Chemistry Reference
In-Depth Information
Oil-water mixture is added to a surfactant. To this emulsion, a short-chain
alcohol (with four to six carbon atoms) is added continuously until a clear
mixture (microemulsion) is obtained. Microemulsions will exhibit very
special properties, quite different from those exhibited by ordinary emul-
sions; the microdrops may be considered as large micelles.
A very typical microemulsion, extensively investigated, consists of a mixture
of
SDS + C
6
H
6
+ water + cosurfactant (C
5
OH or C
6
OH)
The phase region is determined by adding various mixtures (approximately
20 samples) and allowing the system to reach equilibrium under controlled
temperature. In the literature, the following recipe is found (Birdi, 1982):
Mix 0.0032 mol (0.92 g) SDS (mol. wt. of SDS (C
12
H
25
SO
4
Na) = 288) with
0.08 mol (1.44 g) water and add 40 mL of C
6
H
6
. This mixture is agitated
by vigorous stirring, and a creamy emulsion is obtained. To this three-
component mixture, a cosurfactant (C
5
H
11
OH or C
6
H
13
OH) is added under
slow stirring until a clear system consisting of a microemulsion is obtained.
The stability region is found to be a relation between surfactant-water and
surfactant-alcohol. This shows that, at the molecular level, a liquid crystal
structure indeed involved. The size of oil droplets is under a microgram,
and therefore the mixture is clear (Birdi, 1982). These data clearly indicate
that the microemulsion phase is formed at certain fixed surfactant:water
and cosurfactant:oil ratios.
It is important to consider the different stages when producing microemulsions from
macroemulsions. It was mentioned earlier that surfactant molecules orient with the
hydrophobic group inside the oil phase, while the polar group orients toward the
water phase. The orientation of surfactants at the interfaces cannot be measured by
any direct method, although much useful information can be obtained from mono-
layer studies of the air-water or oil-water interfaces.
At present, it is generally accepted that a microemulsion recipe cannot be easily
predicted. However, some suggestions have been made regarding certain criteria,
which may be summarized as follows:
The HLB value of the surface needs to be determined (for deciding the O/W
or W/O type).
The phase diagram of the water-oil-surfactant (and cosurfactant) needs to
be determined.
The effect of temperature is found to be very crucial.
The effect of added electrolytes is of additional importance.
Recently, the phase equilibria of a microemulsion were reported. The phase behavior
of a microemulsion formed with food-grade surfactant sodium bis-(2-ethylhexyl)
sulfosuccinate (AOT) was studied. Critical microemulsion concentration (cμc) was
deduced from the dependence of the pressure of cloud points on the concentration of
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