Molecular dynamics simulations of mixed cationic/anionic wormlike micelles.

Area of Science:

• Colloid and Surface Science
• Materials Chemistry
• Computational Chemistry

Background:

• Wormlike micelles are crucial in various applications, but their stability and formation mechanisms, especially in mixed cationic/anionic systems, require deeper understanding.
• Previous experimental studies have observed phase transitions and structural changes in these systems, necessitating molecular-level explanations.

Purpose of the Study:

• To simulate mixed cationic/anionic wormlike micellar systems across various compositions.
• To elucidate the molecular mechanisms behind the stability and structural transformations of wormlike micelles.
• To investigate intermicellar interactions in systems with zero surface charge density.

Main Methods:

• Molecular dynamics (MD) simulations were performed for mixed cationic/anionic wormlike micellar systems.
• Simulations covered a range of compositions, from pure anionic to pure cationic systems.
• Analysis focused on micelle stability, structural parameters (radius, length), and intermicellar interactions.

Main Results:

• Pure cationic wormlike micelles are unstable, transforming into spherical micelles.
• Stable wormlike micelles are formed upon addition of anionic surfactants with short hydrophobic chains.
• A 50/50 cationic/anionic mixture shows a flattened worm structure, indicating a phase transition to the lamellar phase.
• Increased cationic surfactant concentration reduced worm length, while octyltrimethylammonium chloride addition increased worm radius.

Conclusions:

• The simulations provide a molecular mechanism for experimentally observed phenomena in mixed wormlike micellar systems.
• The 50/50 mixture, exhibiting zero surface charge density, is crucial for studying intermicellar interactions.
• Substantial activation energy is required for micelle merging, even with diminished electrostatic repulsion, highlighting strong cohesive forces.