Page 54 - Introduction to Colloid and Surface Chemistry

44 Kinetic properties
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ratio a /(a+2b) small and allow the Donnan effect to be virtually
eliminated.

Rotary Brownian motion

In addition to translationai Brownian motion, suspended molecules
or particles undergo random rotational motion about their axes, so
that, in the absence of aligning forces, they are in a state of random
orientation. Rotary diffusion coefficients can be defined (ellipsoids of
revolution have two such coefficients representing rotation about
each principal axis) which depend on the size and shape of the
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molecules or particles in question .
Under the influence of an orientating force partial alignment of
asymmetric particles takes place, which represents a balance between
the aligning force on the particles and their rotary diffusion. The
system will therefore become anisotropic. It is possible to draw
conclusions concerning particle dimensions by studying various
changes in physical properties brought about by such particle
alignment.

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Streaming birefringence
The sample is subjected to a strong velocity gradient - for example, in
a concentric cylinder viscometer (Figure 2.12) - and the resulting
molecular or particle alignment causes the previously isotropic
solution to become doubly refracting (birefringent). The magnitude
of the birefringence is related in an elaborate theory to the rotational
diffusion coefficient, and, hence, to the molecular or particle
dimensions.
In certain cases (e.g. iron(III) hydroxide sol) birefringence can be
produced by the aligning action of electrical or magnetic fields.

Dielectric dispersion

When a solution containing dipolar molecules is placed between
electrodes and subjected to an alternating current, the molecules
tend to rotate in phase with the current, thus increasing the dielectric
constant of the solution. As the frequency is increased, it becomes
more difficult for the dipolar molecules to overcome the viscous