film formation

Resisitivity

Acknowledgement

Bio–conjugates


byelectrochemical,


Michael Bredol


Kinetics /Rheology


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


byelectrochemical,


Michael Bredol


Kinetics /Rheology


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


byelectrochemical,


Michael Bredol


Kinetics /Rheology


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


byelectrochemical,


Michael Bredol


Kinetics /Rheology


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


byelectrochemical,


Michael Bredol


Kinetics /Rheology


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 !


byelectrochemical,


Michael Bredol


Kinetics /Rheology


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


byelectrochemical,


Michael Bredol


Kinetics /Rheology


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 . . .


byelectrochemical,


Michael Bredol


Kinetics /Rheology


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


byelectrochemical,


Michael Bredol


Kinetics /Rheology


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


byelectrochemical,


Michael Bredol


Kinetics /Rheology


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.


byelectrochemical,


Michael Bredol


Kinetics /Rheology


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


byelectrochemical,


Michael Bredol


Kinetics /Rheology


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


byelectrochemical,


Michael Bredol


Kinetics /Rheology


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.


byelectrochemical,


Michael Bredol


Kinetics /Rheology


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)


byelectrochemical,


Michael Bredol


Kinetics /Rheology


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

characterization of colloidal materials by … · characterization of colloidal materials by...

Documents