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near their freezing point had indicated that surface tension changes in abrupt steps.
X-ray scattering of liquid surfaces indicated similar behavior (Wu et al., 1993).
However, it was found that lower-chain alkanes (C
16
) did not show this behavior. The
crystallization of C
16
at 18°C shows an abrupt change due to the contact angle change
at the liquid-Pt plate interface (Birdi, 1997). It was found that, in comparison to the
C
16
-air interface, one observes supercooling (to ca. 16.4°C). Each data point corre-
sponded to 1 s, and the data showed that crystallization is very abrupt. High-speed
data (<<1 s) acquisition is needed to determine the kinetics of transition. This kinetic
data would add more information about molecular dynamics at the interfaces, and
the effect of additives to the aqueous phase, such as proteins. The magnitude of IFT
is 12.6 and 4 mN/m for bovine serum albumin (BSA) and casein, respectively. These
difference are described in detail in the literature (Birdi, 2002).
9.3 mIcroemulSIonS
As mentioned earlier, ordinary emulsions as prepared by mixing oil, water, and
emulsifier are thermodynamically unstable. That is, such an emulsion may be stable
over a length of time, but it will finally separate into two phases (the oil phase and the
aqueous phase). They can also be separated by centrifugation. These emulsions are
opaque, which means that the dispersed phase (oil or water) is present in the form of
large droplets (more than a micrometer and thus visible to the naked eye).
A
microemulsion
is defined as a thermodynamically stable and clear isotropic
mixture of water-oil-surfactant-cosurfactant (in most systems, it is a mixture of
short-chain alcohols). The cosurfactant is the fourth component, which effects the for-
mation of very small aggregates or drops that make the microemulsion almost clear.
Microemulsions are also characterized as microstructured, themodynamically
stable mixtures of water, oil, surfactant, and additional components (such as cosurfac-
tants). The study of microemulsions has shown that they are of the following types:
Microdroplets of oil in water or water in oil
Bicontinuous structure
Emulsifier will be found in both these phases. On the other hand, in systems with
four components (Figure 9.4), consisting of oil-water-detergent-cosurfactant, there
exists a region where a clear phase is found. This is the region where microemulsions
are found.
Microemulsions are thermodynamically stable mixtures. The interfacial tension
is almost zero. The size of drops is very small, and this makes the microemulsions
look clear. It has been suggested that microemulsion may consists of bicontinuous
structures, which sounds more plausible in these four-component microemulsion
systems. It has also been suggested that microemulsion may be compared to swollen
micelles (i.e., if one solubilizes oil in micelles). In such isotropic mixtures, short-
range order exists between droplets. As found from extensive experiments, not all
mixtures of water-oil-surfactant-cosurfactant produce a microemulsion. This has
led to studies that have attempted to predict the molecular relationship.
Microemulsions have been formed by one of following procedures:
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