The Colloidal Dynamics ZetaProbe - PowerPoint PPT Presentation

Title: The Colloidal Dynamics ZetaProbe


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The Colloidal Dynamics ZetaProbe
Measuring Zeta Potential in Concentrated Colloids
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Zeta Potential of Concentrated Colloids

• ZetaProbe
• Features Benefits
• No sample dilutionsamples up to 60 volume
• Patented, multi-frequencyElectroacoustic
  technology
• Automatic titrations
• Fast, accurate IEP determinations
• Rugged dip probe sensor
• Easy to clean
• Measures pastes gels
• Automatic correction for particle size effects
• Bottom axial stirring handles high viscosities

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Colloids-background information

• Colloids are suspensions of particles in fluids.
• The particles are typically less than one micron
  in diameter.
• Examples include paints, inks, paper coatings,
  milk, blood, plus many pharmaceutical and food
  products

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Particle charge

• Colloidal particles are electrically charged
• The charge can be controlled by various means,
  including pH adjustment
• Each particle is surrounded by a diffuse cloud of
  ions of opposite charge

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The double layer
Diffuse cloud of ions
Opposite charge on particle surface

• Diffuse cloud surface charge double layer

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The Zeta Potential
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Thickness of the diffuse layer

• Diffuse layer thickness ?- -1 given by

where c is electrolyte concentration in mM
Thus ?- -1 10 nm for 1mM salt, and 1nm for
100mM salt
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The importance of ? potential

• Zeta potential affects
• rheology
• filtration/ dewatering
• shelf life
• colloid stability
• and zeta is a measure of the surface chemistry
  (eg the type of coating on the particle)

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Inter-particle force depends on ?
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The importance of ? - contd
If ? too small (lt 25mV), particles flocculate,
and...
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Zeta Potential (contd)
This affects

• Storage properties

Low ? open floc structure Easily redispersed
Moderate ? dense structure Difficult to
redisperse
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Zeta potential- contd

• Rheology- flowing suspension turns into a paste
  at low zeta

• Particles form elastic networks

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Zeta potential- contd

• The porosity and strength of floc affects
  filtration and dewatering

Flow
Filter cloth
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Controlling zeta potential

• Zeta depends on pH and electrolyte concentration,
  surfactant, polyelectrolyte...

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The Trouble with Dilution

• Most devices for measuring size and zeta only
  work on very dilute colloids
• So most samples require a lot of dilution
• Dilution is time consuming, and you can easily
  alter the zeta potential by the dilution.

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The problem with dilution

• Example Alumina sample diluted with DI water

pH
zeta
Dilution changed zeta from -15 to 60 mV.
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Why did dilution alter zeta?
Dilution path

• Because both pH and ionic strength changed.

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Trouble with dilution- contd

• The very dilute samples are easily contaminated
  by surface active materials

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Measuring without dilution

• Colloidal Dynamics has developed and patented at
  technique known as Electroacoustics for
  measuringZeta Potential in concentrated colloids

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Electroacoustic Measurement Technique

• Applied voltage across colloid generates
  soundwaves
• From soundwaves dynamic mobility of particles is
  calculated

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Measuring ESA in the ZetaProbe
Electric field applied across this gap
ESA measured on internal transducer

• Probe sits in bottom-stirred cell

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Dynamic mobility
Dynamic mobility ? is complex quantity mag(?
)V/E, arg(? )?
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Getting dynamic mobility from ESA
Instrument factor
Particle vol fraction
Solvent density
Acoustic impedance factor
Particle density
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Why is ESA linked to zeta?


The bigger ? the faster the particle moves, and
so they emit stronger sound waves
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Why is ESA linked to size?

Bigger particle has lower velocity, and it lags
behind the field

So the phase lag in the ESA is related to
particle size
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Typical mobility spectra
Mobility Spectrum of 300 nm silica slurry
Magnitudes
Arguments
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Getting size and ? from mobility

• Where ?th is theoretical mobility, a known
  function of size and ?
• p(a) is particle size distribution function.
• We adjust p(a) and ? to get best fit of mobility
  spectrum

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Mobility Spectra of several silica slurries
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ZetaProbe Advantage 3 particle size

• With the ZetaProbe you dont need to enter
  particle size
• In most colloids Dynamic Mobility given by

Inertia factor G(a) depends on radius a

• To get ? from measured Dyamic Mobility, must know
  inertia factor G(a)
• In the ZetaProbe we measure G directly
• The other electroacoustic zeta potential devices
  require the user to enter particle size

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Colloidal Dynamics

• Measurements In
• Concentrated Systems

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Spectra for dilute and concentrated silica

• Dynamic Mobility in a concentrated slurry is
  not the same as a dilute slurry
• Particle-Particle interactions lower particle
  velocity and phase shift
• Concentrated spectra look like smaller
  particles - flatter mag spectra and less phase
  lag
• OBriens theory used in ZetaProbe corrects for
  this effect to give accurate zeta potentials

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Experimental procedures

• To test the measurements, need to make up samples
  in which zeta and size stay constant as
  concentration is varied.
• This involves diluting concentrated, thin
  double-layer systems and keeping the background
  electrolyte the same.

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Supporting evidence- a careful dilution

• Alumina study by Johnson, Russell Scales

Dynamic mobility drops due to particle interaction
s in conc colloid, but...
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Supporting evidence -contd

• Alumina study by Johnson, Russell Scales- contd

Electroacoustic ? potentials are the same for
these suspensions from 3 to 30 volume
Solid line is data from DC electrophoresis data-
using OBrien White mobility formula
Note suspensions were diluted with true
background electrolyte - so ? doesnt change
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Rheology depends on zeta
Alumina study by Johnson, Russell Scales- contd

• Max yield stress
• ?ymax at IEP

• Shear stress ?y
• increases with ?

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Rheology vs zeta- contd

• Curves all collapse onto straight line vs ?2
• So from AcoustoSizer ?
• you can predict floc strength
• Also relevant for filtration
• and dewatering

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Concentrated emulsion example

• Zeta potential vs concentration
• Emulsion diluted with true supernatant

Zeta (mV)
Dilution of a 50 vol sunflower emulsion (Kong et
al)
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ZetaProbe Advantage automated titrations

• Fast and accurate titration measurement

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ZetaProbe advantage Accurate IEP

• Example of auto background correct
• Measurements on 1 and 2 wt Titania- no
  background correction

Uncertain iep?7.8
Different ?s
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ZetaProbe Advantage accurate IEPcont.

• With auto background correct

• Identical ieps (6.9 instead of 7.8) and same
  ?sfor both concentrations

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Optimizing Dispersant Dose
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Optimizng additive dosages

• Addition of Darvan C polyelectrolyte to Alumina
  as function of pH
• Zeta depends on pH and polyelectrolyte
  concentration
• When zeta depends on more than one parameter-
  manual titration would be very time consuming
• With the ZetaProbe, complete surface
  characterization can be completely rapidly

Magnitude of Zeta
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Monitoring non-ionic polymer adsorption

• Nonionic polymer on silica
• Zeta drops because hydrodynamic slipping plane
  moves further from particle surface

Use ? to monitor addition endpoint
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Studying particle coatings
Aluminium Chloride concentration (mM)

• iep changes with alumina coating hence, iep can
  be used to monitor surface coating

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Summary

• The ZetaProbe is a fast and powerful tool for
  zeta potential measurements and surface chemistry
  characterization
• The ability to measure directly in concentrated
  samples ensures accurate zeta potential values
• Automated titrations provide enormous
  productivity advantage over traditional optical
  methods that require dilution