Uncovering the Nanoscopic Phase Behavior of Ternary Solutions in the Presence of Electrolytes: From Pre-Ouzo to Ouzo Regions

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 water-in-oil (W/O) reverse aggregates to oil-in-water (O/W) mesoscopic droplets (∼100 nm) and classical Ouzo droplets (∼1 μm). Experimental results reveal that there is a threshold (∼10^-3 M) for electrolyte levels above which O/W nanodroplet coalescence and phase separation are triggered: screening of electrical double layers and "salting-out" of hydrophobic components drive the hydrotrope into fewer, larger droplets. The total oil volume sequestered in the dispersed phase remains essentially constant (∼0.01%), indicating oil redistribution rather than dissolution. In contrast, W/O reverse aggregates (∼3 nm) 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 (e.g., 2.0 M NaCl) 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.