Vesicle-to-micelle transition in a double chain quaternary ammonium surfactant system: Interfacial behavior and molecular insights

Abstract

Vesicles have emerged as promising drug carriers with excellent compatibility with cell membranes, holding immense potential for biomedical applications. While most reported micelle-to-vesicle transitions involve increasing surfactant or additive concentrations, the reverse vesicle-to-micelle transition remains less explored. In this study, we investigate the vesicle-to-micelle transition of hexyldimethyloctylammonium bromide (C6C8Br) using dynamic light scattering, transmission electron microscopy, and fluorescence spectroscopy. Tetradecyltrimethylammonium bromide (TTABr), a single-chain tetraalkylammonium surfactant with the same chain-length, is studied for comparison. Through the chemical trapping (CT) method, we explore the interfacial composition changes during the vesicle-to-micelle transition. Notably, C6C8Br exhibits a unique characteristic setting it apart from TTABr and other cationic surfactants. At lower concentration ranges, there is a simultaneous increase in interfacial molarities of water and bromide ions, signifying a decrease in interfacial headgroup molarity and surfactant molecule packing. Moreover, our dissipative particle dynamics (DPD) simulation results show that, as the surfactant concentration rises, the interior angle between surfactant chains widens, and the molecular configuration of C6C8Br approaches a straight line. These combined CT and DPD findings underscore the prominent role of steric effects arising from the rigidity of the double-chain. These effects become more pronounced at higher C6C8Br concentrations, leading to looser interfacial packing of surfactant monomers and driving the transition from vesicles to micelles. Interestingly, the breakage of vesicles was not observed when replacing the counterion of C6C8Br with acetate in C6C8Ac solutions. CT results suggest that the elevated interfacial acetate molarity may faciliate the compact packing of surfactant monomers and hinder vesicle-to-micelle transformation. Our study provides a molecular-level understanding of the vesicle-to-micelle transitions in C6C8Br solutions and establishes a theoretical basis for morphological control of surfactant aggregates.