92168.pdf

Kinetic properties 33

Speedometer

-Air turbine

Rotor

chamber

Schlieren optical

system

Figure 2.6 Essential features of an air-driven ultracentrifuge

18 cm diameter); this spins in a thermostatted chamber containing
hydrogen at a reduced pressure. Several mechanisms for driving the
rotor have been investigated - Svedberg, who pioneered this field,
employed an oil turbine; these have been superseded by simpler and
less expensive air-driven and electrically driven instruments.
The ultracentrifuge can be used in two distinct ways for investigat-
ing suspended colloidal material. In the velocity method a high
centrifugal field (up to c. 400 000 g) is applied and the displacement of
the boundary set up by sedimentation of the colloidal molecules or
particles is measured from time to time (Figure 2.7). In the
equilibrium method the colloidal solution is subjected to a much
lower centrifugal field, until sedimentation and diffusion (mixing)
tendencies balance one another and an equilibrium distribution of
particles throughout the sample is attained.

Sedimentation velocity

Equating the driving force on a macromolecule in a centrifugal field
with the frictional resistance of the suspending medium,

2 _ djc

dr