PHYSICAL CHEMISTRY IN BRIEF

CHAP. 14: DISPERSION SYSTEMS [CONTENTS] 459

14.2 Properties of colloid systems

Systems containing colloid particles are in some properties different from systems which are
homogeneous or composed of macroscopic phases.

14.2.1 Light scattering

If light passes through a system containing colloid particles, part of the light is scattered and
consequently the ray passage through the environment can be observed (the Tyndall effect).
For the intensityI of light passing through a dispersion environment we write (compare with
relation (12.16)

ln

I

I 0

=−τ l , (14.1)

wherelis the length of the dispersion environment through which the light passes (optic path),
τ is the turbidity coefficient, I 0 is the intensity of the initial light. A decrease in the light
intensity in the original direction is not due to the absorption of light by the molecules of
the substance, but it is caused by its scattering to all directions by reflection from the colloid
particles. The initial wavelength of the light remains preserved during its scattering.
For the total intensity of scattered lightIscat

Iscat=I 0 −I (14.2)

theRayleigh formulaapplies

Iscat

I 0

= 24π^3

N v^2 particle

λ^4

(

n^21 −n^20

n^21 + 2n^20

) 2

, (14.3)

whereNis the number of particles of the volumevparticlein unit volume,λis the wavelength of
the incident light,n 0 is the refraction index or the dispersion environment, andn 1 is refraction
index of the dispersion ratio. From this relation it is obvious that

• In order for scattered light to originate, the diffraction index of the dispersion ratio and
  that of the dispersion environment have to be different.


• Light scattering is proportional to the number of particles in the system,


• The scattered light is proportional to the quadrate of the particles volume,