8/10/2015 Mannu Kaur 1 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
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Mannu Kaur 18/10/2015
Dynamic Light Scattering for size determination ByMannu Kaur00740801015M.Tech in Nano Science and TechnologyUniversity School of Basic and Applied SciencesGuru Gobind Singh Indraprastha University, Delhi
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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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The Colloidal SystemWhat 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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Properties of Colloids• Heterogeneous Character• Diffusibility• Visibility• Colligative Properties• Optical properties• The Brownian movement
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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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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
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Zeta PotentialDescribing 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 )
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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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Dynamic Light ScatteringIt 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 sizeSample viscosityTemperature
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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.
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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.
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Instrumentation of DLS
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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.
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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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Correlation function
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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 8/10/2015
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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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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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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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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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