A novel iridescent gel phase of surfactant has been found in a
ternary mixture of a double-chain surfactant
(triethanolammonium dihexadecylphosphate), water, and ethanol. The
structure of the iridescent gel
phase has been studied mainly by X-ray diffraction and ultraviolet and
visible light reflection techniques
and determined to be lamellar, having the spacing distance of a
submicrometer. The color appearance
of the solutions results from diffraction of visible light by the
lamellar structure of bilayer membranes.
It is particularly interesting to note in this system that the
iridescent color can be changed with temperature
(0−20 °C), ethanol concentration (15−48 wt %), and surfactant
concentration (1.0−2.0 wt %) as well. The
iridescent color begins to shift to the blue side at a certain critical
temperature when the temperature is
elevated. Turbidity of the solution also starts to increase at the
same critical temperature. When these
critical temperatures are connected as a function of surfactant
concentration, a curve similar to that of
the lower critical solution temperature type phase diagram is obtained.
In the two-phase region of this
diagram, the iridescent color changes with temperature, owing to the
concentration change of the condensed
(lamellar) phase, and does not change at constant temperature even when
the surfactant concentration
is altered. These novel phenomena in the present iridescent
surfactant systems can be ascribed to the
phase separation into a lamellar structure and disordered bilayers of
the surfactant gel phase. This
order−disorder phase transition is substantiated by the
freeze−fracture electron microscopic technique.
This new type of phase separation phenomenon gives us a
challenging fundamental problem on the
interactions between colloidal particles.