Emulsions, Microemulsions, and Lyotropic Liquid Crystals - Surface and Colloid Chemistry: Principles and Applications - page 174

Emulsions
Microemulsions
Liquid crystals (LC; and lyotropic LC)
They are systems in which both water and oil need to be used in an application; it may
be skin treatment, or shining of shoes or a similar activity. In other words, both these
two components (water and oil), which do not mix, should be applied simultaneously.
This also allows the performance of functions that are dependent on water or oil.
In most emulsion systems, there are two liquid phases that are involved, such
as the water and the oil phase. There is thus a need for information regarding IFT
between oil and water, as well as the solubility characteristics of surface-active sub-
stances (SAS) needed to stabilize emulsions.
Microemulsions are microstructured mixtures of oil, water, emulsifiers, and other
substances. Since their structures differ in many ways from that of ordinary emul-
sions, it will be described separately. Liquid crystals (LC) are substances that exhibit
special melting characteristics. Further, some surfactant-water-cosurfactant mix-
tures may also exhibit LC (lyotropic crystal) properties.
Thus emulsion technology is basically concerned with preparing mixtures of two
immiscible substances, oil and water by adding suitable surface-active agents (such
as emulgators, cosurfactants, and polymers).
9.2 emulSIonS (oIl and Water)
When a surface-active substance is added to an oil-water system, the magnitude
of IFT decreases from 50 mN/m to 30 (or lower [less than 1] mN/m. This leads to
the observation that, on shaking, the decreased IFT of the oil-water system leads to
smaller drops of the dispersed phase (oil or water). The smaller drops also lead to a
more stable emulsion. Depending on the surfactant used, either an oil in water (O/W)
or a water in oil (W/O) emulsion will be obtained. These experiments where oil and
water, or oil and water + surfactant are shaken together, are shown in Figure 9.1.
These emulsions are all opaque since they reflect light. Some typical oil-water
IFT values are given in Table 9.1.
One of the trends revealed by these data is that, in the case of alkanes, the decrease
in IFT is much smaller with decrease in the alkyl chain than in the case of alcohols.
9.2.1 o I l -W a T e r e m u l S I oil in S
Emulsions are one of the most important structures prepared specifically for a given
application. For example, a day cream (skin cream) has characteristics and ingre-
dients different from that of a night cream. One of the main differences between
emulsions is whether oil droplets are dispersed in the water phase, or water drops are
dispersed in the oil phase. This can be determined by measuring their conductivity
because conductivity is higher for the O/W than for the W/O emulsion. Another use-
ful property is that O/W will dissolve water while W/O will not, thus showing that
W/O or O/W may be chosen depending on the application area. Especially in the