Uncovering the nanoscopic phase behavior of ternary solutions in the presence of electrolytes: from pre-Ouzo to Ouzo region

TL;DR

This study investigates how electrolytes influence the structure and stability of surfactant-free microemulsions, revealing thresholds for phase transitions and mechanisms of droplet coalescence, with implications for designing tailored emulsions.

Contribution

It provides a comprehensive understanding of electrolyte effects on microemulsion nanostructures, highlighting the threshold conditions and mechanisms driving phase behavior in surfactant-free systems.

Findings

Electrolyte levels trigger phase separation and droplet coalescence.

Water-in-oil nanodroplets are insensitive to ionic strength.

High salt accelerates Ouzo droplet collapse and phase demixing.

Abstract

In this work, we report a comprehensive study of how electrolyte addition governs the structure and stability of surfactant-free microemulsions in a trans-anethol/ethanol/water system. The universal structural response has been validated, spanning the full range of solution dispersed-phase structurings, from sub-10 nm W/O reverse-aggregates to O/W mesoscopic droplets (~100 nm) and classical Ouzo droplets (~1 {\mu}m). Experimental results reveal that there is a threshold for electrolyte levels above which oil-in-water nanodroplet coalescence and phase separation are triggered: screening of electrical double layers and "salting out" of hydrophobic components drives hydrotrope into fewer, larger droplets. The total oil volume sequestered in the dispersed phase remains essentially constant, indicating oil redistribution rather than dissolution. In contrast, water-in-oil nanodroplets in a predominantly organic medium display near-complete insensitivity to ionic strength, owing to low dielectric screening and tight interfacial packing that exclude substantial ion uptake. Finally, addition of high salt to Ouzo droplets accelerates their collapse: large droplets fuse and sediment, leaving only residual nanostructures and confirming electrolyte-driven phase demixing. This insight offers clear guidelines for designing additive-free emulsions with tailored lifetimes and nanostructure architectures across pharmaceutical, food, and materials applications.