Analytical Ultracentrifugation: Concentration Dependence of Sedimentation

The determination of ks, the concentration dependence of sedimentation, is essential if you want to know more about the interparticular interactions in your system. This constant quantifies interactions between sedimenting particles, mostly due to charge effects and shapes differing from spherical. Other effects, like the "hardness" or "softness" of a sphere can contribute to ks.

The evaluation of ks is often neccessary in advanced evaluations to eliminate concentration effects that might effect further results considerably.

But ks is not only an inevitable correction with strong interactions. This property is connected to others, like intrinsic viscosity or the second osmotic virial coefficient by scale relationships. These can be utilized to construct a full-scale hydrodynamic model of the particle in question.

determination of concentration dependence
Figure 1: determination of concentration dependence

A series of six or seven concentrations is sufficient for a reliable determination of concentration dependence. First, sedimentation velocity is determined. A reciprocal plot of mean sedimentation constants versus concentration yields ks. Further, the sedimentation constant at infinite dilution is obtained, a valuable parameter for hydrodynamic calculations.

Although the mean sedimentation coefficient can be evaluated in this way, we examine total distributions to provide higher statistical significance.

This is one of the more expensive measurements on the Analytical Ultracentrifugation, but you should consider:

  • ks contains interesting information on particle properties, it is more than a (sometimes) neccessary correction.
  • the experiments required for ks determination can be otherwise evaluated, for example, for size distributions or density variations.
  • Analytical Ultracentrifugation is an absolute method. This means that some effects must be corrected for and making such measurements neccessary, but on the other hand, no calibration is required.

Often enough, ks correction may be omitted. For instance, this is mostly the case when dealing with hard spheres.

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