Transition from a Sponge-Like to an Onion-Like Nanostructure in the L₃ Phase - Part I

Area of Science:

• Colloid and Surface Science
• Materials Science
• Physical Chemistry

Background:

• The H2O/NaCl - AOT system exhibits complex phase behavior.
• Understanding surfactant structures is key in various applications.
• Previous studies hinted at diverse nanostructures within L3 channels.

Purpose of the Study:

• To investigate the structural transitions within the L3 channel of the H2O/NaCl - AOT system at 25°C.
• To test the hypothesis of a sponge-like to onion-like structural transformation.
• To characterize the phase diagram concerning surfactant and NaCl mass fractions.

Main Methods:

• Phase diagram determination for the H2O/NaCl - AOT system.
• Electrical conductivity and viscosity measurements.
• Nanostructure visualization using freeze-fracture electron microscopy (FFEM) and freeze fracture direct imaging (FFDI).
• Small Angle Neutron Scattering (SANS) for characteristic length scale determination.

Main Results:

• A narrow, one-phase L3 channel was identified.
• Conductivity decreased while viscosity increased with rising surfactant and salt concentrations.
• Experimental evidence supports a structural transition from sponge-like to onion-like structures.
• All observed structures and their coexistences are thermodynamically stable.

Conclusions:

• The L3 channel in the H2O/NaCl - AOT system hosts a structural transition from sponge-like to onion-like phases.
• This transition is influenced by surfactant and NaCl mass fractions.
• The findings provide a deeper understanding of self-assembly in surfactant systems.