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

TL;DR

This study investigates how ionic environment and pH influence the nucleation, growth, and stability of nanodomains in surfactant-free microemulsions, revealing mechanisms like Ostwald ripening and electrostatic effects.

Contribution

It provides new insights into controlling nanodomain behavior in surfactant-free systems through ionic and pH modulation, supported by experimental and theoretical analysis.

Findings

Extreme pH affects droplet size and density.

Ostwald ripening drives droplet growth.

Electrostatic repulsion prevents coalescence in neutral conditions.

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.