Uncovering chemical principles governing nanophase formation in ternary solvents

Nanoscopic phases, such as oil-enriched pockets dispersed in water, have been observed in ternary mixtures of oil, water, and cosolvent in the absence of surfactants. Such nanophases are found across a portion of compositions within the single-phase region of the ternary phase diagram. However, the principles governing the formation of nanophases under certain conditions but not others, regarding both volumetric and chemical makeup, are unclear. Here, the nanophase behavior of ternary mixtures of water with a suite of cosolvents and oils containing strategically chosen functional groups is systematically analyzed to probe the role of intermolecular interactions. Dynamic light scattering is used to quantify the nanophase structure. It was found that stronger classes of intermolecular interactions such as H-bonding or n-Π* interactions between oil and cosolvent notably contribute to forming thermodynamically stable nanophases. Ternary mixtures in which the oil has only van der Waals interactions with the water and cosolvent do not stabilize nanophases detectable by dynamic light scattering. Aromatic groups favor nanophase formation. The most prominent structuring (highest number and largest sizes of nanophases) is found in water-rich compositions near the miscibility gap. This experimental study provides chemical insight into the chemical formulation of ternary solvent mixtures favoring nanophase formation.