This document discusses dynamic light scattering (DLS) for determining particle size in colloidal systems. It explains that DLS measures the hydrodynamic diameter of particles by analyzing how light scatters off particles undergoing Brownian motion. The speed of this motion is related to particle size via the Stokes-Einstein equation. DLS has applications in characterization of proteins, polymers, micelles and nanoparticles. It also allows monitoring of particle aggregation over time.

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Dynamic Light
Scattering for size
determination
By
Mannu Kaur
00740801015
M.Tech in Nano Science and Technology
University School of Basic and Applied
Sciences
Guru Gobind Singh Indraprastha
University, Delhi

2. Outline
• Colloids
• Properties that colloids exhibit
• The Electrical double layer
• Light Scattering
• DLS measurement
• Stoke-Einstein equation
• Sample Preparation
• Applications
• Limitations
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3. The Colloidal System
What is Colloid?
It is a suspension of small particles in a continuous medium.
 The size of the particle present in a colloidal system is in
range of 1-1000 nm.
 It consists of two mediums:
(a) Dispersed phase
(b) Dispersion medium
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4. Properties of Colloids
• Heterogeneous Character
• Diffusibility
• Visibility
• Colligative Properties
• Optical properties
• The Brownian movement
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5. 1. Optical Properties : When a beam of light is passed through a
colloidal dispersion, it becomes visible as a bright streak, this
phenomenon is known as Tyndall effect.
2. Brownian Movement : The particles present in a colloid are in a
constant rapid motion. This ceaseless erratic and random motion is
similar to what is observed in case of pollen grains.
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6. The Electrical Double Layer
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Variation in the ion
density near a charged
surface.
When a charged particle
is present in an aqueous
solution it is influenced
by the ionic strength in
the solution.
• Counter-Ion: Ion of
opposite charge
• Co-Ion: Ion of equal
charge

7. Zeta Potential
Describing Colloidal Stability
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Henry´s Equation:
Used to calculate the Zeta Potential
ζ : Zeta potential.
μ : Electrophoretic mobility.
ε : Dielectric constant.
η: Viscosity
(Zetasizer Nano Series )

8. Light Scattering
The interaction of light with the electric field of a small particle or
molecule results in scattering of light.
Types of Scattering
 Rayleigh Scattering
 Mie Scattering
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9. Dynamic Light Scattering
It is technique for measuring the size of particles and
molecules in suspension.
This technique basically measures the speed of the
particles going under Brownian Motion.
Brownian motion is influenced by :
Particle size
Sample viscosity
Temperature
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10. How DLS works ?
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 In DLS we measured the speed at which the particles are
diffusing due to Brownian motion.
 Speed of diffusion is measured by measuring the rate at which
the intensity of the scattered light fluctuates.
 Small particles causes the intensity to more fluctuate than
larger.
 It measure the diffusion coefficient by using correlation
function.

11. How these fluctuation in scattered light arises ?
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Speckle Pattern for a sample
containing Stationary Particle
For the particle in
Brownian motion a
speckle pattern is
observed where the
position of each speckle
is seen to be in constant
motion Because the
phase addition from
moving particle is
constantly evolving and
forming new pattern.

12. Instrumentation of DLS
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13. DLS signal
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 Obtained optical signal shows random change due to random
change in the position of the particle.
 The “noise” is actually the particle motion and will be used to
measure the particle size.

14. Correlation function
• A correlation function is correlation between random variables
at two different points in space or time, usually as a function of
distance between the points.
• Within the correlation curve all of the information regarding
the diffusion of particles within the sample being measured.
• Correlator construct correlation function G(τ), of the scattered
intensity
G(τ)=<I(t).I(t+τ) ,τ is delay time
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15. Correlation function
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16. Stoke-Einstein equation
• Translational diffusion coefficient defines the velocity of a particle
under Brownian motion.
• The translational diffusion coefficient can be converted into a particle
size using the Stokes-Einstein equation.
Where
Dh = hydrodynamic diameter
kB = Boltzmann’s constant
T = Temperature
η = Viscosity
Dt= Diffusion Coefficient
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17. Hydrodynamic diameter
• The diameter of a hard sphere that diffuses at the same speed as the
particle or molecule being measured.
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18. Sample Preparation
The sample should consist of a dispersion of particles in a
liquid medium
The dispersant should meet the following requirements:
(a) It should be transparent
(b) Refractive Index should be different from particles
(c) Viscosity should be known with accuracy better
than 0.5%
(d) Should be compatible with the particles (i.e. not
cause swelling, dissolution or aggregation)
(e) It should be clean.
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19. Applications
• DLS is used to characterize size of various particles including
proteins, polymers, micelles, carbohydrates, and nanoparticles.
• Stability studies can be done conveniently using DLS.
Periodical DLS measurements of a sample can show whether
the particles aggregate over time by seeing whether the
hydrodynamic radius of the particle increases.
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20. Limitations
• We measure the hydrodynamic radius of the particle, not able to
measure the actual size of the particle.
• The particles having size greater than 1000nm are not measured by
this method.
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