Probing self-assembled micellar topologies via micro-scale diffusive dynamics of surfactants

Abstract

HypothesisSurfactants spontaneously self-assemble in aqueous solutions and are critical in energy, biotechnology, and the environment. The self-assembled micelles may experience distinct topological transitions beyond a critical counter-ion concentration, yet the associated mechanical signatures are identical. By monitoring self-diffusion dynamics of individual surfactants in micelles via a non-invasive 1H NMR diffusometry, we may distinguish various topological transitions overcoming challenges associated with traditional microstructural probing techniques. ExperimentsThree micellar systems based on CTAB/5mS, OTAB/NaOA and CPCl/NaClO3 are considered at various counter-ion concentrations, and their rheological properties are assessed. A systematic 1H NMR diffusometry is conducted and the resulting signal attenuation is measured. FindingsWith no counter-ion, surfactants self-diffuse freely with a mean squared displacement Z2∼Tdiff in the micelles. As counter-ion concentration increases, self-diffusion becomes restricted with Z2∼Tdiffα, and α→0.5. Beyond the viscosity peak, for the OTAB/NaOA system that shows a linear-shorter linear micelle transition, Z2∼Tdiff0.5. Conversely, for the CTAB/5mS system that experiences a linear wormlike-vesicle transition above the viscosity peak, a free self-diffusion is recovered. The diffusion dynamics in CPCl/NaClO3 are similar to those of OTAB/NaOA. Hence, a similar topological transition is surmised. These results highlight the unique sensitivity of the 1H NMR diffusometry to micelles topological transitions.