Unveiling a Rheological Response and Fractal Dimension with Micellization in a Zwitterionic-Anionic Mixed Surfactant System

This study uniquely reveals the hierarchical self-assembly and complex micellar transitions in a selective zwitterionic-anionic mix surfactant system in aqueous solution and the presence of varied salts. A combination of tensiometry, rheology, and small-angle neutron scattering (SANS) experiments was used to investigate these nanoscale transitions in a selective mixed micellar system. The critical micelle concentration (CMC) for single and mixed systems was determined at ambient temperature, and the interaction parameter (β), depicting favorable synergism at a specific mole fraction, was calculated using Rubingh's regular solution theory (RST). Again, various thermodynamic parameters, such as Maeda's free energy of micellization (Δ<i>G</i>_Maeda), the free energy of micellization (Δ<i>G</i>_M), and the free energy of adsorption (Δ<i>G</i>_ads), were calculated. Density functional theory (DFT) calculations were carried out employing the Gaussian 09W software package and visualized using Gauss View 6.0, which yielded varied quantum chemical descriptors that supported the marked influence of the anionic surfactant in favoring the micellization of the zwitterionic surfactant in the examined system. Also, the obtained reduced density gradient (RDG) and noncovalent interactions (NCI) enabled the understanding of the synergistic interactions involved therein, which is affirmed by Two-Dimensional Nuclear Overhauser Enhancement Spectroscopy (2D-NOESY) that explored the self-assembly mechanism in the examined micellar solution. The rheological parameters, such as the modulus of elasticity (<i>G</i>') and modulus of viscosity (<i>G</i>″), described varied solution behavior covering fluidic, viscous, and gel formation, thus attributing viscoelasticity, particularly in the presence of salts. The SANS approach inferred micellar growth with various morphology transitions ranging from spheres, ellipsoids, and cylinders. Interestingly, it also displayed a fractal dimension (<i>D</i>_m) in the examined system, not previously reported for this class of surfactant mixtures, making this study very novel, giving an account of the specific binding of anions and cations from the added electrolyte.