Page 248 - Introduction to Colloid and Surface Chemistry

Colloid stability 23?

where
2 \
+jry+ x
x +xy + x x + xy + x + y \x + xy + x + y )

with x and v defined as follows:

forF 12, x
forF 22, *
forF 12, *

209
Langbein has derived a corresponding expression for this so-
called 'Void effect' which is based on the Lifshitz macroscopic
treatment of dispersion forces.

Steric stabilisation

The stability of many 'protected' colloidal dispersions cannot be
explained solely on the basis of electric double layer repulsion and
van der Waals attraction; other stabilising mechanisms must be
investigated. 'Steric stabilisation' is a name which is used (somewhat
loosely) to describe several different possible stabilising mechanisms
involving adsorbed macromolecules. These include the following:

1. An encounter between particles could involve desorption of
stabilising agent at the point of contact. Since adsorption is a
spontaneous process, AG ads. is negative and AGd es. positive. This
positive free energy of desorption corresponds to particle-particle
repulsion and enhanced stability. However, for polymeric material,
adsorption and desorption processes tend to be slow compared
with the time of a typical particle-particle encounter" and so, in
such cases, the attainment of a 'primary minimum' coagulated
condition is unlikely.
2. When particles collide, their adsorbed layers may be compressed
without penetrating into one another. This 'denting' mechanism
will reduce the configurations available to the adsorbed polymer
molecules; therefore, there will be a decrease in entropy and an
increase in free energy, and stability will be enhanced by an elastic