The effect of added decane, m-xylene, or squalane on
the phase behavior of the polyoxyethylene dodecyl
ether (C12EO
n
) −water system
was investigated as a function of polyoxyethylene (EO) chain length at
25 °C.
When the surfactant is relatively lipophilic (more lipophilic than
the balanced state), the type of liquid crystal
is changed to the more lipophilic one upon addition of decane. The
lamellar (Lα) to lipophilic reverse
hexagonal (H2) transition takes place in the
C12EO3 system. On the other hand, the
hexagonal (H1) to
hydrophilic discrete cubic (I1) liquid crystal transition
occurs in the hydrophilic C12EO7 system.
There are
two kinds of effects of oil on the self-organizing structures. One
is the “penetration effect”, in which oil
molecules penetrate into the surfactant palisade layer and expand the
effective cross-sectional area, a
S.
The
other is “swelling effect”, in which oil molecules are solubilized
in the core of aggregates and expand the
volume of aggregates. In this case, the a
S
is almost constant. Due to the swelling effect, the
H1−I1 phase
transition takes place in the
decane−C12EO7 system, whereas the
Lα−H2 transition occurs in the decane−lipophilic C12EO3 system due to the
penetration effect. This causes the opposite tendency of the
change in
the curvature of the surfactant layers in liquid crystals on each side
of the balanced state. As a result, the
phase behavior of surfactant is quickly changed from forming micelles
to reverse micelles within a narrow
range of the EO chain in the presence of oil. On the other hand,
m-xylene tends to penetrate the surfactant
palisade layer, and the H1−Lα transition occurs even in
the hydrophilic C12EO7
system.