Brownian Motion First observed in 1827 by the botanist Robert Brown. He was looking at pollen grains under a microscope. The force of life? Desaulx in 1877: "In my way of thinking the phenomenon is a result of thermal molecular motion in the liquid environment (of the particles).” The mathematical theory of Brownian motion was developed by Einstein in 1905. Jean-Baptiste Perrin verified Einstein's analysis (Nobel Prize 1926).
Brownian Motion Explanation: A suspended particle is constantly and randomly bombarded from all sides by molecules of the liquid. If the particle is very small, the number of hits it takes from one side at a given time will be stronger than the bumps from other side. This make the particle jump. These small random jumps are what make up Brownian motion. Stoke-Einstein relation:
Applications of DLS Size: Using Stoke-Einstein equation DLS can be used to easy, fast and accurate determination of the hydrodynamic radius of particles. Typically range: 1 nm – 1μm. Shape: Ellipsodial particles results in a small fraction depolarized scattered light. Can be used for estimation of ellipticity of the particles. Difficult!
Examples Some examples of sub-micron systems: –Micro-emulsions –peptides –micelles –macromolecules –polymers –paint pigments –bacteria, viruses
The influence of pH on silica nanoparticles Distribution of sizesExp Data
Estimation of Ellipsodial objects Lysozome Hydrodynamic size Calculated hard sphere Calculated ellipitic shape
Size Distribution of Insulin Diameter (nm) Intensity
The influence of pH on Insulin Diameter (nm) Amplitude
Research using DLS Determination of size on complex systems: –“water-in-oil” –bio-molecules –cellulose Glass transition dynamics Polymer dynamics
Relaxations and Conductivity in Polymer Gel Electrolytes Nernst-Einstein equation …there is a close connection between the fast diffusive process and the ionic conductivity!
Outlook: X-ray PCS Exemplary correlation functions of colloidal silica suspension measured at q ~ 7.6 × 10 -4 Å -1 using three different X-ray energies as indicated.
Summary: DLS technique Probes density and/or concentration fluctuations. Time scales:~10 -6 - 10 3 s Wave vectors:~10 -3 Å -1 Standard characterisation techniques for particles –Determination of size1 nm - 1 m –Estimation of ellipticity and/or swelling Research –Polymer dynamics –Glass transition dynamics
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