What are the differences between stokes radius, radius of gyration and hydrodynamic radius ?? and how can they be measured (size exclusion chromatography, DLS, SAXS) ??
Strongly recommend that you look up each term on the web. That way you can learn about them and answer any questions that you may have. Learning how to research questions on your own is a critical skill that is best acquired as early as possible in your studies.
Hi, Mohammad.
I also know about this definition problem. As far as I know there is a big difference about the methods in use and the subject of interest. Rules of thumb often do not work out for proteins and most of the definitions were set up for Polymers. Typically, radius of gyration is obtained from Neutron scattering and hydrodynamic Radii are obtained from DLS. The Stokes Radius should be intrinsically related to the hydrodynamic Radius and the Radius of gyration Deals more with the inner Compactness of a molecule that also Depends on the axial Ratio of the shape. As William said, a thorough Literature survey would be the best choice. Then you will get a feeling for these values. Good luck.
Best, joerg
The Stokes radius or the Stokes-Einstein radius of a solute is the radius of a hard sphere that diffuses at the same rate as that solute. Hydrodynamic radius (Rh) and Stokes radius are synonims. It is the radius of a sphere with the same diffusion coefficient as the sample. Dynamic Light Scattering (DLS) measures the translational diffusion coefficients Dt of nanoparticles and colloids in a solution by quantifying the dynamic fluctuations in scattered light. DLS utilizes the time-dependent fluctuations of scattered intensity, which arise from Brownian motion, in order to determine the diffusion constant. Sizes and size distributions are calculated with Stokes-Einstein equation, from the diffusion coefficients, in terms of hydrodynamic radius.
On the other hand, MALLS examines the angular dependence of the time-averaged scattering intensity to determine the mass-averaged root mean square radius, so-called "radius of gyration" (Rg). For its determination no refractive index increment (dn/dc) is needed.
Thanks Mr. Simulescu for this clear explanation.
Can we perform DLS for any solution?? or it is just restricted to colloids or suspension (as nanoparticles)?
In other words, can we determine hydrodynamic radius for whatever solute?
In principle, because DLS and MALLS measures the scattered light, you need dipoles which scatter the light, so you need particles in suspension or polymers in solution.
The Stokes radius Rs is also sometimes defined as the hydrodynamic radius at infinite dilution.
As long as there is sufficient scattering, and you have (freely) moving objects in a liquid, DLS should work, it is amazingly versatile:
http://www.materials-talks.com/blog/2017/03/07/can-i-use-dls-for-xyz/
The smallest you can easily memasure yourself is sucrose in water: it is about 0.6nm in diameter.
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