Kinetic properties via the ultracentrifuge

The sedimentation method based on gravity has a practical lower limit of about 1 pm. Smaller colloidal particles sediment so slowly under gravity that the effect is disturbed by the mixing tendencies of diffusion and convection. Even the largest macromolecules have effective radii of not much more than 10 m (10 nm), so that their sedimentation cannot be observed in the gravitational field of earth. 

A typical numerical example is given here for estimation of the angular velocity and the acceleration of the centrifugal field (remember that g = 9.80 m s ). 

The distance from the axis of rotation, r, is 6 cm. The angular acceleration, a, can now be calculated as a = ap-r= (23rl000 s- ) (0.06 m) = 2.36x 10 ms- = 240000g With such high accelerations even molecules as small as sucrose can sediment at measurable rates. 

Sedimentation experiments using an ultracentrifuge are usually presented based on the obtained values of the sedimentation coefficient (s). For a particle moving in a circle of radius R with an angular velocity of the centrifuge co, then the acceleration in the direction of this radius is o/R. 

In the above definition of s, x is the distance of the particle from the rotation axis, and it has dimensions of time. It is often expressed in Svedberg units (1 S = 10 s), in honour of T. Svedberg who pioneered this field and introduced the use of ultracentrifugation in the study of colloidal systems. 

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