Multiscale Insights into the Counterion-Mediated Self-Assembly of CTA+ Micelles: Bridging Molecular Interactions and...

As a cationic surfactant, the diverse self-assembly behaviors of cetyltrimethylammonium bromide (CTA⁺) are influenced by the structure and type of counterions. However, related issues have not been systematically studied. Therefore, this paper employs multiscale simulations using density functional theory and coarse-grained molecular dynamics to simulate the interaction and dynamic behavior between CTA⁺ and different aromatic counterions. The results show that the stronger the hydrophobicity of the hydrophobic group of the counterion, the easier it is for the counterion to "insert" into the micelle, thus effectively shielding the electrostatic repulsion between the surfactant headgroups. The arrangement of CTA⁺ headgroups on the micelle surface tends to be hexagonal with closest packing. The reason why sulfonate counterions promote the growth of wormlike micelles more easily than carboxylate counterions is that the stronger interaction between the sulfonate group and the quaternary ammonium contributes to the more effective shielding of electrostatic repulsion and facilitates micelle growth. Additionally, we found that the stability of the CTA⁺-BSA micelle system with sulfonate counterions is better than that of the CTA⁺-BzO system with carboxylate counterions. The research results deepen the basic understanding of the counterion-mediated micellization process and provide actionable guidelines for the engineering design of viscoelastic surfactants.