92168.pdf

32 Kinetic properties

Dynamic light scattering (see page 61)

The ultracentrifuge

There are a number of experimental techniques (for example, the
Wiegner tube and the Oden balance)^29 which make use of sedimenta-
tion under gravity for fractionating or determining particle-size
distributions in systems that contain relatively coarse suspended
material such as soils and pigments. A popular method of carrying
out a cumulative sedimentation size analysis is the balance method, in
which the weight of particles settling out on to a balance pan is
recorded as a function of time. Sedimentation under gravity has a
practical lower limit of c. 1 /xm. Smaller (colloidal) particles sediment

Table 2.2 Sedimentation rates under gravity for uncharged spheres of density 2 g cm"^1

in water at 20°C, calculated from Stokes' law

Radius Sedimentation rate

1(T^9 m (1 nm) 2.2 x 1<T^12 m s"^1 (8 nm h'^1 )

10~^8 m (10 nm) 2.2 x lO^ms"^1 (0.8/Amh"^1 )

Kr^7 m(100nm) 2.2 x KT^8 m s"^1 (80//.m h"^1 )

lQ~^6 m(l^m) 2.2 x lO^ms"^1 (8 mm h"^1 )

10~^5 m (10 /am) 2.2 x 10~^4 m s"^1 (0.8 m h"^1 )

so slowly under gravity that the effect is obliterated by the mixing
tendencies of diffusion and convection.
By employing centrifugal forces instead of gravity, the application
of sedimentation can be extended to the study of colloidal systems,
and has been used, in particular, for the characterisation of
substances of biological origin, such as proteins, nucleic acids and
viruses^30 "^31. The driving force on a suspended molecule or particle
then becomes m(l—vp)o>^2 jt, where «is the angular velocity and x the
distance of the particle from the axis of rotation.
An ultracentrifuge is a high-speed centrifuge equipped with a
suitable optical system (usually schlieren or interference optics, the
latter being particularly useful when low concentrations are involved)
for recording sedimentation behaviour and with facilities for eliminating
the disturbing effects of convection currents and vibration. The sample
is contained in a sector-shaped cell mounted in a rotor (usually c.