4? 106 J?m^3. In other words, full coalescence would lead to an increase of
the temperature of the system (provided it is isolated) by 1.5 mK. Hence the
free energy decrease involved in changes in dispersity generally is quite
small.
The most important conclusion is, however, that the stability is not at
all related to the value ofDG. If the aqueous phase has a viscosity like that
of water, creaming will spontaneously occur and be clearly observable
within a day. On the other hand, the emulsion may be stable against
significant coalescence and oil oxidation for several years. The time needed
for visible aggregation to occur may vary from one to 10^6 minutes, or even
longer. We thus need to study the kinetics of the various processes. They
may be slow because of a high activation free energyDG{, or because the
transport process involved is impeded, e.g., owing to a high viscosity. Only
in a limited number of cases is the change slow because the driving force is
small.
It may finally be noted that in many systems instability—expressed as
a rate of change—may vary with time, e.g., owing to some reaction causing
a change in pH or viscosity.
13.2 AGGREGATION
Two particles are said to be aggregated if they stay together for a much
longer time than they would do in the absence of colloidal interaction forces.
In the absence of such forces, the time together (i.e., the time needed for two
particles to diffuse away from each other over a distance of about 10 nm)
would often be on the order of some milliseconds. It may be noticed that the
termsflocculationandcoagulationare also used, where the former is, for
instance, used to denote weak (reversible) and the latter strong (irreversible)
aggregation.
In this section, the kinetic aspects of aggregation will especially be
discussed. Most of the theory derived is valid for the ideal case of a dilute
dispersion of monodisperse hard spheres. Most food dispersions do not
comply with these restrictions. Where possible, the effects of deviations from
the ideal case will at least be mentioned. Some consequences of aggregation
are also discussed.
Note Particles can aggregate because of the attractive free energy
acting between them. However, aggregation will decrease the
translational mixing entropy of the particles, and that would
counteract aggregation. It can be derived that the average free