Spontaneously formed multiscale nano-domains in monophasic region of ternary solution

Ternary mixtures, containing one hydrotrope and two immiscible fluids, both being soluble in the hydrotrope at any proportion, exhibit unexpected solubilization power and unusual mesoscopic properties, which are a subject of long-standing controversies for decades.

This work investigates the entire monophasic region of ternary trans-anethol/ethanol/water system, where multiscale nanostructurings with correlation lengths exceeding dimensions of individual molecules are identified by dynamic light scattering. The physical properties of the ternary mixture are characterized, with revealing the compositional dependence of refractive index and dynamic viscosity.

In surfactant-free microemulsion (SFME) regime, the single phase consists of two distinct nanoscopic domains in equilibrium, one trans-anethol-rich aggregate at the molecular scale (∼1 nm) and one mesoscopic droplet at the mesoscale (∼100 nm). However, only a tiny fraction of the hydrophobic component trans-anethol (<∼0.025%) are initially incorporated in mesoscale structures, and the vast majority are molecularly dissolved or in the form of aggregates. The concentration of mesoscopic droplets increases sharply when shifting toward the miscibility gap, but their size exhibits a weak compositional dependence. The nanoscopic domains exhibit long-term physical stability, and the Ostwald ripening is still the primary mechanism governing such mesoscopic droplet aging, but with a rather low ripening rate.