Ionic environment-modulated nucleation and stability of multiscale nanodomains in surfactant-free microemulsions

Abstract

Nano-clustering occurs in the monophasic "pre-Ouzo" region of ternary liquid mixtures without the use of surfactants. This study is proposed to elucidate the nucleation and stability of multiscale nanodomains in a surfactant-free microemulsion (SFME) system composed of trans-anethol, ethanol and water, tuned by aqueous ionic environment. We examined direct- and reverse-SFME structures with different compositions performed by dynamic light scattering and nanoparticle tracking analysis. Our findings demonstrate that extreme pH and ionic strength modulation significantly affect the nucleation and growth of nanodomains, resulting in larger sizes and lower number density of mesoscopic droplets in O/W structuring, whereas the reverse aggregates (W/O structuring) remain stable. Our findings clarify that Ostwald ripening is the primary mechanism to drive the droplet growth. Both theoretical calculation and experiment results match well and reveal that droplet ripening is significantly affected by extreme acidic (pH<3) and alkaline (pH>10) conditions. Electrostatic repulsion in a neutral ionic environment can prevent the coalescence of droplets induced by collision. This research provides an insight into the behavior of multiscale nanodomains in SFME under extensive pH control for applications in material synthesis, drug solubilization and gel preparation.