1. WALLIS - Zeta Potential Analyzer
Laser Doppler Electrophoresis (LDE) principle
for Zeta potential measurements
2014/04/10

2. Why measuring Zeta potential?
Colloidal solution stability ?
Charged particles
repel each other
Stable systems
Flocculations
Uncharged particles
are free to collide and
aggregate
Coagulation
Sedimentation
Electrostatic or charge stabilization : This effect uses more natural interactions between particles through the distribution of charged species (ions) in the solution.

3. Zeta Potential: a phenomenological approach
stable
Positive Zeta
+ 30 mV
unstable
0 mV
IEP : IsoElectric Point
- 30 mV
stable
Negative Zeta pH
If surface has - charge, then Cations are attracted to the NP surface
Anions attracted to Cations, builds electric double layer
Slipping/shear plane: distance from particle surface where ions move with particle
Zeta Potential = potential (mV) at slipping plane

4. Zeta Potential determination
z: Zeta Potential (mV)
µe: electrophoretic mobility
e: medium permittivity
h: medium viscosity
a: particle radius
k: invert of double layer thickness
-1 : Debye length
f (k.a) : Henry’s function
z = µe f (k.a) h e
Calculated
Measured
Zeta is dependent of the k.a factor value
k is dependent of the solvent
General cases
1 ≤ f (k.a) ≤ 1.5
99% cases !!!

5. Electrophoresis & Electrophoretic mobility
µe: electrophoresis mobility
E: electric field
V : voltage applied
q : electric charge
Fe : electric force
Ff : friction force V
Applying an Electric field: the particles move with its surrounding layers.
Particle velocity directly related to its electrophoretic mobility and E

6. Laser Doppler Electrophoresis (LDE)
Heterodyne optical Interferometer : retrieving doppler low frequency
Splitter
d = 17°
Laser
flaser + fdoppler
flaser + fmod
Photon Counting
Module
Combiner
fdoppler
fmod
Attenuator
Optical Path Modulation
fmod
Spectrum from FFT
fmod ± fD
µe = fD
llaser
E sin d
µe: electrophoretic mobility (µmcm/Vs)
E : applied electric field
fD : Doppler frequency

7. Wallis Measurement Sequence
Heterodyning Signal
Measured
Computed
Double Layer model
FFT
Zeta
Potential z
Electrophoretic
Mobility µe
Doppler shift
Huckel ?
Smoluchowski ? fD
By LDE
(Laser Doppler Electrophoresis)
f(k.a)
Henry function

8. What defines the measurement resolution?
Detected signal fmod
Sampling Freq ≥ 2 x fmod (Shannon criteria)
Nber of sampling points limited by : buffer size, acquisition duration, calculation time

9. WALLIS : High-Resolution Zeta Potential Analysis
Purely designed and optimized for Charge/zeta potential measurement
Complementary tool to VASCO for colloid characterization
High resolution analysis down to 0.5 mV

10. Why WALLIS has higher resolution than others?
Correlator
(2 in 1 concept Zeta+DLS other supplier)
High-speed digital acquisition: Wallis
256 to channels
Fmod=256Hz
8192 sampling points
Fmod=8 kHz
G2(t)
Intensité t t
FFT
Resolution 3-5 mV
Resolution < 0.5 mV

11. Sample cell configuration : the dip cell approach
Electro-osmosis effect
Capillary 3 mm
Vitreous Carbon Electrodes
Connector
Laser beam
Hellma Cell
Quartz, Glass or
polystyrene polished
No electro-osmosis
Field proven concept
Custom made electrodes
Dip cell 10 x 10 mm
(WALLIS)
No need to focus at any ”stationary plane”

12. Benefits of WALLIS sample cell configuration
Easy to handle
Easy filling, no risk of bubbles ! (vs capillary cell/viscosity)
Stand alone electrode holder-> easy to replace
Vitreous carbon electrodes: extreme chemical resistance (No oxidation); low electrical resistance
Easy cleaning of electrodes (rinsing and ultra sound bath tub)->No cross contamination
Reusable quartz cell-> no consumable
Easy to clean
Excellent optical quality
Compatible with organic solvent
Compatible with standard cell (10x10 mm2) …
sample volume ≈750µL

13. ZetaQ software :the brain of WALLIS
Unique and proprietary software
User friendly and intuitive GUI
SOP and programmable experiments (Time, T°, pH)
Experiment storage in data base
Many other advanced functionalities

14. Performances example #1
Résolution < 0,5 mV !!!
Résolution : 3.7 mV !!!
with correlator
(other suppliers)
with fast acquisition
(WALLIS)

15. Performances example #2
IEP characterization of an industrial surfactant
WALLIS High resolution allows precise measurement of IEP even with sharp Zeta transition !

16. Further reading!
[1] Hunter R, Zeta potential in colloidal science; Academic press, New York, 1981
[2] Reliang Xu; Particle Characterization: light scattering methods; Kluwer Academic Publishers
[3] ISO13099B1,Colloidal systems—Methods for zeta potential determination Part 1:Electroacoustic and electrokinetic phenomena
[4] V. DELGADO et al.; measurement and interpretation of electrokinetic phenomena; IUPAC Technical Report , Pure Appl. Chem., Vol. 77, No. 10, pp. 1753–1805, 2005

17. Thank you of your attention!
Lab support:
Dr. Benoit MAXIT
Mail:
CORDOUAN TECHNOLOGIES
Cité de la photonique
11, Avenue Canteranne
33600 Pessac FRANCE
Customer support:
Eng. Boris PEDRONO
Mail:
CORDOUAN TECHNOLOGIES
Cité de la photonique
11, Avenue Canteranne
33600 Pessac FRANCE
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