A Surfactant-Free Middle-Phase Microemulsion System Formed by <i>n</i> -Hexadecane, <i>n</i> -Butanol, and Water

Surfactant-free microemulsions (SFMEs) are unique microemulsions, which form from immiscible liquids (generally called oil and water components) in the presence of amphi-solvents rather than traditional surfactants. Our understanding of SFMEs is still limited. One of the questions to be explored is whether the surfactant-free systems can form the middle-phase (or Winsor III) microemulsions like the surfactant-containing systems. In the current work, the phase behavior of the ternary mixture of n -hexadecane (HDA), n -butanol (BuOH), and water was investigated at different temperatures (20–90 °C), where amphi-solvent BuOH is completely miscible with HDA (oil) but partially miscible with water. The single-phase, two-phase, and three-phase patterns were observed for the ternary mixture, depending on its composition. The area of the three-phase region in the ternary phase diagram decreases with an increase in T (20–55 °C). The mesoscale structuring of the ternary mixture samples with different existing states was determined using dynamic light scattering, electrical conductivity, and cryogenic and negative-staining transmission electron microscopy techniques. The middle phase of the three-phase systems was identified as the bicontinuous (BC) microemulsion, demonstrating that the middle-phase (or Winsor III) microemulsion can be formed without traditional surfactants. In addition, the Winsor I, II, and IV microemulsion systems were also identified in the ternary mixture. This work provides a better understanding of the features of SFMEs.