54 Optical properties
The Tyndall effect-turbidity
All materials are capable of scattering light (Tyndall effect) to some
extent. The noticeable turbidity associated with many colloidal
dispersions is a consequence of intense light scattering. A beam of
sunlight is often visible from the side because of light scattered by
dust particles. Solutions of certain macromolecular materials may
appear to be clear, but in fact they are slightly turbid because of weak
light scattering. Only a perfectly homogeneous system would not
scatter light; therefore, even pure liquids and dust-free gases are very
slightly turbid.
The turbidity of a material is defined by the expression
/f// 0 = exp[-T/] (3.2)
where / 0 is the intensity of the incident light beam, /t is the intensity of
the transmitted light beam, / is the length of the sample and T is the
turbidity.
Measurement of scattered light
As we shall see, the intensity, polarisation and angular distribution of
the light scattered from a colloidal system depend on the size and
shape of the scattering particles, the interactions between them, and
the difference between the refractive indices of the particles and the
dispersion medium. Light-scattering measurements are, therefore, of
great value for estimating particle size, shape and interactions, and
have found wide application in the study of colloidal dispersions,
association colloids, and solutions of natural and synthetic macro-
molecules.
Light scattering offers the following advantages over some of the
alternative techniques of particle-size analysis:
- It is absolute - no calibration is required.
- Measurements are made almost instantaneously, which makes it
suitable for rate studies.
- There is no significant perturbation of the system.
- The number of particles involved is very large, which permits
representative sampling of polydispersed samples.
