(16d) Probing Microstructure of Self-Assembled Micellar Solutions Via 1D 1 h NMR Diffusometry

Surfactants molecules have a hydrophilic head and a hydrophobic tail. When added with salt in aqueous solutions, they self-assemble into various interesting microstructures, including spherical, rod-like, vesicles, linear worm-like, and branched networks. These surfactant solutions have many applications, such as household cleaning and self-care products. A microstructural transition is usually observed in surfactant solutions beyond a critical salt to surfactant concentration ratio. A range of micellar solutions with a constant surfactant concentration of Cetyl trimethyl ammonium bromide (CTAB), octyl trimethylammonium bromide (OTAB), and cetylpyridinium chloride (CPyCl) and a range of salt concentrations of 5-methyl salicylic acid (5mS), sodium oleate (NaOA) and sodium chlorate (NaClO3) dissolved in D₂O, are considered. Our rheological measurements indicate that these micellar solutions exhibit a viscosity peak beyond a critical salt to surfactant concentration ratio, and each are known to undergo different microstructural transitions; linear to vesicles (CTAB/5mS), linear to linear (OTAB/NaOA) and linear to branched (CPyCl/NaClO3). The final properties of the self-assembled micellar solutions are directly tied to their microstructure. The goal of this work is to assess the microstructure of various micellar solutions using a non-invasive NMR diffusometry method. After sample preparation and rheological characterizations, we investigate the diffusion dynamics in micellar solutions with the help of 1D ¹H NMR diffusometry. Our results show that for the CTAB/5mS system, which shows linear to vesicle transition, at low salt concentrations, self-diffusion follows a 1D restricted diffusion with an apparent diffusion coefficient that is smaller than that measured for water. As salt concentration increases and worm-like micelles are formed, the diffusion follows a 2D pattern. Finally, for even higher salt concentrations but lower viscosity due to vesicles formation, diffusion seems to be not restricted (3D) but slower than the water molecules diffuse. We have also measured the mean square displacement and apparent diffusion coefficient for other systems, where the observed diffusion behavior corresponds to the expected microstructure.