High-resolution structural elucidation of extremely swollen lyotropic phases

Area of Science:

• Colloid and Surface Science
• Materials Science
• Soft Matter Physics

Background:

• Lyotropic phases, particularly microemulsions, are crucial in formulation science.
• Swollen microemulsions are achieved by increasing the bending modulus of surfactant films.
• Previous studies reported cubic phases in specific pseudo-quaternary systems.

Purpose of the Study:

• To reinvestigate the phase behavior of a specific pseudo-quaternary system (decane, brine, octanol, sodium dodecyl sulfate) under conditions favoring swollen microemulsions.
• To clarify the structural characteristics of lyotropic phases in this system using advanced scattering techniques.
• To determine if the previously reported cubic phase exists in this region of the phase diagram.

Main Methods:

• High-resolution small-angle X-ray scattering (HR-SAXS) for detailed structural analysis.
• Rheo-small-angle X-ray scattering (rheo-SAXS) for sample homogenization and structural unit orientation.
• Measurements spanning over three orders of magnitude in scattering vector.

Main Results:

• The formation of a swollen two-dimensional hexagonal phase was confirmed.
• Structural features indicative of a cubic phase were not detected.
• Long correlation lengths, exceeding 2000 nm for the hexagonal phase, were observed.
• The structure is characterized by elongated, swollen emulsion droplets forming ordered arrangements.

Conclusions:

• The reinvestigation did not detect the cubic phase previously reported in this specific microemulsion system.
• A swollen hexagonal phase with unusually large lattice spacings is the predominant structure.
• The findings suggest an organization of elongated emulsion droplets, forming ordered hexagonal morphologies.