Liquid–liquid phase separation and self-assembly of hexanoic acid and ethyl hexanoate in ethanol–water systems: a model for aged colloidal Baijiu

Liquid-liquid phase separation (LLPS) in the pre-Ouzo region governs the early-stage aggregation of amphiphiles, dictating the non-equilibrium evolution of soft materials. However, classical turbidimetry, which defines phase boundaries by macroscopic cloudiness, is blind to this metastable regime. Here, we introduce an integrated fluorescence-microscopy ternary phase mapping approach that directly probes the pre-Ouzo region in a canonical ethanol-water-amphiphile system. This method reveals critical aggregation concentrations that are 2-3 orders of magnitude below the binodal-a regime inaccessible to turbidimetry. Applying this diagram to naturally aged Baijiu (1-20 years), a dynamically evolving colloidal system, uncovers a pronounced aging-enhanced solubilization: the dissolution ratios of key amphiphiles (hexanoic acid and ethyl hexanoate) exceed 96%, with dissolved concentrations far surpassing static equilibrium predictions. Mechanistic investigations show that this phenomenon arises from the synergistic restructuring of ethanol-water hydrogen-bond networks and the expansion of hydrophobic microdomains. Our work not only provides a high-resolution tool for mapping non-equilibrium phase behavior but also establishes a direct link between slow microstructural evolution and the emergence of kinetically stabilized, supersaturated states in complex fluids.