Structural transitions of CTAB micelles in a protic ionic liquid

Area of Science:

• Materials Science
• Physical Chemistry
• Soft Matter Physics

Background:

• Ionic liquids offer unique solvent properties for self-assembling systems.
• Understanding micellar behavior in ionic liquids is crucial for developing novel materials.
• Hexadecyltrimethylammonium bromide (CTAB) is a common surfactant used in self-assembly studies.

Purpose of the Study:

• To investigate the phase behavior of CTAB in the protic ionic liquid EAN.
• To characterize the isotropic (L(1)) to hexatic (Hex) phase transition in CTAB/EAN micellar solutions.
• To explore the relationship between structure, rheology, and conductivity across different concentration regimes.

Main Methods:

• Shear rheology to measure viscosity changes.
• Polarizing optical microscopy (POM) for texture analysis.
• Small angle neutron scattering (SANS) for structural characterization.
• Conductivity measurements to probe ionic mobility.

Main Results:

• Three concentration regimes were identified: dilute, semi-dilute, and concentrated.
• A reversible, temperature-dependent L(1)-Hex phase transition was observed in the concentrated regime.
• The transition is marked by increased viscosity, formation of focal conic textures, and enhanced crystalline order (Bragg reflections in SANS).
• Ionic conductivity remained unaffected by the L(1)-Hex transition.

Conclusions:

• The study provides the first experimental observation of the predicted L(1)-Hex phase transition in CTAB/EAN.
• The observed transition is a structural rearrangement rather than a topological one, as indicated by conductivity measurements.
• This work advances the understanding of self-assembly in ionic liquid-based systems and their potential applications.