92168.pdf

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 , *

Langbein^209 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, AGads. is negative and AGdes. 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