Phase Transition of a Quaternary Ammonium Gemini Surfactant
Induced by Minor Structural Changes of Protic Ionic Liquids
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Abstract
The aggregation behaviors of a Gemini
surfactant [C<sub>12</sub>H<sub>25</sub>(CH<sub>3</sub>)<sub>2</sub>N<sup>+</sup>(CH<sub>2</sub>)<sub>2</sub>N<sup>+</sup>(CH<sub>3</sub>)<sub>2</sub>C<sub>12</sub>H<sub>25</sub>]Br<sub>2</sub><sup>–</sup> (12-2-12) in two
protic ionic liquids (PILs), propylammonium nitrate (PAN) and butylammonium
nitrate (BAN), were investigated by means of several experimental
techniques including small and wide-angle X-ray scattering, the polarized
optical microscopy and the rheological measurement. Compared to those
in ethylammonium nitrate (EAN), the minor structural changes with
only one or two methylene units (−CH<sub>2</sub>−) increase
in cationic chain length of PIL, result in a dramatic phase transition
of formed aggregates. The critical micellization concentration was
increased in PAN, while no micelle formation was detected in BAN.
A normal hexagonal phase was observed in the 12-2-12/PAN system, while
the normal hexagonal, bicontinuous cubic, and lamellar phases were
mapped in the 12-2-12/BAN system. Such aggregation behavior changes
can be ascribed to the weaker solvophobic interactions of 12-2-12
in PAN and BAN. The unique molecular structure of 12-2-12 is also
an important factor to highlight such a dramatic phase transition
due to the PIL structure change
