Emulsions, Microemulsions, and Lyotropic Liquid Crystals - Surface and Colloid Chemistry: Principles and Applications - page 180

O:W

Emulsion

O:E

FIGure 9.3 Emulsion region based on ratio of oil (O):water (W) versus oil (O):emulsifier (E).

This system is a very useful example for understanding what phase equilibria
are involved when three components are mixed. Some characteristics are noticeable
in this system that point out the significance of the ratios between KC and O. For
example, the aqueous phase region is extrapolated to 1 mol octanol:2 mol K-caprate.
This shows that the 1:2 ratio dominates the phase region. It has been found from
other studies (such as monolayers on water films of lipids) that such mixtures are
indeed found. The three-phase region is extrapolated to show that the ratio is 1 mol
octanol:1 mole KC. It is found that, in such complicated phase equilibria, some sim-
ple molecular ratios indicate the phase boundaries. Thus, in general, it may safely be
concluded that these molecular ratios will be useful when working with emulsions.
The observation that exact ratios exist between different components at the phase
lines suggests that some kind of molecular aggregates are formed. These correspond
to the formation liquid-crystalline structures. Confirmation of these molecular aggre-
gates has been found from monolayer studies of mixed films spread on water (Birdi,
1984, 1989). A similar conclusion was reached when investigating microemulsions
(as described in the following text).
Further, in practice, a given emulsion needs to be prepared with some specified
range of ratio between oil and water. In such cases, it may be more useful to study
mixtures of oil (O), water (W), emulsifier (E), as plots of ratios (Figure 9.3). The region
of the most suitable emulsion can be determined by studying varying mixtures.

9.2.3 e m u S l I o n S T a b I l I T y a n d a n a l y S e S

The stability of emulsions is dependent on various parameters such as size of drops
and interactions between drops. These are described in the following text.
Emulsion drop size analyses: Since stability and other characteristics (such as viscos-
ity and appearance) are known to be related to the drop size, they need to be measured.
There are commercial instruments that can be used for such analyses. These are

Coulter counter: This is the most common type where one simply counts the
number of particles or drops passing through a well-defined hole. A signal
is produced, which corresponds to the size of the particle.