Self-assembly and micellar transition in a CTAB-sodium oleate mixed system: experimental and molecular dynamics validations

Abstract

Mixed micellar systems are critical in academic research and industrial applications owing to their tunable interfacial and solution-phase properties. This study investigated the nanoscale self-assembly in a mixed system consisting of two oppositely charged surfactants, namely, cetyltrimethylammonium bromide (CTAB) and sodium oleate (NaOl), in aqueous solutions. A combination of tensiometry, dynamic light scattering (DLS), and small-angle neutron scattering (SANS) techniques elucidated the physicochemical characteristics of the system in single and mixed micellar environments. Tensiometric measurements determined the key interfacial parameters, where the observed critical micelle concentration (CMC) values of the mixtures indicated the non-ideal behavior of the mixed system. Thermodynamic parameters were quantified using regular solution theory (RST), revealing a non-ideal mixing and the composition-dependent interaction parameter (β), which have not been previously reported for this specific mixed micellar system, thereby making our study exclusive. Notably, the energetics at a CTAB mole fraction (X_CTAB) of 0.25 exhibited a more negative value of β attributed to the enhanced hydrophobic–hydrophobic interactions, depicting synergism and thereby indicating favorable micelle formation at X_CTAB = 0.25. At a constant total surfactant concentration of 300 mM, the mixtures of CTAB and NaOl exhibited diverse solution behaviors, including low-viscosity clear fluids, viscoelastic fluids, and gel-like phases, which were further complemented by the scattering studies that inferred morphology transitions, including ellipsoidal and cylindrical micelles and vesicular structures. These experimental observations were further validated using molecular dynamics (MD) simulations, which provided atomistic validation of the nanoscale self-assembled architectures. The GROMACS 2021.1 software package was utilized to run the MD simulations with the help of CHARMM36m force fields. The TIP3P potential was used to model water molecules around the surfactant molecules. The simulations shed light on the nanoscale organization of mixed micelles, providing additional molecular-level insights, such as radial distribution function (RDF), radius of gyration (R_g), root mean square deviation (RMSD), and solvent-accessible surface area (SASA).