Page 71 - Introduction to Colloid and Surface Chemistry
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Optical properties 61
o Experimental points
• Extrapolated points
. o o .
sin-'— + kc
Figure 3.8 A Zimm plot
Particles which are too small to show a series of maxima and
minima in the angular variation of scattered light are frequently
studied by measuring the dissymmetry of scattering (usually defined
as the ratio of the light scattered at 45° to that scattered at 135°). The
dissymmetry of scattering is a measure of the extent of the particles
compared with A. If the molecular or particle size is known, it can be
related to the axial ratio of rod-like particles or the coiling of flexible
linear macromolecules.
The application of light scattering to the characterisation of
colloidal systems has advanced rapidly over the last few decades. This
has been made possible by the development of (a) lasers as intense,
coherent and well-collimated light sources, (b) sophisticated electronic
devices for recording data, and (c) computers for the complex data
processing that is involved.
Dynamic light scattering
The precisely defined frequencies associated with laser sources makes
it possible to exploit light scattering to study the motion of colloidal
particles. Light scattered by a moving particle will experience a
Doppler shift to slightly higher or lower frequency depending on
whether the particle is moving towards or away from the observer.
For a collection of particles moving at random by virtue of their
Brownian motion, a Doppler frequency broadening will result.
