Hydrodynamic radius - what is it?
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Hydrodynamic radius - what is it?

Molecules in solutions are undergoing constant diffusion. This diffusion is the result of thermal fluctuations in the suspension and is often referred to as “Brownian motion”. The diffusion coefficient is related to the size of the diffusing object. It was found that the diffusion coefficient of a spherical particle with radius RH in a medium of viscosity η and at temperature T will diffuse with the coefficient:

D = kT / 6πηRH

This is also known as the Stokes-Einstein relationship. If the diffusion coefficient is measured and the viscosity of the solvent is known, then the hydrodynamic radius can be calculated from the Stokes Einstein equation. The hydrodynamic radius is the effective size of the molecule as detected by its diffusion.

 
  
Presentations:
 
 


Requires free registration and loginOn demand presentation on "What is the hydrodynamic radius?".
In dynamic light scattering (DLS) intensity fluctuations of the scattering light are analyzed. The fluctuation rate is related to the diffusion coefficient of the scattering species. The radius of a hard sphere with the same diffusion coefficient is the hydrodynamic radius RH. Additional radii such as the rotation radius, a mass radius and a radius of gyration are also defined. These different radii are shown for a lysozyme molecule. Comparison of the hydrodynamic radius of larger proteins (insulin, immunoglobulin G, thyroglobulin) shows the range of values for biomolecules.
 
 


Requires free registration and loginOn demand presentation on: Dynamic Light Scattering (DLS) Primer.
Scattered light is emitted from scattering objects in dispersion. The Brownian motion of the scattering objects produces intensity fluctuations in the signal. Analysis of these fluctuations using the correlation function yields the diffusion coefficient or the hydrodynamic radius (using the Stokes-Einstein equation) of the objects. Small molecules diffuse quickly and produce a fast decay in the correlation function, larger particles diffuse more slowly. The decay rate of the correlation function is related to the z-average particle size. A multi-exponential analysis can produce a size distribution, providing insight into the presence of different species inside a sample. DLS is non-invasive, highly sensitive and requires very little sample volume.
 
 


Publication: A Primer on particle sizing using dynamic light scattering.