Shape control through molecular segregation in giant surfactant aggregates

Area of Science:

• Colloid and Surface Science
• Materials Chemistry
• Supramolecular Chemistry

Background:

• Cationic and anionic surfactants can self-assemble into complex colloidal structures.
• Controlling the morphology of these self-assembled structures is crucial for various applications.
• The role of surfactant ratios in dictating colloidal morphology is not fully understood.

Purpose of the Study:

• To propose and validate a general mechanism for how surfactant ratios control colloidal structure.
• To investigate the molecular segregation process during cocrystallization.
• To understand the relationship between molecular composition and emergent macroscopic shape.

Main Methods:

• Cocrystallization of cationic and anionic surfactants at various ratios without added salt.
• Freeze-fracture electron microscopy (FFEM) to visualize colloidal structures.
• Fluorescence confocal microscopy to confirm molecular segregation of surfactant components.

Main Results:

• Surfactant ratio dictates colloidal shape, ranging from nanodiscs to punctured planes.
• Excess surfactant accumulates on edges or pores, not within crystalline bilayers, via molecular segregation.
• Identified three distinct colloidal states and their coexistence regimes using FFEM.
• Demonstrated consistent shape reproducibility upon thermal cycling, indicating thermodynamic stability.

Conclusions:

• The initial mole ratio of surfactants is the primary determinant of colloidal shape.
• Molecular segregation is the key mechanism driving shape control in these systems.
• The icosahedral shape represents a stable minimum of bending energy for facetted structures with segregated excess material.