Analytical Ultracentrifugation - density variation

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The technique of density variation allows simultaneous determination of the size and density distributions of particles. This experiment is neccessary, when

  • you want to obtain particle size distributions for particles of unknown density
  • you want to determine particle densities
  • you want to obtain size distributions for multimodal mixtures containing species of different density
  • your system exhibits excess properties
  • ...

The procedure bases on measurements of sedimentation velocity, which makes the experiment rather cheap and quick. Sedimentation velocity is measured in two solvents of different density; combination of the resulting sedimentation coefficient distributions yields

  • a particle size distribution and
  • a particle density distribution

simultanously, where sizes and densities are unambigously assigned to each other.

integral and differential particle size distributions
Figure 1: integral and differential particle size distributions
The neighboring Figure 1 shows the integral and differential particle size distributions in black and blue, respectively. The corresponding densities are plotted in red on the right axis. In this case, a monomodal distribution with nearly constant density is observed.

From these data, a molecular mass distribution can be calculated, if the frictional properties of the particle are known. This is most easily done for spheres.

The evaluation procedure is very stable and was validated on a large variety of systems. The only prerequisite is the existence of a second solvent of different density, where the particles must behave in an identical way. Most often, the deuterated analogue is used for reasons of chemical equivalence.

An interesting aspect in dealing with hard spheres, for example polystyrene latices, is that the diameter obtained is the geometric diameter. From the raw data, the hydrodynamic diameter is obtained. This means that the experiment provides the thickness of the hydration shell.

If the particle is swollen with solvent, the obtained density is the bulk density of the pure material. This applies, for example, for polymer coils and for proteins. For the latter, this means that the most relevant property of hydration is accessible through density variation, a parameter that is most important for understanding the functionality of such molecules.

With increasing complexity of a particle, auxiliary methods must be included for total characterisation. Highly complex aggregates, such as micelles, and probably even exhibiting excess properties, can be fully characterised and yielding parameters as the diameter of the micelle core or quantising the excess properties.

Concentration and diffusion can take considerable effect on the results of a density variation evaluation. We have examined these effects and know how to handle them. Describe your system to us and we will optimize the experiment. This technique is most powerful and reliable.

More on the theoretical background can be found on our scientific website under (in German).