Structural behaviour of mixed cationic surfactant micelles: a small-angle neutron scattering study

Area of Science:

• Physical Chemistry
• Colloid and Surface Science
• Materials Science

Background:

• Cationic surfactants self-assemble into various micellar structures in solution.
• Understanding surfactant self-assembly is crucial for applications in detergents, drug delivery, and materials science.
• The influence of surfactant structure and salt concentration on micelle morphology and transitions is complex.

Purpose of the Study:

• To investigate the self-assembly behavior of mixed cationic surfactants with varying tail lengths and salt concentrations.
• To elucidate the relationship between surfactant composition, bending elasticity, and micelle growth dynamics.
• To compare the micelle-to-bilayer transition behavior in different surfactant mixtures.

Main Methods:

• Small-angle neutron scattering (SANS) was employed to study micelle formation and structure.
• Mixtures of cetyltrimethylammonium bromide (CTAB) and dodecyltrimethylammonium bromide (DTAB) were investigated.
• Mixtures of dodecyltrimethylammonium bromide (DTAB) and didodecyldimethylammonium bromide (DDAB) were studied, with and without added salt (NaBr).

Main Results:

• Mixed CTAB/DTAB micelles unexpectedly transitioned from prolate spheroids to tablet-like structures with increasing CTAB concentration.
• Mixed DDAB/DTAB micelles elongated significantly, forming wormlike structures before transitioning to bilayers, especially in the presence of NaBr.
• Added salt (NaBr) shifted the micelle-to-bilayer transition in DDAB/DTAB mixtures, indicating a reduction in spontaneous curvature.

Conclusions:

• The distinct micelle growth behaviors in CTAB/DTAB and DDAB/DTAB mixtures are attributed to differences in bending rigidity and saddle-splay constants.
• DDAB/DTAB mixtures exhibit lower bending rigidity and higher saddle-splay constants, influencing their elongated growth and bilayer transition.
• The study provides insights into controlling micelle morphology and phase transitions through surfactant structure and salt concentration.